3 Minute Marks on Pilot’s Chronographs: (Partially) Debunking Myths and (Some) Dead Reckoning

This is long one (very TL;DR) , but it's intended to begin a settling of what seems a long and ongoing debate about wristwatches that needs settling, and takes a bit of debunking to clear the underbrush. When I started my inquiry, I wish I'd have googled into such a discussion. So here it is.

Vintage chronograph watches are often found with either (or a combination of) unique 15-minute totalizers (opposed to 30-minute totalizers) and/or with pronounced 3-minute increment demarcations (opposed to or in addition to the more typical 5-minute demarcations).



There are several (what seem to me) myths online about the origins of the 3-minute increment demarcations, including that they relate to timing phone calls (either long distance, or pay phones) in the early or middle part of the 20th century. These types of explanations are unsatisfactory to the extent such 3-minute demarcations are:

- Disproportionately found on watch models marketed toward pilots (did only pilot’s time their long-distance phone calls?); and

- Were often found on watches with movements intentionally modified to have a 15-minute totalizer (would a manufacturer go so far as to modify the movement in a pilot’s watch to assist in the timing of a pay phone call?);

While I suppose we could never rule out that some manufacturer placed 3-minute incremental demarcations on a chronograph for purpose of timing phone calls [EDIT: Indeed, see below and later in this forum thread that almost certainly this was the case!], it seemed exceedingly unlikely that – for just one example – Breguet would in the mid-1950’s produce watches available exclusively to military Air Force units and (depending on the exact model/design) either (1) place elongated and lumed markers at 3 minute increments on a 30 minute totalizer, or (2) go so far as to alter the movement to have a 15-minute totalizer with only 3 minute increments displayed (and lumed).]



But at the same time, the internet did not serve up a wealth of explanations behind why Breguet and others may have done this for pilots. And so, absent more concrete aeronautical explanations, the myths of telephone call timing, etc., persist.

THE BEGINNING (FOR ME)

I first noticed an oddity in the minute totalizer on pilot’s chronograph when I purchased my Universal Geneve Aero-Compax 890101/01. I purchased the watch from the original owner, who was a professional helicopter and fixed wing pilot. He purchased the Aero-Compax in 1968, because at the time he needed the chronograph to time his helicopter’s unequal burn rates in doing many stop-and-go jobs.

This Aero-Compax displayed an atypical 15-minute totalizer where usually there was a 30-minute totalizer. The underlying movement, the Venus 178, was used in many watches of the period and - like most chronographs – was set up for a 30-minute totalizer. UG (or Venus on behalf of UG) had purposefully altered the movement to offer a dial with a 15 minute instead of 30-minute totalizer.




This intentional alteration of the function of the Venus 178 was almost certainly intended to appeal to pilots. Similarly, the UG 890101/01 had altered the Venus 178 to be a 24-hour movement, with dual 24-hour time zone capabilities between the dial and bezel. This watch was intended as a pilot’s watch through-and-through.

And, they made a sister version without the 15 minute totalizer having instead the more standard 30 minute totalizer.



The original owner of my UG 890101/01 could not remember nor tell me why a pilot may have valued a 15-minute totalizer over a 30-minute totalizer in 1968. That is just the sort of mystery of horology that gets under my skin.


THREE MINUTE DEMARCATIONS

Once the 15-minute totalizer of my UG Aero-Compax was under my skin, I took to the internet to learn about these 15-minute totalizers. Quickly I was seeing all the early pilot’s watches with 3-minute incremental demarcations on their dials, with the most “intentional” being those with a 15-minute totalizer divided in equal 3-minute increments. In those instances, the manufacture had made the choice to both alter the movement for a 15-minute rather than 30-minute totalizer, as well as to alter the dial design to highlight 3-minute rather than 5-minute increments.

In addition to these early pilot’s watches with 15-minute totalizers broken into 3-minute increments, there were many other models with more standard 30-minute totalizers that included prominent dial design demarcations for 3-, 6-, and 9-minute increments. While less intentional seeming than models with a 15-minute totalizer showing only 3-minute increments, these 30-minute totalizers none-the-less demonstrated that the manufacturer took affirmative steps to assist the wearer in noting 3-minute increments.



These 3-minute demarcations, once seen, cannot be unseen. You’ll notice these 3-minute demarcations everywhere you turn, either in vintage chronographs of the early- and mid-20th century, or in their more modern reissues. You’ll also notice, less often but requiring extra attention, 15-minute totalizers.

THE UNHELPFUL INTERNET

Now under my skin were not just 15-minute totalizers, but also these 3-minute demarcations. I turned further toward the internet for an explanation. I found three main purported explanations. The prevalence of each explanation I found to be inverse to the explanation’s reasonableness.

TIMING OF LONG-DISTANCE CALLS

The “long-distance calls” explanation is both the most prevalent explanation for 3-minute demarcations to be found on the internet but also the most patently silly. [EDIT: Actually, a limited version of this explanation appears to explain some or many such dial designs: see below.]

On this theory, long-distance calls from home were once measured and charged in 3-minute increments, and so watch makers denoted 3-, 6-, and 9-minute increments on chronograph dials in order to assist people in timing their long-distance calls to know when the next automatic charge would be incurred.

While this theory has the appeal of being both easily understood by even non-pilots as well as feeling old-timey (always a draw to the sentimental), it does not hold up to history regarding long distance calls.

In short, while it is true that long distance calls were historically initiated with an initial 3-minute charge (and so once connected there was a flat fee for the first 3 minutes), long distance calls were thereafter charged in additional 1-minute increments. [EDIT: As uncovered later in this forum thread, while the above-described charge methods were true in the U.S. and many other countries around the world, it appears that charge methods were different in the UK and potentially some other European countries, where long distance calls do appear to have been charged in 3-minute successive increments. Accordingly, as discussed later in this forum thread, it appears there can be at least two different convergent stories regarding these types of markers on watches in general, as there may be a concurrent/separate explanation for different design approaches on pilot's watches in particular.]

Accordingly, if watch manufacturers broadly catering to those wishing to time their long-distance telephone calls, at best an initial 3-minute increment would be of marginal utility – but any subsequent 3-minute increments on a dial would be irrelevant. [EDIT: This would remain true for the U.S. and many other countries that, unlike the UK and possibly some other parts of Europe, did not bill in successive 3-minute blocks.]

[To be fair, I think this “long distance calls” myth is really a misunderstanding of the pay phone myth discussed below, which is more defensible (but still not right).]

See also the list of additional counterpoints noted further below.


TIMING OF PAY PHONE CALLS

In a variation on a theme, the second most prevalent explanation for 3-minute demarcations on chronograph dials is that they were used to time not long-distance calls, but instead long-distance pay phone calls. Indeed, in many geographic locations and periods of time, long-distance pay phone calls did appear to toll in 3-minute increments.

This myth is recounted everywhere.

However, here again, despite the appeal of being easily understood by even non-pilots, this theory does not hold up to history regarding long distance pay phone calls.

In short, long-distance pay phone calls did not allow for going over the minutes paid for, and so a chronograph timer was essentially irrelevant. To make a long-distance pay phone call, one would need to first supply the necessary money to be connected for 3 minutes; at the end of those 3 minutes there would be temporary disconnection and a prompt to insert sufficient money for subsequent 3-minute intervals. (Early on, those 3-minute prompts may have been performed by operators, whereas later on they were performed electronically.) In other words, long-distance pay phone calls did not require the assistance of timing 3-minute intervals because a caller would be prompted to either pay for an additional 3 minutes or instead the call was disconnected for lack of paying.

Note also that these “interruptions” to request and receive additional funds for additional 3-minute increments may themselves take up several dozen seconds (the requesting of additional funds, the fumbling for and insertion of several quarters, etc.), meaning that a chronograph’s demarcation of back-to-back 3-minute increments would become unhelpful. By the time a caller had reinserted additional funds for the next 3 minutes, his chronograph would be running well into the next 3-minute increment and no longer tell him when the phone call’s next 3-minute increment was up.

A more favorable read of this “pay phone calls” theory would say that the caller could use the chronograph’s 3-minute increments to know when to pre-pay for the next 3-minute increment before arriving at the payment prompt. On this version of the theory, the caller did not look to utilize the auto-prompt by the phone company to insert more money, but instead looked to avoid the brief interruption of the call by paying in advance for the next 3-minute increment; so, the caller would watch his chronograph and, say, just before the first 3-minute mark begin to insert the additional money for the next 3-minute increment. Alternatively, if the caller did not wish to extend the call, he could use his chronograph’s 3-minute demarcations to know when to begin saying his “goodbyes” before being cut-off unceremoniously.

This latter version of the “pay phone calls” theory (anticipating the end of a 3-minute call interval) does not suffer any logical fallacy (unlike, say, the “long-distance call theory”). And, if a person had a chronograph with a 3-minute interval denoted it would appear to have this marginally helpful use-case: a person making a long-distance call from a pay phone who wanted to avoid the pay phone’s auto-prompt for additional funds and/or say his “goodbyes” before being unceremoniously disconnected.

While this limited use case might incidentally find helpful a chronograph with 3-minute demarcations, I cannot believe that this limited use case was the reason that watch manufacturers were altering their dials and movements to make prominent and legible 3-minute increments.


ADDITIONAL COUNTER-CONSIDERATIONS TO ANY TELEPHONE-RELATED (AND MANY OTHER TYPES OF) EXPLANATION FOR 3-MINUTE DEMARKATIONS

A fulsome historical refutation of, for example, the various telephone hypothesis described above would be best served with a novella, including supporting documentation of historical charge rates, etc. I’ve avoided that sort of depth here for reasons of both brevity and necessity, as I think that the points made above already cast sufficient doubt on those theories “from the armchair.” But there is more to be said in that vein and I want to briefly enumerate these other counter-considerations to any such telephone-related (or similarly anecdotal and non-pilot related) explanations for 3-minute demarcations on chronographs, in that they:

·tend to ignore that such 3-minute demarcations were overwhelmingly seen in watches marketed to pilots (and there’s no reason to believe only pilot’s preferred to time their telephone calls) or even on watches made solely for military Air Force pilots

·typically come from commentors in North America, and never demonstrate that (for example) a 3-minute telephone charge convention was standardized across the globe (such as to explain global watch manufacturers’ standardization in this design feature to all markets)

·provide no good reason that 30-minute totalizers with 3-minute demarcations show only the 3-, 6-, and 9-minute marks (when telephone calls, etc., undoubtedly could last 12 minutes!)

·provide no reason a watch manufacturer would go so far as to alter a movement to create a 15-minute totalizer (most closely associated with pilot’s watches denoting only 3-minute increments) in place of a 30-minute totalizer

·seem unlikely to explain why some watch manufacturers further emphasized the legibility of these 3-minute increments by, e.g., creating “big eye” dials, including lumed indices and hands, etc., suggesting legibility at-a-glance was intended (when presumably legibility in, e.g., the telephone booth was not so critical an issue)

[EDIT: Here, I now believe that many of the above critiques do not apply to many watches that merely took a standard movement and dial design and simply added the 3/6/9 emphasis in the minute totalizer; for many such watches, it would appear that manufactures could in fact have been providing buyers in the UK and potentially elsewhere in Europe an on-wrist "telephone timer." However, I also now believe there is essentially a second category of watches marketed to pilots, that went further in not only design but even movement construction, in order to cater specifically to pilots, as discussed further below.]




THE THIRD, MORE ESOTERIC, EXPLANATION: PILOTS, NAVIGATION, AND DEAD RECKONING

In various watch forums discussing the 3-minute increments, inevitably a pilot chimes in with a suggestion that these demarcations were used by pilots for navigation. Some such pilots go so far as to briefly mention some esoteric navigation concepts related to the 3-minute increments.

Inevitably, these brief injections are overwhelmed by the musings of non-pilots who are not immediately convinced by any such esoteric suggestion. The non-pilots appear more drawn to explanations regarding telephone calls, and what not – perhaps because it’s most digestible.

But for me, these injections by pilots, under-argued or under-explained as they may be, made the most immediate and compelling case notionally. Any explanation rooted in aeronautic navigation would explain the things left broadly wanting by alternative theories of telephone calls, etc., including:

·why such 3-minute demarcations are overwhelmingly found in watches marketed to pilots (including those watches made exclusively for military Air Forces)

·why manufacturers from across the globe were uniformly focused on 3-minute increments (aeronautic navigation principles being largely uniform by mathematic necessity and shared units of measurement)

·why manufactures may go so far toward legibility as to create 15-minute totalizers, lumed indices in such 3-minute increments, etc. (legibility-at-glance in pilot’s watch being a very familiar concept to watch collectors)

·Etc., etc., etc.

In short, it stood to reason that pilots’ chronograph manufacturers had made such noise about 3-minute increments for good reason; something of broadly applicable importance to pilots in the early to mid-20th century. But unlike the telephone explanations, the actual utilization of the 3-minute increments was not readily apparent to average-joe.

This sent me down a rabbit hole of aeronautical navigation as relates to 3-minute increments. I’m not a pilot, so this is not my expertise. And, in the modern age, even those pilots I asked did not seem altogether agile with the navigational desires of pilots in, say, the 1930’s and 1940’s.

But, esoteric as it may seem at first blush, I have satisfied myself these chronograph demarcation of 3-minute increments (and all of the mechanical or dial design decisions around it) were to provide pilots with the most basic of tools for aeronautic navigation.



BASICS OF NAVIGATION

Still today, but moreso in the early and mid-20th century, pilots require “rules of thumb” by which complicated calculations can be simplified into (literally) “on-the-fly” educated estimates. Making calculations of absolute accuracy, absent the assistance of computers, requires a degree of attention, equipment, and time that simply aren’t practical when one’s hands must stay “on the stick.”

Meanwhile, it should be uncontroversial even to non-pilots that the wristwatch (or clocks in general) was of paramount importance to pilots and aeronautic navigation. This, afterall, is why there are “pilots watches” as a class.

As one white paper from an aviation resources website put it: “It is generally agreed that the compass is a pilot’s primary navigation tool. But when it comes to specifying the second most valuable such device in the cockpit … those with more experience vote for the clock. After all, when a fuse blows or the left-right needle behaves like a metronome gone berserk, a pilot must resort to basics. The reliable compass and clock become his primary weapons in a battle of wits against the elements. The compass indicates where he’s going, and the clock tells him how far. Without either of these allies, a pilot can get lost, very fast, especially when above the clouds or when over terrain where checkpoints are confusingly few and far apart.”

https://www.gofir.com/aviation_accident_prevention_program/docs/pdf/dead_reckoning_navigation.pdf

Specifically, the compass and clock are utilized together for so-called “dead-reckoning navigation.” The white paper goes on to say:

“[D]ead-reckoning navigation, however, is slowly becoming a lost art as increasingly more reliance is placed on electronic guidance…. Dead-reckoning, or DR navigation, is a relatively painless procedure that can and should be combined with radio navigation so that a pilot is aware of his approximate position at all times…. [T]he term originated with maritime navigation… [and] is a method of [deducing] en route progress based on the direction of flight and the estimated ground speed since last known position.”

I’ve placed my own emphasis on “the estimated ground speed” above, as it relates specifically to the utility of 3-minute and 6-minute increments applied to aeronautic (or maritime) navigation.


3-MINUTE AND 6-MINUTE INTERVALS AND ESTIMATED GROUND SPEED

Now, I’ll admit I’m not “a math person.” So, forgive the following mathy-like discussion as originated from my brain. I invite all more mathematically inclined people to offer better explanations.

First, it will be obvious even non-pilots that rates of speed are uniformly expressed in per-hour units. There are “miles per hour,” or “kilometers per hour,” or where maritime/aeronautic navigation are concerned there are “knots per hour” and “nautical miles per hour.”

Second, still easily understood by even non-pilots, is that a 6-minute interval is exactly 1/10th of an hour; so, for example, if someone knows a distance that they have traveled in a 6-minute interval, they may (assuming constant speed) quickly deduce their “per hour” distance traveled by merely moving the decimal one place over: if in 6 minutes I traveled 0.6 nautical miles, then I am traveling 6 nautical miles in an hour (if maintaining that same speed).

Third, and now less obvious to non-pilots, is that air speed in aeronautics (and at sea, in maritime applications) is expressed in knots. One knot equals a rate of speed to cover one nautical mile per hour.


Accordingly, if a person knows:

·their present air speed in knots (say 50 knots), then they can quickly deduce how far they will travel in ground distance at 6 minutes (5.0 nautical miles), or 3 minutes (2.5 nautical miles), or 15 minutes (12.5 nautical miles) in virtue of the time units being easily calculable proportions of a 60-minute hour

·or instead, that they want to initiate a turn after traveling an additional 8.0 nautical miles, then they may set their air speed to 80 knots and then travel for 6 minutes (or, similarly, set their air speed to 160 knots and then travel for 3 minutes), or any number of other easily divisible proportion of a 60-minute hour

With just this much, it may begin to seem reasonable that 3-minute and 6-minute increments on a chronograph are useful because they provide various sorts of “rule of thumb” guidance to a pilot making “on the fly” calculations that utilize convenient proportions of an hour (3- and 6-minutes) in the context of determinations that involve per-hour rates of speed or distance.

This type of usefulness basically underpins the various methodologies of either maritime or aeronautic “dead-reckoning.” As quoted above, DR navigation can be summarized as “a method of [deducing] en route progress based on the direction of flight and the estimated ground speed since last known position.” And when it comes to estimating ground speed, the easiest conversions from knots can occur as demonstrated in the attached video.


Basically, because of the underlying divisibility units of knots, yards, nautical miles, etc., together with the easy divisibility of per-hour rates, it turns out that:

THE “RULE OF 3” (TO QUICKLY ESTIMATE GROUND SPEED IN YARDS): the “rule of 3” states that the distance a craft will travel in 3 minutes in yardsis equal to speed in knots [multiplied by] 100.

THE “RULE OF 6” (TO QUICKLY ESTIMATE GROUND SPEED IN NAUTICAL MILES): the “rule of 6” states that the distance a craft will travel in 6 minutes in nautical miles is equal to the speed in knots [divided by] 10.

So, a pilot that knows his air speed in knots, can reference either 3 minute or 6 minute intervals to quickly determine an estimated ground speed (and so distance traveled), and/or to time (using 3 or 6 minute intervals and their larger integers) longer ground distances needing traveled.

As put by a different navigation white paper (in the maritime context), “The six-minute rule is a great tool for the cockpit navigator and is useful for a navigator looking for a fast way to lay down a series of short-interval DR positions. The 6-minute rule is also a vital part of any radar collision avoidance plot. The rule states that the distance traveled in six minutes is equal to one-tenth the distance traveled in an hour. Sounds almost too simple to bother with until faced with trying to calculate the distance traveled in five minutes (how many of us can quickly calculate one-twelfth of eight nautical miles?) …. The 6-minute rule really comes into play in close-in harbor approach navigation, when precise position information and navigation decisions must be derived quickly. It is much easier to law down a DR track at 6-minute intervals (or a multiple of 6, since the rule can be used easily for 12-, 18- or even three-minute intervals) than to struggle to find the calculator or the circular slide rule. With the six-minute rule the navigator can do all such calculations in his or her head.”

The bold emphasis above is again mine, because…


THE BREITLING PROBLEM

When considering the “mystery” of the 3-minute and 6-minute increments on pilot’s chronographs, it’s almost comical to consider the role of the various slide-rule or ‘computer’ watches that are widely recognized as being intended for pilot’s navigational purposes. A Breitling Navitimer/Cosmonaute is a classic example for which there is little debate the company intended the watch to be useful to pilots, and utilized for making various calculations.

In the same breath, many – I think including pilots – will comment on how overly wrought with information such “computer” bezels are. So, while a Navitimer is unarguably designed to assist pilots with navigation calculations, it is at the same time almost unhelpfully complete in that effort.



Ironically, the far more simplistic approach of a highly visible 15-minute totalizer with 3-minute indices, is almost so elegant in its design-to-utilization approach that over time it has become lost on us why anyone might ever offer this feature to a pilot’s chronograph.

In a way, finding an agreed explanation of the utility of 3-minute indices on a chronograph may have been hampered by “the Breitling problem” of thinking that an aeronautic navigation watch must look far more complicated than is even useful to many pilots. For many pilots, their cockpit compass plus a chronograph with 3- and 6-minute increments goes a long way toward helpful navigational tool.



THE 9-MINUTE MARK PROBLEM (AND ITS VARIENTS)

The skeptical might at this point be wondering about the 9-minute demarcations found on many chronographs, given that the explanations above focus so heavily on 3- and 6-minute “rules.” It’s a good question, and one that I think can be largely explained by two things in conjunction.

First, several of the most serious pilot’s watches do not have only 3-, 6-, and 9-minute demarcations, but instead their minute totalizer is entirely displayed in 3-minute increments. We might take these examples as really going the distance in providing legible demarcations in increments of time useful to navigation.

Second, regarding the many chronographs that have an otherwise normal minute totalizer except for elongated or highlighted 3-, 6-, and 9-minute indices: these are arguably the bare minimum indications needed for the totalizer to become helpful. With a 9-minute demarcation, a pilot traveling at 160 knots for 3 minutes will have traveled approximately 8 nautical miles, for 6 minutes 16 nautical miles, for 9 minutes 24 nautical miles, for 15 minutes 40 nautical miles, etc. Put differently, if a pilot can string together 3 contiguous 3-minute runs (totaling 9 minutes), or instead a single 15-minute run, he or she may have covered the waterfront of the bare minimum tool needed for the sort of calculations the chronograph can help with. Remember, the rest of the chronograph with this design approach (the 3 highlighted indices at 3-, 6-, and 9- minutes) will still otherwise denote 10-, 12-, 15-, 30- minute or hour-long intervals, as may be needed.

[EDIT: It now would seem more plausible that "simple" watches that merely highlight the 3/6/9 second marks on the minute totalizer were in fact intended to assist in long distance calling in the UK and possibly other parts of Europe that charged in successive 3-minute blocks; why they chose to only mark up to 9 minutes, when calls could certainly last longer, may have been purely an aesthetic or practical decision.]

ACTUALLY IT'S NOT ABOUT PILOTS OR ABOUT TIMING TELEPHONE CALLS

While not an entirely academic presentation, I hope the above much information will cast serious doubt on whether early- and mid-20th century pilot’s watches highlighted 3-minute intervals for anything other than aeronautic navigation. I haven’t posted all sources or information I came across to support this, but instead just enough to both (1) send others on their own way of confirming for themselves, and (2) dispel, finally I hope, the ongoing myths that manufacturers redesigned the mechanics and dial layouts of pilot’s chronographs for anything other than their utility to pilots in navigation.

That said, I believe the explanation provided above can also explain why seemingly non-pilot’s watches may also contain these 3-, 6-, and 9-minute increments on their dials without needing any reference to timing telephone calls. [EDIT: retracted!]

Somewhat ironically, the following is the best and most simplistic explanation for the 3-minute demarcations on a chronograph that I've not otherwise seen set out in the debates around telephone calls, navigation, etc.

Specifically, 3- and 6-minute increments are useful in calculating rates that are otherwise expressed in hours. If a person is timed to build 8 widgets in 3 minutes, then she can be estimated to build 160 widgets within an hour (or be said to be building widgets at a rate of 160/hour). Similarly, should I drive my car at 50 mph for 6 minutes, I’ll have driven 5 miles. In effect, these 3-minute demarcations provide, in a way, some related functionality to a tachymetre but for events that last longer than 60 seconds (or that need backing into).

Notice, then, that these 3-, 6-, etc., minute demarcations provide to a chronograph dial more legible indices at those minute markers that are incredibly useful to not only pilots, but anyone else looking to quickly estimate rates or distances/units that are otherwise divisible by an hour. The standard chronograph’s emphasis on 5- and 10-minute indices are not terribly helpful/visible in this way.

Which is all to say: even where pilots are not concerned, that watch manufacturers denoted 3-, 6-, and 9-minute indices to time telephone calls, etc., appears wildly debunked when weighed against the mere utility of merely highlighting the useful proportions of an hour. [EDIT: Retracted, especially for such "simple" dial highlights in watches not marketed to pilots.]

Let me put it a different – hopefully conclusive - way: it’s not that phone companies randomly chose 3-minute increments and so watch companies later noted that on their chronograph dials, but instead that telephone companies picked 3-minute charge increments because it is 1/20th of an hour and provides a convenient division of an hourly rate, which is only incidentally related to why watch companies also denoted these convenient increments of an hour on their watch dials.

Many things expressed as a rate expressed in time may benefit from the ability to discern 1/20th or 1/10th of an hour.

THE PROBLEM WITH MY UG AERO-COMPAX

Having for weeks fretted over all the above rabbit holes of 3-minute demarcations, dead-reckoning navigation, etc., I then return to my UG Aero-Compax which has a 15-minute totalizer but without particularly highlighted 3-minute increments. What’s the deal, UG!?

I assume that this dial design decision was borne of the Venus 178’s rather small sub-dial design. If the minute totalizer were to instead demonstrate 30 minute total, then differentiating single-minute increments at a glance would be particularly difficult. Whereas when the minute totalizer is reduced to a 15-minute totalizer, each minute is now – in effect – twice as easy to read.

Admittedly still, for as much of a pilot’s-pilot watch as is the UG 890101/01, it now irks me a bit to look at that sub-dial and see the 5-minute marks emphasized.


CONCLUSION

Despite my UG’s dial design now irking me, I do otherwise find a LOT of solace in having satisfied myself of the origins and utility of 3-minute demarcations found on both pilot’s and non-pilot’s chronograph watches. It’s as simple as the fact that 3-minute increments (3, 6, 9, 15, etc.) are useful divisions of 60, and so the basic equation of [distance = speed x time] may be easily manipulated by a chronograph user to derive one variable when the other two are known, at easily calculated fractions. [EDIT: My own conclusion (YMMV) is that there are essentially two different explanations for two different categories of watches intended for two different use-cases:

-> first, for more 'generalist' chronographs, simply highlighting 3/6/9 on the minute totalizer of an otherwise standard dial/mechanical design was in many (all?) cases directed toward the UK and possibly other European markets where long distance calls were measured in successive 3-minute increments (but would have been otherwise of minimal/no utility in the U.S. or many other countries, where long distance calls were instead charged in only an initial 3-minute minimum charge, but followed by single-minute charges)

-> second, for watches marketed to air or sea pilots (or specifically built for military pilots, such as the Breguet discussed above), for which the dial design and even mechanics of the watch were altered to, for example, create a "big eye" minute totalizer, a 15-minute (rather than 30-minute) totalizer, and to emphasize only 3-minute (rather than 5-minute) increments, including with lumed hands and indices, I continue to believe these were all changes intended to allow a pilot to quickly and easily note 3 minute increments (or multiplies of 3) in connection with many basic navigational calculations (such as dead reckoning, plotting, etc.).

That said, this second category could use and I'll continue to search for primary source confirmation and additional information.]

That is arguably more useful to any chronograph user than are the more standard 5- and 10-minute demarcations; in fact, it seems like rather than wondering why there are 3-minute demarcations on a chronograph dial we may have been better off asking why there are 5-minute demarcations.

This point was not lost on Pilots in particular, who more than the average user of a chronograph had great need for “on the fly” deductions of ground speed, distance traveled, etc., especially in the early and mid-20th century. Indeed, not only did these pilot’s watches therefore often emphasize 3-minute increments, but they would also go so far as to alter the movements (e.g., 15-minute totalizers) and dial designs (e.g., foregoing 5-minute demarcations for complete 3-minute demarcations) to further emphasize the more important timing increments in aeronautic navigation.



And these demarcations while now less obvious in use are still brought forward in modern reissues of historical pilot’s watches





FINAL CAVEAT

Yes, yes, there are some watches that denote 2-minute and 4-minute increments; and these also arguably have some utility to aviation with respect to turn rates, etc. But these exceptions to the rule will have to be dealt with another day.

Amazing research! I found this fascinating, and it reminds us that chronographs were also used by foremen and supervisors to time productivity on assembly lines.

Thanks so much! My two mid-century chronos:

I learn something new every day. Well done! 👍

Brilliant exposition, thanks. Having only recently bought a couple of mid century chronos, the three minute marks had been puzzling me.

Your well written and persuasively argued paper provides the very answer i was looking for. Five gold stars from me.

And how exactly is a pilot supposed to measure the number of yards they covered in three minutes. The tachymeter scale is precisely there for measuring speed, a pilot doesn't need the subdial for that. And there is nothing special about 3 or 6 minutes when it comes to the distance one travels. That part didn't make any sense to me. No pilot is going to set their speed to a particular value just so they can travel a certain distance in 3 minutes. Maybe there is something I'm missing, but the pilot explanation doesn't feel satisfying.

I think it would be good to search for period advertising. Maybe some of the manufacturers promoted the markings as a feature. Or maybe one could find a book from the 1930s. That would be much less speculative.

Nice research!! 👍

This is exactly the point of “the 3 minute rule”: your air speed in knots X 100 = the distance (in yards) you’ll travel in 3 minutes.

If a pilot instead prefers to know nautical miles, the 6-minute rule (or a derivative in 1/2 over 3 minutes) may be used in the alternate.


I didn’t quite say he did, did I? After all, he’ll ideally have a working air speed gauge in knots (that as a separate matter may need conversion to ground speed and/or ground distance traveled).

But in any event, the tachy is a separate matter. It measures something related but different, and only for events that occur at an interval of less than 60 seconds. Furthermore (or for the same reasons), a tachy cannot be used to back into a desired ground distance to be traveled over a period of time (to say nothing of a period of over 60 seconds).


Perhaps either it’s worth a re-read, or instead you’re incredibly adept at performing quick calculations in your mind with more complicated proportions. I quoted a navigation primer that specifically addressed why, when navigating, simple proportions are easier to use than more complicated proportions: see the quote above RE “how many of us can quickly deduce 1/12th of 8 nautical miles”


It’s my understanding that, in a pinch and certain other scenarios, this is exactly wrong.

For example, DR navigation can be used to quickly plot courses of intersection (or avoidance); I didn’t get into it, but this sort of navigation was/could be used in war to achieve various flying objectives, for just one example. With some information about the heading of another craft, one could use DR to quickly determine how to intersect or avoid that craft. Throttling one’s own speed to achieve this would absolutely be amongst the methods.

Similarly, if one flew over a known rail road crossing at a certain bearing (ignoring wind) and knew that a turn should occur in, say, 20 nautical miles, the pilot might absolutely choose to achieve that by traveling at a given speed for a given time: it seems rather plain to me.


Maybe. I’ve linked to two different primers on navigation and dead reckoning which themselves either provide far more info, or the means to search and find more.

I admitted I did not give this a full journalistic treatment, and instead that I’ve given enough I think for people to spend their own hours, should they care to.


All good if not obvious ideas, which I’d hope to do more of some day.

Though, I did review several period navigation books that also emphasized the 3 and 6 minute rules, which perusal was enough for me, for present purposes.

I also saw other’s claim of seeing a watch manufacturer’s claims regarding, e.g., that 15 minutes was the time a proper pilot out to complete pre-flight check. But, such claims in my looking around ran into (1) the reality of comments from pilots (no such standard transcended all decades, all craft, etc., such as to remotely make sense of the prevalence of 15 minute totalizers), and (2) my general weariness if manufacturer’s own historical understanding of their products.

In any event, the post is intended to invite just this sort of critique so that, if nothing else, the internet does not rest on suggesting it has something to do with timing long distance phone calls (except incidentally).

Please do feel free to pile on.

Sorry, I don't follow. 🙁

Back to the pay phone explanation for me. 👍

One compounding thing is that the proliferation of wristwatch chronographs (~1913), flight (first use in battle ~1911), and long distance telephoning (first transcontinental call ~1914) all share somewhat overlapping timelines. So at least where wristwatches are concerned, correlation and causation are alone, muddy.

But as for pre-existing stopwatches or pocket watch chronographs: certainly better experts could (and I hope will) chime in, but I chased this lead for a brief moment and was discouraged.

On one hand, the earliest chronographs were most commonly cited for timing horse races, astronomical events, and any manner of things; whereas on the other hand, the “invention” of a wristwatch was historically tied very closely to taking these clocks into a cockpit, with chronographs becoming seemingly most popular first with pilots (for related reasons of navigation).

Put differently, that the proliferation of wristwatch chronographs were closely tied to pilots I think is well established; this 3-minute interval question, then, is really about whether that aspect of chronographs was also an extension of this established relationship between chronographs and pilots.

That said, perhaps it’s worth reiterating or reframing the intended conclusion of the above long-windedness: as it turns out, chronographs noting 3 or 6 minute intervals provide a convenient dissection of a 60 minute hour, which convenient dissections may be used for estimating any number of calculations that are relevant to an hour. Accordingly, the particularly close association between pilots and 3-minute intervals is borne not of any exclusive relationship between pilots and 3-minute intervals, but instead only a particularly useful or important relationship between pilots and needing to quickly and easily make “back of napkin” type calculations around the equation [distance = speed X time] or it’s derivatives.

Which brings me back to a version of your very good lead that could use further chasing: basic aeronautical navigation such as dead-reckoning is derivative of its predecessor maritime navigation. So, one could look to ship’s equipment, etc., for some additional precedent.

That said, a ship’s size and relatively slow speed allowed for far more equipment or calculation time on hand. -: compared to early or mid-century pilots. Nonetheless, early maritime navigation books, and the coast guard navigation slide provided above, etc., all still point to maritime utilization of 3/6 minute rules when charting. Indeed, one of the navigation primers I linked to above was a maritime navigation primer.

Would love the pocket watch or stopwatch aficionados to weigh in!

Which, amongst the choices available, is among the most anecdotal, least supported, and uniformed of conclusions. Ironic, given the only critiques you’ve suggested.

Which altogether suggests that really you’re just feeling more spunky and contrarian, and less contributive. 👎

I read your first post a couple of times, and honestly it seems very speculative. What may seem sensible or logical to you may not seem sensible or logical to me. Which is why I suggested doing historical research. While there are arguments against the pay phone explanation in some cases, that doesn't mean it wasn't the rationale in other cases.

Maybe there is a navigation reason for 3 and 6 minute increments, but your explanation made no sense so something must be missing. First you mentioned estimating ground speed, but that would obviously be done using the tachymeter. And if you know your speed, converting time to distance is trivial. The idea that a pilot would change speed in order to make the math easier may seem logical to you, but it seems ridiculous to me. "Let's see, I need to go 20 miles and I'm traveling 300 miles-per-hour. So instead of just continuing for 4 minutes, I'll slow down to 200 miles-per-hour so I can get there in 6 minutes." 🙄

There’s really two things you’re saying here: first, navigation itself has nothing to reasonable (in your opinion) to do with 3 and 6 minute intervals, and second, that even if it did it doesn’t explain the markings on chronographs.

Regarding your rather bizarre dubiousness of whether navigation has anything to do with 3 and 6 minute rules, perhaps you’re best progressed with the suggestion of starting by reading the two white papers, video, and US coast card slide deck linked, each from essentially professional navigators, and then write them your observations of how they don’t know what they’re talking about? I could also to a few navigation books I reviewed for these concepts, but I suspect you’re not interested in reviewing those more involved discussions any more than you are the already linked simplified versions.

This at something like this might address your rather bizarre armchair assertions that 3 and 6 minute increments dont have anything to do with navigation in the first place (quite aside from whether they relate to the markings on the watch).

By the way, can I ask that we clear up whether you yourself are an experienced pilot (air or sea) with experience in dead reckoning?

Please don't put words in my mouth to fuel your Straw Man argument. I get it ... you don't like it when someone disagrees with you and you try to snow them under with aggressive lengthy posts. Anyway, that's not at all what I said, and it's silly to argue about this. You are speculating, I am giving my opinion. It's all guesswork. I won't be re-visiting this thread, so carry on.

It should be noted that a number of chronographs that were intended to be used by pilots do not have these 3, 6, 9 minute markings. These chronographs span a number of decades (1930s-1960s), were made by a number of brands (e.g. Tutima, Hanhart, Longines, Breguet, Dodane), and were used by a number of Air Forces (e.g. German, Romanian, French). If such markings were useful to pilots, I wonder why they would be omitted? The omission is especially puzzling given how ubiquitous such markings are on chronographs at large. See examples below.


https://monochrome-watches.com/tutima-grand-flieger-classic-chronograph-review-price/
https://www.vintage-portfolio.com/shop/hidden-gems/vintage-1944-military-style-hanhart-chronograph/?marke[]=0
https://omegaforums.net/threads/longines-13zn-auctions-originality-authority.40453/#post-473641
https://www.phillips.com/detail/BREGUET/CH080118/120
https://shop.hodinkee.com/products/1960s-dodane-type-21-with-military-engravings?variant=22124825604

Feel free to please contribute substantively, whenever.

I may have caused confusion somewhere between my explanation/understanding of "dead reckoning", on one hand, vs, on the other hand, my various other "examples" of math that can be easily performed "on the fly" using 3 or 6 minute increments where any two of three of the following variables are known: distance = speed X time.

Regarding dead reckoning proper, my understanding just as I believe you are describing: with known air speed and time, one can determine ground speed and so ground distance covered.

I wish I had been clearer in that, and also that I had included enough qualifications regarding both my math and my pilot experience to have garnered some charitable reading, and genuinely welcome the additions/clarifications.


It's an interesting comment, as I sort of gather from the materials that I've reviewed that dead reckoning or other uses of the rules of 3/6 are rooted in times where either (1) there was less reliable access to radio or other fixes, and/or (2) one must be prepared for that eventuality?