Assistance: 3-minute marks on pilot’s chronographs watches

[Coleson Bruce]

Hello forum,

I am an interloper to the topics of early aviation, coming instead to the forum as a collector of early pilot’s chronograph watches. I’ve come seeking your expertise, specifically about some “mystery” design features of early pilot’s chronographs.

THE “STANDARD” CHRONOGRAPH:

A “standard” chronograph watch typically includes a minute register (or minute “totalizer”) that displays 30 or sometimes 45 minutes maximum; on these “standard” minute totalizers, there are also typically dial markings that emphasizie increments of 5 minutes (i.e., a standard 30 minute chronograph register may have dash marks for each minute, but then additionally display the numerals at minutes 5, 10, 15, 20, 25 and 30).

THE PILOT DESIGN FEATURES:

Between (roughly) the 1920s and into the late 1960s, chronograph wristwatches were marketed toward and utilized by pilots for innumerable purported utilities in the cockpit, I would imagine.

Some chronographs of the period marketed towards pilots or designed on spec for military aeronautics would alter the “standard” approach to the minute totalizer design. Rather than being designed to “emphasize” 5 minute increments, they instead emphasize 3 minute increments. So, rather than containing numerals and emphasis at the 5 minute marks, there would instead on these chronographs seemingly geared towards pilots be emphasis placed on the 3 minute marks.

Were this “emphasis” of showing 3 rather than 5 minute intervals all, it might be written off as mere design happenstance. But, many such chronograph manufacturers went even further in this apparent emphasis on the 3 minute increments. For just some examples:

-> some manufacturers altered the dial design to enlarge the minute register for even greater legibility of these 3-minute demarcations, creating so-called “big eye” chronographs (a name due to the asymmetrically enlarged minute register featuring prominently on the dial)

-> they or other manufacturers may also go so far as to alter the mechanical function of the watch to have not a 30 minute register, but instead a 15 minute register (with 3-minute markings), which 15 minute register again appearing to be intended to make more legible these three minute intervals

-> they or others may also make the “hand” and 3-minute marks of the minute register contain luminous material, again to further increase the legibility of these 3-minute demarcations (whereas other “standard” chronographs rarely made luminous the minute register)

When all these design decisions were combined in one watch (sometimes), and marketed towards pilots (or created for military flying applications), it causes one to believe that pilots may have had some particular reason to “need” to easily see 3 minute increments when running their chronograph. (Or, at least, that watch manufacturers believed this to be useful enough for pilots that they altered their dial designs and even the movements themselves).

THE “MYSTERY”

I have not come to the forum completely empty handed of research on this. I do have some theories. But, I don’t want to share them prematurely and then potentially bias any responses (for now) - except to say my research has pointed me towards reasons having to do with navigation calculations.

I hoped the pilots or historical aeronautics buffs here in this forum might do me the wonderful favor of attempting to help with the “mystery” around the probable utility of these early and mid-20th century pilot’s watches that emphasize 3-minute intervals of time.

Put more simply: why might early and mid-20th century pilots have valued a wrist chronograph that made clear(er) to read the elapsed 3-minute time interval? (Any period photo/writing evidence is ultimately where I’m headed.)

A “THANK YOU”

A long first post, only to ask for help, for which I apologized and thank you for your patience.

I’ll also and meanwhile be digging around the forum for accidental answers!

 

[Bell206]

why might early and mid-20th century pilots have valued a wrist chronograph that made clear(er) to read the elapsed 3-minute time interval?

SWAG: Given the 1920s was the beginning of regulated air travel, the 5 min to 3 min change could be due to regulatory procedural changes. And also maybe due to aircraft incrementally flying faster in that same time frame when performing those procedures. A quick search found the reference below to "3 minute" and "5 minute" increments. Another route to take is to contact the Smithsonian as they have an extensive collection of chronographs/watches (to include the ones used on Gemini and Apollo flights) and more importantly the staff behind them that will probably have an answer to your question. I never contacted them about watches but have contacted them when researching old aircraft and they provided some very remarkable feedback and assistance. Good luck.
https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_1.5.html

 

[GaryM]

I'll throw out a wild guess: 3 minute intervals make it easier to track decimal hours.

 

[Shepherd]

What GaryM said.

 

[Coleson Bruce]

I'll throw out a wild guess: 3 minute intervals make it easier to track decimal hours.

This is something like my working hypothesis. But, since I’m not a pilot, much less bit a pilot familiar with early- and mid-20th century pilots that would find a wrist chronograph useful, I’m hoping to find some better authority for this hypothesis.

Specifically, one thing I keep running into are references to the “3 and 6 minute rules” in both maritime and aeronautical navigation. As best I can tell, these “rules of thumb” were utilized in navigation as short hand and ease-of-use calculations when, for example, doing dead reckoning, otherwise eatimating ground speed, or estimating how long one must fly at a given speed in order to cover a certain amount of ground.

Basically, the “6-minute rule” is as simple as noting that 6 minutes is 1/10th of an hour (and 3-minutes 1/20th), and so a pilot may easily do all types of mathematical estimates quickly when trading in units of 6 (or 3).

One example is that your ground speed over a 6 minute period will be roughly your air speed in knots divided by 10. So if traveling at 170 knots, then over 6 minutes you’ll have covered 17 nautical miles. (And, over a 3 minute period your air speed multiplied by 100 will tell you roughly the yards covered on the ground.)

Just the same but doing the reverse calculation, if you wanted to travel 17 nautical miles further ahead, you could set your air speed to 170 knots and hit your chronograph for a six minute interval.

That said, here I am the least knowledgeable person in the room attempting to describe what to me is essentially jargon.

Perhaps someone could help translate more what is meant by “decimal hours”, how that was used, why it may have been useful on a wrist chronograph, etc.? It certainly sounds to be the mathematical explanation for why the “6 minute rule” was a useful “rule of thumb”: it’s much easier to perform quick estimates for a 1/10 than - say - a five minute interval which is a 1/12th fraction (who wants to do THAT math quickly?).

 

[Sac Arrow]

Install Foreflight on the iWatch.

Boom. Done.

 

[Half Fast]

So if traveling at 170 knots, then over 6 minutes you’ll have covered 17 nautical miles. (And, over a 3 minute period your air speed multiplied by 100 will tell you roughly the yards covered on the ground.)

Just the same but doing the reverse calculation, if you wanted to travel 17 nautical miles further ahead, you could set your air speed to 170 knots and hit your chronograph for a six minute interval.

That's an interesting theory, but I think you should leave airspeed out of it. Airpseed and groundspeed are rarely the same because of the effects of wind. BUT, looking at your theory a bit differently, if a navigator were to spot his positions on a map over a 6 minute interval he would easily know his actual speed over the ground.

That might have been useful prior to the invention of the flight computer (the E6-B is the most common example). I think the earliest flight computers began to appear in the 1930s, so it might be interesting to know when 3-minute and 6-minute markings became less common.

 

[Coleson Bruce]

That's an interesting theory, but I think you should leave airspeed out of it. Airpseed and groundspeed are rarely the same because of the effects of wind. BUT, looking at your theory a bit differently, if a navigator were to spot his positions on a map over a 6 minute interval he would easily know his actual speed over the ground.

Thanks for the response! It’s hard for me to strike the right balance between brevity and clarity.

For further clarity, as I understand it, the “3 and 6 minute rules” are are “rules of thumb” intended to achieve quick, shorthand, calculations. As such, they ignore for present purposes the effects of wind, and instead provide a quick approximation/reminder that ground distance traveled in 6 minutes ~= knots/10, etc.

But your additional example is spot on, in that really the 6 minute rule simply provides a quick way to manipulate D=S/T in its various configurations but using 6 minutes as T.

That might have been useful prior to the invention of the flight computer (the E6-B is the most common example). I think the earliest flight computers began to appear in the 1930s, so it might be interesting to know when 3-minute and 6-minute markings became less common.

The apparent trend of pilot-focused chronographs emphasizing the 3 or 6 minute intervals (in some form or another) in the minute totalizer can be seen really throughout the 20s to late 60s (maybe even early 70s).

But, I do not think the advent of the EB-6 would have materially affected the potential use of the 6 minute rules, due to it being merely a shorthand “rule of thumb” to use in stead of the EB-6, or as a redundancy measure, etc.

I’m brought to mind a few quotes I’ve come across in pilot navigation primers or manuals that I believe to have relevance to why a pilot may look to trade in 3 or 6 minute intervals on their wrist chronograph despite other equipment onboard. In an FAA primer on navigation, in the section of emergency situations, the stringency of accurate calculations appears to take a backseat to pragmatism, even for the FAA:

“Computing course, time, speed, and distance information in flight requires the same computations used during preflight planning. However, because of the limited flight deck space and because attention must be divided between flying the aircraft, making calculations, and scanning for other aircraft, take advantage of all possible shortcuts and rule-of-thumb computations. When in flight, it is rarely practical to actually plot a course on a section of a chart and mark checkpoints and distances.”

Or as put by a maritime navigation primer “The six-minute rule … 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?)…It is much easier to lay 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…”

Or from an aviation navigation primer on DR: “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.”

(I’m sorry for not posting links to these source materials, but I don’t yet have that privilege.)

None of this I hope is taken as disagreement, as I’m here to be educated - instead as mentioned from the top, in trying to balance brevity of my posts with clarity there was absolute room to push back - and maybe the materials above help better clarify the possible arena in which a chronograph trading in 3 minute increments was useful.

More than the FAA’s imagined emergency situations, I think to possible WWII pilots making quick decisions, quickly roughing out either interception or avoidance paths, while watching the clouds for enemy aircraft, etc., and start to imagine that reaching for a EB-6 simply wasn’t practical, and a rough sketch would do (even ignoring wind effects on airspeed).

Still, all this deduction doesn’t mean much if seasoned pilots familiar with flying at that time can’t vouch for the inferences!

 

[BladeSlap]

NASA seems to have went with 5min

 

[Salty]

And a phallic symbol at noon.

 

[NoHeat]

Totalizer — I was unfamiliar with that term. So is it a little dial on the watch face, used like a stopwatch?

Nowadays I can imagine uses for a 1 minute timer for legs of a modern holding pattern (4 minutes total for one circuit of the pattern), 2 minutes for a standard rate turn, and a 30 minute timer for switching fuel tanks.

 

[Coleson Bruce]

NASA seems to have went with 5min

Yes, many chronographs of the time stuck with conventional approaches to the minute totalizer; namely, an either 30 or 45 min max, and typically highlighting 5 minute increments.

However, the Speedmaster in particular was designed for and primarily marketed to automobile sports; that it was ultimately selected by NASA as a reliable chronograph was incidental (and I suspect navigation from space a bit different )

That said, it did not stop Omega from later marketing to the maritime and aeronautic navigator - below are example issues of Boating and Flying magazine and Omega’s campaign approach in ~66-69

 

[Coleson Bruce]

Totalizer — I was unfamiliar with that term. So is it a little dial on the watch face, used like a stopwatch?

Yes; there are various terms, all correct, for these registers that display the chronograph functions. To make a cartoon simplistic dichotomy, a “stopwatch” does not tell time and run continuously, but instead is started and stopped to record the duration of an event; whereas a chronograph is this “stopwatch” functionality incorporated into a time-telling clock. So, you can think of the small registers on a chronograph wristwatch as being the stopwatch displays.

 

[GaryM]

And a phallic symbol at noon.

That's the Dr. Evil rocket model

 

[Coleson Bruce]

I think I’ve come to understand that historical DR perhaps differs from the modern concept of DR, and that the historical sense may be more relevant to these 3 and 6 minute increments.

I’ve uncovered the following “sage” advice article from an author who appears to know his stuff; he’s basically arguing that all the newfangled gadgetry is not only unnecessary but even unhelpful to a pilot. I’m adding it here in full, with the relevant portions RE 6 minute increments being at the final page middle column (near the bottom) and running into the far right column (toward the top).

It would be great to also find more first-hand accounts of this type of approach to navigation being common enough to cause watch manufacturers to adjust their chronograph designs to reflect the importance of the 6 minute (or 3 minute increments). I also suspect that this style of flying described in the article, particularly where there was not the luxury of time in cockpit for better accuracy, was particularly useful to military pilots (the author of this pop piece is the military instructor afterall) - possibly explaining the mil spec watches that went above and beyond to emphasize the 3/6 minute increments.

 

[Sluggo63]

That's an interesting theory, but I think you should leave airspeed out of it. Airpseed and groundspeed are rarely the same because of the effects of wind. BUT, looking at your theory a bit differently, if a navigator were to spot his positions on a map over a 6 minute interval he would easily know his actual speed over the ground.

That might have been useful prior to the invention of the flight computer (the E6-B is the most common example). I think the earliest flight computers began to appear in the 1930s, so it might be interesting to know when 3-minute and 6-minute markings became less common.

Not really related to the original question, maybe sorta.

Obviously like people have said, aviation math becomes really easy when you use multiples of 3 or 6.

To your point of IAS/TAS/GS, when we flew low levels in flight school (I have no idea how the tactical guys do it now with INS/GPS) in basic aircraft (T-37/T-38) students would plan the LL routes days before and plot them out and put tick marks at minute intervals. Then, when it came to fly it, you would "reverse" plan it at a certain GS (which was a multiple of 3). We used 210 knots in the T-37 and 360 knots in the T-38 (if I remember correctly). Starting with your GS, you would backward wind it to find the IAS you needed on that leg to achieve the desired GS (210 or 360). So, each time you turned to a new heading, you would reset your speed to get that GS.

That way you know if you were flying VR1014 it would take 37 minutes from entry to target. Every time. So, if you wanted a TOT of 1500, you made sure you entered at 1423 and flew it as planned. If you were early or late, it was a matter of slowing down or speeding up 30 or 60 knots and quick math to get back on time.

 

[Warlock]

Funny we used 90 knots in a UH-1 for the same reason for night cross country navigation in flIght school…

 

[Coleson Bruce]

Not really related to the original question, maybe sorta.

Obviously like people have said, aviation math becomes really easy when you use multiples of 3 or 6.

To your point of IAS/TAS/GS, when we flew low levels in flight school (I have no idea how the tactical guys do it now with INS/GPS) in basic aircraft (T-37/T-38) students would plan the LL routes days before and plot them out and put tick marks at minute intervals. Then, when it came to fly it, you would "reverse" plan it at a certain GS (which was a multiple of 3). We used 210 knots in the T-37 and 360 knots in the T-38 (if I remember correctly). Starting with your GS, you would backward wind it to find the IAS you needed on that leg to achieve the desired GS (210 or 360). So, each time you turned to a new heading, you would reset your speed to get that GS.

That way you know if you were flying VR1014 it would take 37 minutes from entry to target. Every time. So, if you wanted a TOT of 1500, you made sure you entered at 1423 and flew it as planned. If you were early or late, it was a matter of slowing down or speeding up 30 or 60 knots and quick math to get back on time.

Funny we used 90 knots in a UH-1 for the same reason for night cross country navigation in flIght school…

Agree completely that the heart of what you’re saying still does comport with the same rationale for the “3 and 6” minute rules (or as the Naval Air instructor in the article above called it, “the other ‘rule of 60’” - basically, trading heavily in units that are relatively easy to manipulate given the fundamental 60-based (rather than 100-based) increments of time that are pregnant within a rate of speed.


Basically, my hypothesis is that certain chronographs of the period marketed to pilots attempted to make more legible and apparent 3 minute intervals of chronograph computations in order to assist certain pilots flying in a certain style at the time be able to easily (1) track longer elapsed times either down to a 3 minute interval (eg the chronograph would clearly at a glance total 1hr and 6 minutes) or (2) to use the chronograph to time successive or 3 or 6 (or 15) minute intervals.

I can imagine a scenario where, for example, a pilot discovered they are off course in a fashion needing a correction leg of 6Nm; they could as a “rule of thumb” adjustment turn toward the correction, set their air speed to 60kts, hit their chronograph, and know that at the 6 minute mark on the chronograph they would have roughly closed the desired 6km and make the corresponding turn back to course.

that is, of course, a hypothetical described by a non-pilot, but - whether I get the jargon exactly right - does appear to be they type of pragmatic flying style described in various period materials such as the article posted above, and pretty consistently trading in these 3 and 6 minute increments.

I would love to find more period support, in documentation, or testimonials, but especially where the testimonials include discussion of the use of the chronograph (whether onboard, or on wrist).

But all of these discussions here have been extremely helpful to my understanding already, and I think you!

 

[theowilder]

This is something like my working hypothesis. But, since I’m not a pilot, much less bit a pilot familiar with early- and mid-20th century pilots that would find a wrist chronograph useful, I’m hoping to find some better authority for this hypothesis.

Specifically, one thing I keep running into are references to the “3 and 6 minute rules” in both maritime and aeronautical navigation. As best I can tell, these “rules of thumb” were utilized in navigation as short hand and ease-of-use calculations when, for example, doing dead reckoning, otherwise eatimating ground speed, or estimating how long one must fly at a given speed in order to cover a certain amount of ground.

Basically, the “6-minute rule” is as simple as noting that 6 minutes is 1/10th of an hour (and 3-minutes 1/20th), and so a pilot may easily do all types of mathematical estimates quickly when trading in units of 6 (or 3).

One example is that your ground speed over a 6 minute period will be roughly your air speed in knots divided by 10. So if traveling at 170 knots, then over 6 minutes you’ll have covered 17 nautical miles. (And, over a 3 minute period your air speed multiplied by 100 will tell you roughly the yards covered on the ground.)

Just the same but doing the reverse calculation, if you wanted to travel 17 nautical miles further ahead, you could set your air speed to 170 knots and hit your chronograph for a six minute interval.

That said, here I am the least knowledgeable person in the room attempting to describe what to me is essentially jargon.

Perhaps someone could help translate more what is meant by “decimal hours”, how that was used, why it may have been useful on a wrist chronograph, etc.? It certainly sounds to be the mathematical explanation for why the “6 minute rule” was a useful “rule of thumb”: it’s much easier to perform quick estimates for a 1/10 than - say - a five minute interval which is a 1/12th fraction (who wants to do THAT math quickly?).

Hello. Anyone have suggestions for the best "bang-for-your-buck" aviators watch? I've looked at quite a few options, but can't decide which features I desire in a watch. For those with E6B watches, do you ever actually use the E6B? I think at the least I want something with local and Zulu time. I found some nice watches by Torgoen, but there are cheaper aviator watches by Casio for less than $100. All suggestions are welcome!

 

[Half Fast]

Anyone have suggestions for the best "bang-for-your-buck" aviators watch?

Casio G-Shock.

 

[TCABM]

Apple watch with a GMT face.

 

[ColinD]

Something 45mm or larger and very shiny so there's no confusion who's the acting PIC.

 

[Larry in TN]

My requirements are an analog watch which easily switches time zones. I've used a variety of Citizen Eco-Drive models.

 

[Zeldman]

What is a watch.??

I have been using my cell phone to tell time and as a timer for years. It also does other things. Sometimes I even use it to make phone calls...

 

[Half Fast]

...but can't decide which features I desire in a watch.

That would be step 1.

For those with E6B watches, do you ever actually use the E6B?

No. The numbers are too small to read easily, especially in the cockpit, and half the calculations will require you either to remove the watch from your wrist or read the scale upside down, which is error prone. Wristwatch E6Bs are a gimmick and bling, not really utilitarian.

For the most part, you can think of pilot watches as falling into three groupings.

1 - The utility watch. These are watches like G-Shocks which can be bought for a hundred bucks or so. The G-Shock you already own is likely perfectly acceptable for flying. Don't own a G-Shock? Get one! G-Shocks are an essential part of basic kit, as ubiquitous and versatile as a pocket knife, a 12ga shotgun, or a crescent wrench. My G-Shock is the watch I wear when doing farm chores, fishing/hunting, working on the car, scuba diving, etc., etc. Solar powered, sets time from the atomic clock, all I have to do is wear it. Accurate, displays multiple time zones, has stopwatch, countdown timer, and alarm functions. Rugged as a rock, they're popular with military pilots and with astronauts. It will do everything you need, last forever, and it's inexpensive.

2 - The flying watch. While the G-Shock can do just about everything, it's not necessarily ideal for any one thing. You may decide you want a more specialized watch that better suits your personal needs. Some of the "...cheaper aviator watches by Casio for less than $100..." you mention fall into this category, and I use one as my regular flying watch. It has analog hands and chronometer dials plus a rotating timing bezel. One thing I dislike about my G-Shock is it takes multiple button pushes to use each function, and I have to scroll through displays to see what I want. With my analog Casio, I use the chrono to time my fuel tank changes and the timing bezel to time flight legs. Without pushing buttons, I can tell at a glance the time of day, my fuel time, and time en route. YOUR NEEDS MAY BE DIFFERENT, so you should select the watch that best suits them. You may want to fly for a while to figure out what's best for you.

3 - The bling watch. These are luxury watches. This is the watch you wear to brag, to show off at a party, to let the world know you're a pilot by gawd. It encompasses watches like the Breitling Navitimer, the Omega Speedmaster, and the Rolex GMT Master at the high end (multiple thousands of dollars) and ones like the Citizen Blue Angels Skyhawk and Seiko Flightmaster at the low end ($500-$1000). It's generally poor form to buy one of these yourself; they're supposed to come to you exclusively as gifts, often as a token of an admirer's affection. These watches may have E6B bezels (the Navitimer was the original, I believe), but as I said above they're more for looks than practical use. These watches can certainly be used in the cockpit, but a $100 G-Shock will get the job done just as well.

So figure out first what features you want, then set your budget, and the answer will fall out.

 

[Half Fast]

I have been using my cell phone to tell time and as a timer for years. It also does other things. Sometimes I even use it to make phone calls...

I tried that, but with the cell phone strapped to my wrist it's tough to reach into my pocket.

 

[Piperonca]

My requirements are an analog watch which easily switches time zones. I've used a variety of Citizen Eco-Drive models.

Good discussion of the different GMT and time zone adjusting watches

"In other words, a “true GMT”, you can independently move the hour hand (quick set hour) without moving the minute hand or stopping the watch. The Rolex Master GMT 2 is a true GMT. Whereas in an “Office GMT” (most affordable GMTs) you are able to independently move the GMT hand and the minute hand always moves with the hour hand."

 

[Shepherd]

Practically a necro post but I just wanted to mention that I STILL, to this day, fold my maps as outline in the post by Coleson Bruce.
I was taught this technique by an instructor back in the mid 1960's.
The 1948 Flying Magazine article was a year before I was born, so I had never seen it before, but I copied the article just to have it.
Thanks Coleson.