[Crusader]

We - like many a forum - have had several discussions on the origin and the purpose of the elongated three-minute-markers on chronograph 30-minute subdials otherise marked in 5-minute intervals (confusingly, JohnF has come up with a vintage elongated-4-minute-marker variant).

A number of explanations have been advanced for the elongated three-minute markers, with the need of users (even military ones) to time American long-distance calls measured in three-minute intervals looming quite large.

Looking anew at a Longines A-7 (a U.S. Army Aircorps issued aviation chronograph) from the mid 1930s on MWR (http://www.mwrforum.net/forums/showthread.php?t=22713) and seeing that its 30-minute chronograph subdial is marked ONLY in 3-minute intervals, I am increasingly convinced that the purpose of the elongated three-minute marks is rather basic - its purpose is to make reading the elapsed time easier ... looking quickly at a 30-minute subdial marked with 5-minute intervals only, one may easily be mistaken between, say, 3 and 4 minutes past the interval. Not so with the additional elongated 3-minute markers.

Another way to address the same problem is the 15-minute register as used in the German aircraft clocks of WWII (they switched to 15-minute subdials from 30-minute subdials), and by Breguet for the French Type XX/21 chronographs after the war.

The thesis is open for discussion. ;-)

 

[tempus edax rerum]

Re: On The Three-Minute Markers on Chronograph Subdials

We - like many a forum - have had several discussions on the origin and the purpose of the elongated three-minute-markers on chronograph 30-minute subdials otherise marked in 5-minute intervals (confusingly, JohnF has come up with a vintage elongated-4-minute-marker variant).

A number of explanations have been advanced for the elongated three-minute markers, with the need of users (even military ones) to time American long-distance calls measured in three-minute intervals looming quite large.

Looking anew at a Longines A-7 (a U.S. Army Aircorps issued aviation chronograph) from the mid 1930s on MWR (http://www.mwrforum.net/forums/showthread.php?t=22713) and seeing that its 30-minute chronograph subdial is marked only in 3-minute intervals, I am increasingly convinced that the purpose of the elongated three-minute marks is rather basic - its purpose is to make reading the elapsed time easier ... looking quickly at a 30-minute subdial marked with 5-minute intervals only, one may easily be mistaken between, say, 3 and 4 minutes past the interval. Not so with the additional elongated 3-minute markers.

Another way to address the same problem is the 15-minute register as used in the German aircraft clocks of WWII (they switched to 15-minute subdials from 30-minute subdials), and by Breguet for the French Type XX/21 chronographs after the war.

The thesis is open for discussion. ;-)

That longine is an interesting watch with its rotated dial! Nice for telling time with your arm in a very neutral position!

I totally agree with you re: the purpose of elongated 3-minute markers, btw.

 

[lysanderxiii]

That has been my hypothesis all along.

 

[HappyJack]

I used to be a Royal Navy Navigator, and we always used to mark our charts for projected future times along our track by DR (dead reckoning) or EP (estimated positions) calculation, in time intervals that were simple multiples or fractions of six minutes. The reason was very simple - a 6 minutes is 1/10th of an hour. So if you ship is doing 15 knots (15 nautical miles per hour), in 6 minutes it will do 1.5 nautical miles. Depending on the scale of the chart in use, you might want to mark the chart every 3 minutes, or every 12. In the same way, we'd aim to fix our position at 6 minute (or 3 or 12 minute) intervals since we could then very quickly calculate the speed made good - as being 10 times (or 20 times or 5 times) the distance covered between adjacent fixes.

So I suspect that the 3 minute interval markings on the chrono may be to assist with navigation calculations...

 

[lysanderxiii]

I used to be a Royal Navy Navigator, and we always used to mark our charts for projected future times along our track by DR (dead reckoning) or EP (estimated positions) calculation, in time intervals that were simple multiples or fractions of six minutes. The reason was very simple - a 6 minutes is 1/10th of an hour. So if you ship is doing 15 knots (15 nautical miles per hour), in 6 minutes it will do 1.5 nautical miles. Depending on the scale of the chart in use, you might want to mark the chart every 3 minutes, or every 12. In the same way, we'd aim to fix our position at 6 minute (or 3 or 12 minute) intervals since we could then very quickly calculate the speed made good - as being 10 times (or 20 times or 5 times) the distance covered between adjacent fixes.

So I suspect that the 3 minute interval markings on the chrono may be to assist with navigation calculations...

When these making first start to show up, the average aircraft when anywhere from 2 to 5 miles in one minute, three minute cues on the watchface would be rather coarse.

However, it does show that the readability of the watch must be something that can be performed as rapidly as possible, as pilot work-load had alway been rather high, because as technology allows more things are automated, technology creates morethings for the pilot to monitor/control.

 

[HappyJack]

When these making first start to show up, the average aircraft when anywhere from 2 to 5 miles in one minute, three minute cues on the watchface would be rather coarse.

:-s Hmm...not sure what you mean by "rather coarse". When you practice visual navigation you choose the scale appropriate to your speed, so at 300 knots (600 km/h), you'd cover 30km in 3 minutes. That's 12cm on a 1:250000 scale navigation chart, which is a hand's width - knowing you do a hand's width every 3 minutes will help a pilot work out, very quickly, where he is.

 

[Crusader]

Hmm, I think it has to do with the fact that an airplane, even in the early days when the three-minute markers were introduced, would move at a multiple of the speed of a ship, thereby increasing navigational errors substantially.

Unless the three-minute interval was in use in shipborne navigation prior to its use in aviation, I would find it difficult to see navigational requirements as the principal cause of the three-minute marks.

 

[whifferdill]

With regard to the A7 in the link above, anyway, I tend to agree with HappyJack - in aviation as well as nautical navigation - the pilot / navigator is always interested in distance per hour or distance per minute or multiples thereof - everything hinges on fractions or multiples of 60 - including position over the earth ( minutes and seconds of arc ).

A useful aviation time piece, especially one from the days when practical dead reckoning was employed, might reasonably be expected to highlight fractions of the 60 as a whole - hence the 30 minute sub-dial and highlights of tenths of that ( 3, 6, 9 etc ).

Knowing groundspeed and direction, one might plot position at 1/10 of an hour intervals on a chart. Easier to do when this time segment is highlighted on whatever you are using to time the leg.

I think the above is probably separate to the elongated 3 minute interval marks found on other chronographs such as the Benrus Skychief etc, but for those involved in aviation and navigation, working in multiples or fractions of 60 rather than 100, is just natural and normal, and will therefore be at the fore of the mind when looking at features like this on any watch designed for aviation / navigational use.

So in a way - it is almost certainly done for legibility, but I agree with happyJack that what lies behind this might well have its routes in dead reckoning navigation.

 

[Crusader]

So in a way - it is almost certainly done for legibility, but I agree with happyJack that what lies behind this might well have its roots in dead reckoning navigation.

If that were the case, one would need an explanation for the four-minute markers on the vintage pieces which JohnF has come up with ... of course, it could be so easy as one dialmaker who was misinformed, or clueless, about the purpose of the elongated three-minute marks ... :think:

 

[JohnF]

Hi -

Contemplate this: while the 3-6-9 markings are good for initial timing for a 20x division (60/3=20), the 4-8-12 markings are good for initial timing for a 15x division (60/4=15). That makes as much sense as the 12x division (60/5=12) that we're all used to...

But I've checked and checked and checked and checked. There really seems to be no meaningful, "why, of course!" explanation that I've been able to find. I am convinced that it has to be a military/aviation thing, given that we simply do not see this on any other kinds of watches: I just don't know why. I still like my true headings navigation trick theory, but have no explanation of why this doesn't also work using three-minute legs.

I'm also close to acquiring a Benrus SkyChief with that elusive dial, but the logistics are not looking good right now. :-(

JohnF

 

[whifferdill]

If that were the case, one would need an explanation for the four-minute markers on the vintage pieces which JohnF has come up with ... of course, it could be so easy as one dialmaker who was misinformed, or clueless, about the purpose of the elongated three-minute marks ... :think:

4 minute marks may well originate from the advent of instrument holding patterns - done as a 'race leg' pattern over an NDB or VOR beacon and if flown absolutely spot on, should take 4 minutes to complete in total from overhead the beacon to back overhead the beacon.

Only speculation but a possibility.

 

[JohnF]

Hi -

That's a similar explanation to what I found: that at least the Australian Air Force trained their pilots to do a 4-minute dogleg to measure true bearing from indicated bearing, by flying a box with 4 minute legs, reversing on their original direction, with any difference between indicated and true thus measured. Designed to teach pilots who would ordinarily fly over extensive areas with no effective landmarks (Austrialian desert, Pacific Ocean).

You would make the first 90° turn and start the stopwatch, second 90° at 4 minutes, third 90° at 8 and final 90° at 12 where you'd compare your original heading to the indicated heading to determine how far crosswinds were blowing you off course.

The problem: this could also be done with 3-6-9. I simply haven't been able to find any actual documentation that it was so trained beyond hearsay. :-(

JohnF

 

[whifferdill]

Hi -

Contemplate this: while the 3-6-9 markings are good for initial timing for a 20x division (60/3=20), the 4-8-12 markings are good for initial timing for a 15x division (60/4=15). That makes as much sense as the 12x division (60/5=12) that we're all used to...

JohnF

If you look at the A7 - each 3 minute interval on the 30 minute sub-dial is highlighted - not just the first 3 as in other chronos.

With the A7 - this highlight is definitely conducive with navigation.

Perhaps on other watches - the first 9 minutes only are highlighted for very short navigational legs that require greater accuracy of timing?

When I was learning visual navigation ( albeit on helicopters ), we students flew times and headings along a route to an IP ( Initial Point ) from where we would conduct a final, short leg into a 'target' ( terms and method derived from military flying but applicable for helicopters in civilian roles ) - this would be of very short distance - typically about 3-6 miles and flown at a reduced ground speed and altitude to better pick up features that would help guide you in to the 'target' - this would ideally take between 3 and 6 minutes ( it's neater that way ).

As HappyJack has pointed out - it's all a matter of scale - this final 'run in' could be 12 or 30 miles, but would last in the region of three + minutes no matter how fast you're flying - you pick the distance and the scale of chart appropriate for your speed of travel and the nature of the target. It could apply to jets doing 300 knots as much as it might apply to a little helicopter doing 60 knots. Timing these short legs acccurately is critical - if you haven't spotted your target by the elapsed time, you've flown over it!

Also - many private pilots now still draw on 6 minute marks ( based on the relevant groundspeed ) across their intended track lines on their navigation charts so that they can cross reference where they are in relation to these as they progress on their flight and thereby keep an eye on how close they are to where they planned to be! This basic method is taught to all pilots early on.

So, based on my own experience, I think that these markings must have their routes in navigation - especially on watches designed or marketed for pilots, like the Skychief etc and especially the A7.

 

[whifferdill]

Hi -

That's a similar explanation to what I found: that at least the Australian Air Force trained their pilots to do a 4-minute dogleg to measure true bearing from indicated bearing, by flying a box with 4 minute legs, reversing on their original direction, with any difference between indicated and true thus measured. Designed to teach pilots who would ordinarily fly over extensive areas with no effective landmarks (Austrialian desert, Pacific Ocean).

You would make the first 90° turn and start the stopwatch, second 90° at 4 minutes, third 90° at 8 and final 90° at 12 where you'd compare your original heading to the indicated heading to determine how far crosswinds were blowing you off course.

The problem: this could also be done with 3-6-9. I simply haven't been able to find any actual documentation that it was so trained beyond hearsay. :-(

JohnF

Interesting!

the 4 minute leg I refer to is one designed to be flown in cloud, when the pilot's won't be visual with the ground and is flying 'on instruments' in reference to the radio beacon. It's still used today at busy airports but is slowly fading from the requirements.

if you've ever been in a holding pattern as a passenger coming into a busy airport - you will have been flying a race-track pattern like this, that should approximate 4 minutes in duration. Try timing it - start your chrono as the first turn is commenced ( or any turn ) - keep it running as the aircraft levels, then starts another turn and levels again. At the start of the next turn, stop the chrono - it should be pretty close to 4 minutes.

These days, it's as likely to be over a GPS waypoint as it is above a radio beacon, but the principal is the same. The timing may be out as the pattern will have been flown by the aircraft systems, programmed by the crew, and that most likely adjusts the pattern to make the inbound leg to the fix exactly 1 minute, which skews the timing overall, but it should be within 30 seconds or so of 4 minutes, still.;-)

 

[steponone]

Can any Member please provide the names Vintage Chronograph Brands with the 3 minute graduations on the 30 min Subdial.
Longines has recently introduced the Heritage Avigation Replica with this configuration.
I am interested in this .
Thanks .Steponone