Military aircraft compasses
 

I have a bit of a puzzle on my hands, and wonder if any of you can help.

Somebody tells me that some military aircraft had their compasses graduated in Mils rather than degrees.

I'd never heard of Mils, but for those that haven't either they should be a thousandth of a Radian, but since that would give an awkward 6,283 in a full circle, it's been rounded to 6,400. As far as I can tell, the main reasons that they're used are for greater accuracy when shooting at longer distances and for the quite useful fact that anything 1,000 units away from you measures one Mil on the scale when it is one unit long - so one foot at one thousand feet range (slightly inexact because of the rounding). One degree is about 17.8 Mils, so you can see how handy that accuracy is if you're trying to hit something small a mile away.

My question then is whether any of you have ever seen or heard of an aircraft compass graduated this way? Would Army aircraft or maybe the RN have used them and so possibly the FAA? If it helps, the era that he was referring to is really mid 60s to late 80s.

Nothing I've ever heard of.
I'm thinking maybe a gunsight mounted near the compass or on the compass.

Army prismatic compasses (ground use) were available in mils, degrees, or some had both on a split scale. Artillery and surveyor types used the additional accuracy of mils, average squaddies who struggled to tell the difference between the sun and the moon were generally fine with degrees.

I can't speak for aircraft compasses, but I only ever saw issued prismatic compasses in mils (R SIGNALS). Bit of a waste when the calibration marks on the antennas were in degrees!

The A-4 Gunsight was as good as a compass and it had MILS for accuracy - dial on left. LIBRAscope reticule below (bat****crazy)

Librascope Reflector Gunsight used in early Douglas Skyhawks.
OLD Style Gunsight SITE: http://www.aircraft-gunsights.com/reflector-sights/

http://i98.photobucket.com/albums/l2...m.gif~original
http://i98.photobucket.com/albums/l2...S.gif~original
http://i98.photobucket.com/albums/l2...m.jpg~original

Mils are extensively used by ground troops as has been said, for accuracy but also a useful tool for range estimation. Your forefinger held up at arms reach is about 30 mils, therefore, if the target is at 2,000m and the fall of shot is two fingers away, the correction is "right 120". Further, the graduations in binoculars are 10, 5 and 2 mils, therefore, if you sight a BMP-1 (2m to the top of the hull) and it fills the 2 mil graticule, you know it to be 1,000m away.
Or zap it with a laser range finder.....and his laser warning receiver will tell him you are looking at him and attract unwelcome attraction.
Interestingly, the Soviet Bloc use mils also but rounded down to 6,000 mils in a circle.

Tha Army uses mils calibrated Silva compasses for teaching navigation

https://www.army.mod.uk/documents/ge...tion_jun11.pdf

although the compass does have an inner scale in degrees
https://www.goarmy.co.uk/silva-compa...tish-army-prod

Every aircraft compass that I have ever seen has been calibrated in degrees. Gunsights, on the other hand, have their depression measured in mils.

Thanks for the replies so far, and in particular to SpazSinbad for the lengths he's gone to.

I should have made it clear that my interest is in compasses used for steering. And, more specifically, the fact that I may have cornered a 'Walt'.

He claims to have flown Sea Vixens and also mentions Meteors. I had my doubts, so mentioned that I had an E2.B (a standby compass that he would probably have sat behind). He blustered a bit over what an E2.B might have been and so I told him it was a compass. His immediate response was to ask whether it was graduated in Mils or degrees, and went on to explain that you had to know which it was when being given a course to fly. At which point the rat became a bit smellier.

I reckon you've got your Walt. The E2b is/was in degrees. Ordinary pilots know nothing else. In fact IIRC, the E2b is only graduated in ten-degree increments. Now, work out the most embarrassing way to out him. Could be fun.

First page of this pdf has this photo of the 'roger - standby' Sea Vixen compass:

http://www.seavixen.org/images/docum...e_Feb_2011.pdf (1Mb)

http://i98.photobucket.com/albums/l2...M.jpg~original

Might want to ask him what the main compass was...... Mk. 5FT...

Sea Vixen FAW Mk 2. Pilots Notes.

http://www.seavixen.org/images/docum...esscan0001.pdf

Never known a steering compass, aircraft or marine, to be graduated in mils. The only occasion I've seen them was, when in REME attached to an artillery regiment, the gunners used them for gunlaying.

It used to be a regular on 1st April----the decimal compass. Here it would be explained that the military were going over to decimal compasses so East would change from 090 to 025 and so on. Some excellent facsimiles of decimal compasses would be produced, some quite convincing.

Ask any navigator; they would have done a course on them.

The reason that I have an E2B is that my Pa designed an aircraft compass and consequently had a bit of a collection of others, that I now have. With that background I've flown a little, sailed a lot, and could see that introducing a second number system would be a) a bountiful source of interesting moments, and b) thought to be an excellent idea by someone in a Ministry office somewhere.

I wonder if a bunch of Sea Vixen pilots fancy a day out in Surrey?

Other than surveying and gun sighting, it's hard to imagine anything that could be done to an accuracy of a degree or better; certainly not steering a moving object of any kind.

Off topic: In my sailing days, we used to use gimballed compasses that were weighted to compensate for the dip of the magnetic field. This meant that you had to swap them over when you went to the Southern Hemisphere...

They still are weighted pasta, and I don't know if you noticed but it leads to a problem which is known about in flying and more or less ignored in sailing. That added weight gets left behind if you move sideways. In boats it makes steering South harder, but in a side-slipping aircraft it's possible to fly in a complete circle with the compass apparently motionless.

My Pa's idea was to remove the weight but to stop the needle from dipping.

That's the old joke, isn't it?

Nav: "Starboard 1 degree".

Pilot:"I can't adjust by just one degree".

Nav: "Roger, starboard 5 degrees".

Pilot: "Steady".

Nav: "Roger, port 4 degrees"......

I'm pretty sure that should read "precision" rather than "accuracy". The difference is subtle, but oh so important.

Which was something I used to ponder on when doing a compass swing on a raw Norfolk February morning out on the airfield. I cant remember the accuracy of the headings I passed to the nav inside the aircraft after reading them off the Watts Datum Compass, but I am sure it read to less than 0.1 of a degree. Could never see the point of that, nor the laborious Fourier's Analysis which followed the swing. But then I was only a hack squadron nav - maybe somebody more versed in the mystical art of navigation might enlighten me!