Probably a bone question but I'll ask anyway

[x213a]

I started a thread ages ago about "Bombs gone" phraseology and a few links were forthcoming. One of them was of a Canadian Lancaster raid in 1943:

YouTube - Canadian Lancaster Crew Voice Recording (http://www.youtube.com/watch?v=8nV8gn-gaLw)

Could somebody please explain the significance of the "303 magnetic" call just after bomb release?

Once again, apologies if it's bone:}

Cheers.

[taxydual]

Possibly giving the pilot a heading to steer to get away from the target?

[FATTER GATOR]

Yep, Escape heading, post drop.

[Pontius Navigator]

The aircraft probably had a gyro magnetic remote reading compass as well as a direct reading pilot compass. The remote sensing compass would be corrected for variation and deviation through a VSC - a manual variation setting control. AVSC (automatic) didn't really come in to widespread use until the 60s.

The remote reading compass was dependent on the accuracy of the gyro output and the magnetic input from a remote sensing fluxgate in the wing. As the aircraft would possibly be doing violent manouevres in the target area the gyro compass of the day would not necessarily have maintained its accuracy. The direct magnetic compass OTOH would not have been subjected to electrical failures and could be more accurate during an escape manoeuvre.

Also, where there was a significant difference between true and magnetic north, variation, it was necessary to specify which heading the pilot was to use. I don't know the 'magvar' over Germany in the 1940s but recall values of IRO 10 deg in UK in the 60s. Over Germany 20 years earlier it ws probably 10 degrees.

[L J R]

Showing your age again Pontius.....

[cornish-stormrider]

Pontius - how was it to work with the Royal Flying Corps(e). Beags - feel free to chip in with your experiences too!

Old Duffers that you are :E

[Pontius Navigator]

LJR, and proud of it.

C-S, oddly our navigation course was entirely devoted ontechniques in how to navigate a 4-engined bomber to Berlin. Only once we graduated were we re-trained in modern equipment from the late 40s :}

As for the GIVB and P12; if you ain't swung one you ain't lived.

[teeteringhead]

Another connected fairly stupid question from a simple airframe driver - which Pontius may also be able to answer ;)

How would the loading and subsequent dropping of lots of metal (ie bombs) affect the deviation of the aircraft compasses? Would a previous "bombless" swing be valid again once the load was dropped? And if so, how erroneous might the mag compasses be in transit to the target with all the ordnance on board...??

[maxburner]

The magnetic flux detector is usually way out on the wing or tail. That way it's a long way from the bomb load and that minimises the effects.

[Pontius Navigator]

Similarly the direct reading compass was probably sufficiently far forward.

However several known potential effects were ignored. When more than one engine was changed we were supposed to do a compass swing. When there was a mjor component change, ie the wing, or part of the system, or when there was a large change of 'magnetic latitude' eg a move from UK with a 40 degree dip to the equator with a zero dip.

Now the latter was ignored on transport aircraft and we successfully ignored it for bombers too. Similarly we made no correction for bomb loads either.

I remember once seeing a UK chart of local magnetic variation (1949 survey IIRC) that showed maximum variation around major cities like London, Birmingham and Manchester. This would suggest that there was a significany change in compass accuracy near the cities although this would reduce with altitude.

As far as bombing accuracies were concerned, a 0.1 deg error between true and magnetic (allowing for variation and deviation) would translate to a bombing error of only 51 feet for an offset aiming point 5 miles from the target.

In navigation terms this would have been insignificant as other unknowns - start position, fixing accuracy, wind, speed etc would all have had a greater effect.

[CRM Monkey]

I think PN has got is wrong. The Gyro would have maintained accuracy during the manouvering.

[Pontius Navigator]

CRM, in theory you are correct, but you must remember you are talking 66 years or more ago when gyro-magnetic compasses were in their infancy. A free-running gyro, uncoupled from the long-term stability of a flux-valve, was accurate in the short term but had very high drift rates in today's terms.

When Smith's were asked the expected error after 5 minutes of manouevring in a Vulcan in the 60s they said 'not more than 180 degerees'. In practice, IIRC, a heading error within 10 degrees of the required heading was not unusual.

In 1944 I would guess 10 degrees or more would be the norm. It would then take upwards of 2 minutes for the GM to stabilise. Whilst the P12 may also be subjected to lag and swirl it could settle sooner.