A photo of an emergency compass was recently sent to the Institute with a request for information about it. Interesting details came to light during
A photo of an emergency compass was recently sent to the Institute with a request for information about it. I sent the enquirer a tentative answer to the points he raised but further details came to light during the search so this forms a more detailed comment.
The emergency compass was designed at the Admiralty Compass Observatory in 1946 for use as an emergency compass in the then newly emerging jet aircraft.
For future reference the engineering details are as follows. It is an edge reading compass with a card graduated at 10deg and mounted on a light system with a total weight of 2.8gm. The magnets are of relatively low moment, 37-46 cgs units, and are made of 35% cobalt. The bowl is a Diaklon moulding and houses 4 corrector magnets for control of B and C coefficients. The compass is mounted on a wedge plate and attached near the pilot’s line of sight – frequently on the canopy frame or the cockpit coaming. There are no anti-vibration mounts since it is intended for jet aircraft which have low vibration. It is filled with silicon fluid of viscosity 3 centistokes and damped such that the time to swing through 85deg after a 90deg deflection is 2-3sec.
The Air Ministry compasses were made as three models depending upon the illumination. Model E2A had no illumination, E2B had red illumination and the E2C had blue/white illumination. These differences might be irrelevant in the real world because probably the most likely cause for using the compass would be a total power failure so that the pilot’s pocket torch probably be the only illumination anyway.
These compasses appear to have been fitted in most RAF aircraft since 1946. I have a diagram of it mounted high on the canopy frame of the Lightning F6.
It seems that the compasses were also adopted by the civil aircraft manufacturers. At some time the same compass was marketed by Kelvin Hughes (a division of Smiths Industries) as the KCA0104W series. These appear to be identical to the E2 series, just a change in the markings. Again, there are three models. The marks on the top will be KCA0101W for the one with no illumination, 104 for the red illumination and 105 for the white illumination.
It is interesting to see that if you use Google to search for “KCA0105W” you find amongst the many hits that the compass is still (in 2007) on the current parts list for the Embraer EMB110 and the Airbus 300. At www.sirs.co.uk you will also find that in 1994 the UK company SIRS Navigation Ltd bought from Smiths Industries the "Aircraft Standby Compass known typically as the range E2B or KCA 0104W”. SIRS still market the E2B today.
So, this modest little compass has been in production for 61 years and can still be bought off the shelf – the only change being the option of NVG compatible illumination. How’s that for product durability.