Stores Ref. No.:6A/5463
Attachment:Clamp, instrument securing type C.12956


Cabin pressure displayed as an altitude in feet. Used to inform the aircrew of their breathing oxygen requirements, and also to warn of malfunction of cabin sealing / leak (along with an independent warning light).

This is a purely mechanical device, and on the surface would appear to be the simplest of all indicators, however, inside it is a clockwork piece of art.

It measures cabin altitude using a sealed aneroid capsule, exposed to cabin pressure by holes in the casing; the capsule acts upon a mechanism to operate the pointer.


Being a mechanical device, my interface will have to result is some form of mechanical actuation.

Subjecting it to a sub-atmospheric pressure, simulating cabin pressure, is not possible as the casing has no fitting / connection for this, and is designed to leak, so another method is to be explored.

Where the aneroid capsule acts upon the mechanism, there is a location to add a bent piece of wire to push the mechanism parallel to the aneroid capsule. This wire can exit the indicator at the rear and be actuated externally by a servo motor. This will likely require a new casing to be made, to allow the wire out and mount the motor to, unless one of the existing pressure sensing holes can be utilised. Thus a solution can be found with no modification needed.


A copy (scans are fine) of A.P.112G-1005-1 which describes this altimeter.


Indicator general and technical information:
A.P.1275A, Vol.1, Sect.22 Chap.16, A.L.9, Aug. 1957 - Cabin Altimeter, Mk21 Series

T Mk.5 electrical "Vol. 1":
A.P.101B-1005-1B, Sect.7, Chap.6, A.L.62, Feb. 74 - miscellaneous instruments - Paragraph 11