CABIN ALTIMETER MK. 21A
LEADING PARTICULARS
| Stores Ref. No.: | 6A/5463 |
| Manufacturer: | Smiths |
| Attachment: | Clamp, instrument securing type C.12956 |
DESCRIPTION
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.
SIMULATION
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.
WANTED
A copy (scans are fine) of A.P.112G-1005-1 which describes this altimeter.
REFERENCES
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