Second, you can obtain backup altitude information from the cabin pressure altimeter. This instrument
should be considered accurate to +/- 500 ft and is primarily useful at altitudes above 5,000 ft AGL where
the radar altimeter is useless. In order to use the cabin pressure altimeter above 5,000 ft MSL, you will
have to depressurize the cockpit.
Cross-check your other pitot static system instruments (A/S and VSI) to ensure that they are operating
Because the standby barometric altimeter includes an IFF altitude encoder, you may need to disable the
IFF altitude function by placing the M-C MODE SELECT/TEST switch on the IFF control panel to OUT.
The procedure for dealing with an altimeter failure is:
1. Check that PITOT HEAT is ON.
2. Report the failure to ATC.
3. Use the radar and the cabin pressure altimeters to determine altitude (depressurize cockpit if
4. Be aware that the IFF altitude may also be in error.
5. Land as soon as practicable.
If you lose the VSI, use the clock to gauge the amount of altitude change occurring over a specific period
of time. For example, if you were to descend 200 ft in 15 seconds, your rate of descent would be 800 ft
per minute. You can also use this procedure to check the accuracy of a suspect VSI.
It is also important that you determine whether other instruments in the pitot static system are operating
correctly. What may appear to be a stuck or erroneously indicating VSI could be part of a larger pitot
static system problem.
NOTE: With a failed VSI, your PAR approach capability will be severely limited.
The procedure for dealing with a VSI failure is:
1. Check standby VSI.
If standby VSI is also inoperative.
2. Check that PITOT HEAT is ON.
3. Cross-check the altimeter and clock for vertical velocity reference.
4. Watch for other indications of possible pitot static problems.