The altimeter is an air driven instrument, and it requires a source providing it with undisturbed atmospheric pressure, in order for it to work.
The instrument case is fed with static, undisturbed air pressure which causes:
- A stack of sealed aneroid capsules (which contain a small partial pressure) to expand and contract with changes in atmospheric pressure from the static source.
- The mechanical linkage translates these changes into pointer movements on the indicator.
Altimeter – Freezes at the last sensed altitude, it does not move at all when you try to climb or descend.
VSI – Also freezes and doesn’t move.
ASI – Airspeed shows slow in a climb and fast in a descent.
If you see this, then there is a problem with the pitot-static system, and more exactly the static port. The solution is quite simple – either your aircraft has a switch called ‘Alternate air’, which will feed cabin air directly into the system, or, if your aircraft is not so equipped, you will need to break the glass cover of the VSI. This is also effective in providing cabin air pressure to these instruments. However, even though we will now have a reading on the altimeter, it will be in error, showing us a higher altitude than we really are, because the cabin pressure is lower than the actual static pressure outside. The altimeter will sense the pressure as if we were at a higher altitude (lower pressure) than we actually are.
The second scenario, would be a malfunction in the altimeter itself. This can be identified by the fact that only the altimeter is not working. In this case, some piloting skills will be necessary. In VFR conditions you fortunately still have visual reference to maintain your attitude for straight and level flight, however, you have no indication of what your actual altitude is. Luckily, you can determine whether you are maintaining, climbing or descending using your VSI.
If we need to descend, then we would do the same, but this time with the needle 500fpm below the zero.