Before we begin, the intention here is not to get everyone testing their flying skills unnecessarily, and becoming the next self proclaimed Chuck Norris, so please listen up. The altimeter is an essential instrument when flying an aircraft, and should always be in a serviceable, working condition before you take off. However, as with all human inventions, it is fallible, and should it fail in flight, having a back up plan of how to fly without it, is a skill worth having.

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 altimeter measures the height of the aircraft above a given pressure level.

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.

There are two obvious reasons why your altimeter would stop working. The first, would be a blocked static port. Remember that in a pitot-static system, if the static port becomes blocked then three instruments will miss-behave, namely the airspeed indicator (ASI), altimeter and the vertical speed indicator. Signs of this:

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 place the aircraft in the climb attitude until the VSI indicates a 500fpm climb, then time for two minutes, while carefully maintaining this indication, then we will climb 1000ft, and increase our altitude by 1000ft.

If we need to descend, then we would do the same, but this time with the needle 500fpm below the zero.

 

The VSI is a very useful instrument, especially if your altimeter fails, but it is very sensitive and tends to lag behind your actual attitude changes, so it will require steady hands to maintain a steady rate of climb or descent. The key to managing these oscillations is good power control. Knowing your power settings for the cruise, climb and descent, are also very important in trying to obtain the correct altitude when you must determine altitude using the VSI. Steady hands and knowing your power settings will see you through.