Frequently Asked Altimeter Questions
The pointers on my altimeter are very jumpy and stick sometimes.
What is wrong?
The altimeter is exposed to the outside atmosphere. This includes all of the dirt and dust present in that atmosphere. Dirt and dust will get into the gears and cause them to stick and bind, the vibration from the aircraft will help the gears to overcome this problem but they will be very jumpy and become worse as the unit gets older. This unit needs an overhaul.
Can I convert my milli-bar altimeter to InHg or vice versa?
Yes, altimeter dials can be converted, provided that the manufacturer has published a procedure for doing so. If there are no published, FAA Approved, procedures then, no, this modification cannot be accomplished.
How often does my altimeter need to be calibrated?
The FAA requires that the aircraft static system be tested and certified biannually. The altimeter is a part of that system.
The altimeter ID plate says that it is a 35,000 Ft. altimeter, but the repair shop that overhauled my altimeter marked the unit as being certified to 30,000 Ft. Why the difference?
As altimeters get older and the parts wear the unit will become inaccurate at the higher end of its range. This does not mean that the altimeter can no longer be used; it just has to be used at the lower altitude. Therefore, it will be sold only to customers who request a lower altitude unit, typically general aviation.
What is a “car” altimeter?
Sometimes, if a unit is so old and worn that it can no longer be used in an aircraft then these units are sold at a discounted price for use in other than aircraft, typically people will use them in motor homes, cars, and boats.
Along with my altimeter, I received a correction card. What is that?
Altitude is a non-linear function. It is impossible to calibrate an altimeter to be absolutely accurate at all altitudes. Therefore, a certain amount of error is allowable. The correction card advises the users of the amount of error in a particular altimeter. As each altimeter will have its own characteristics, the error card is identified with the unit’s serial number.
What is a TSO?
TSO stands for Technical Standard Order. This is an FAA document, which defines how a specific type of instrument should work in order to be considered airworthy. Altimeters manufactured by the following companies typically are manufactured to TSO standards: United Instruments, Kollsman, Garwin, and Aerosonic. Altimeters manufactured by the following companies typically are not qualified to TSO specifications:UMA & Falcon. If an instrument is certified to a TSO, it must state such on the ID plate. If it does not state the TSO on the nameplate then you must assume that it is not qualified to the TSO.
What are the typical failure modes of an altimeter?
- Sticky/jumpy pointers
- Inability to properly adjust the Kollsman window
- Out of calibration
- Worn pivots and/or jewels
- “Oil-canning” of the aneroid
