Sea Level | TGH Aviation

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

How to Read a 3 Pointer Altimeter

A three-pointer altimeter, as its name implies, has three different pointers on the front dial. They are the 100-foot pointer, the 1000-foot pointer, and the 10,000-foot pointer. The medium length pointer is the 100-foot pointer, the shortest pointer is the 1000-foot pointer, and the longest pointer is the 10,000-foot pointer. The altimeter dial has 10 major indices numbered 0 through 9. In between each major indice are 4 minor indices. The value of these indices is dependent on the pointer being read. When reading the 100-foot pointer each minor indice equals 20 feet, each major indice equals 100 feet. When reading the 1000-foot pointer each minor indice is equal to 200 feet, each major indice is equal to 1000 feet. When reading the 10,000-foot pointer each minor indice is equal to 2000 feet, each major indice is equal to 10,000 feet. The altimeter in figure 1 is indicating 11,520 feet and is read as follows:

The 10,000 foot pointer is past the 1 and not yet up to the

2 and so it is read as: 1 x 10,000 = 10,000 +

The 1,000 foot pointer is past the 1 and not yet up to the

2 and so it is read as: 1 x 1,000 = 1,000 +

The 100-foot pointer is 1 minor indice past the 5 and so

Therefore, it is read as: 5.2 x 100 = 520

The indicated altitude is the sum of the pointers: 11,520

Figure 1: Three-Pointer Altimeter

Getting to know your Kollsman Window and Barber Pole

Reading the Kollsman Window ( Barometric Reading)

The Kollsman window is located at the 3 o’clock position on the altimeter dial. This window allows access to read a sub-dial, which contains the barometric readings. The arrowhead indice located precisely at the 3 o’clock position on the altimeter’s main dial is used as the reference point for reading the barometric sub-dial. Most altimeters will have a sub-dial, which covers the readings from 28.1 InHg (inches of mercury) to 31.0 InHg. On the sub-dial each major indice is read as 0.1 InHg, each minor indice is read as 0.02 InHg.

The Altimeter in Figure 1 reads 29.92 InHg

As previously, stated, weather conditions will greatly affect the pressure of the atmosphere (the barometric reading). Altimeters report altitude as a function of atmospheric pressure. Typically pilots will obtain a local barometric reading from the nearest airport. They will then set the Kollsman window to the setting that they received. This action will adjust the altimeter reading, eliminating error due to local weather conditions.

Some altimeters will have a Kollsman Window, which reads out in milli-bars in lieu of InHg. These are usually altimeters designated for use in Europe. However use of milli-bars has become more common in the U.S. during the last few years. Milli-Bars is just another unit of measurement, 1013.2 milli-bars = 29.92 InHg.

The Barber Pole

The “Barber Pole” on the face of the altimeter is visible only when the altitude is above sea level. When the altitude is below sea level the barber pole is no longer visible. This is provided to avoid the error of reading –1,000 Ft. as being +10,000 Ft.

Understanding Altimeters

In its purest form, an altimeter is simply an absolute pressure gauge. This means that it is displaying the pressure being exerted by the atmosphere at its current location.

The earth is surrounded by an atmosphere. This atmosphere is the air that we breathe. The atmosphere is held in place by the earth’s gravity. The atmosphere has a specific weight. The weight of the atmosphere is approximately 14 pounds of weight for every square inch of earth when measured at sea level on an average day.

An accurate method of measuring this weight is to use a barometer. A barometer is a reservoir filled with mercury. The reservoir has two openings; one opening is exposed to the atmosphere and the other empties into a glass tube. The atmosphere pushes down on the mercury within the reservoir causing the mercury to fill up the glass tube. How far the mercury goes up into the glass tube is directly proportional to the weight of the atmosphere pushing it. This is why barometric pressure is normally expressed in terms of “Inches of Mercury (InHg)”.

At sea level, on an average day, the barometric pressure is 29.92 InHg. However, this will vary constantly depending on the weather. Stormy weather tends to pull the atmosphere away from the earth’s surface causing lower pressure. Hot, dry weather pushes the atmosphere down causing higher pressure.

The weight of the atmosphere also changes depending on altitude. The closer to sea level that you are, the more air there will be, consequently the atmosphere will weigh more. As you go higher in altitude, the less dense the atmosphere will be, therefore less weight or pressure is exerted. An altimeter measures this change in atmospheric weight as expressed in terms of pressure or feet of altitude.