THE PRESSURE ALTIMETER

A pressure altimeter is the DR reference for the vertical distance at which an aircraft is flying (Fig. 1.3). The instrument is a descendant of the meteorological barometer and, up to now; the construction and operation of the two instruments are still the same. For example:

1.The aneroid capsule (fig. 8.5) is used by both the altimeter and barometer.

2.The unit of either millibars or inches of mercury (fig. 10.1) is used by both the altimeter and barometer.

3.The change in the weight of the air- column above the aneroid capsule (i.e. ) is used by both the altimeter and barometer to determine the height.

When the pressure altimeter was put into service, it immediately earned a bad record. For nearly twenty years, the majority of air- traffic accidents and incidents were traced

Help

Question 1.

a.An aircraft is at F.L.80. The regional QNH is 990mb. What approximate clearance has the aircraft over ground, which is 4187ft above mean sea level?

(Assume 1mb =30ft.)

b.Give two reasons; apart from the altimeter being subject to instrument and position errors, why in answer (a) you are only able to give the approximate clearance.

C.P.L. [10]

Question 2.

An aircraft flies at constant flight level from A to B. The mean sea level pressure at A and B are the same. The mean air temperature between sea level and the aircraft is greater over A than over B.

Is the aircraft height above sea level greater when over A or B? Give the reason for your answer.

A.T.P.L. [10]

Solution

Drg. 10.3. The altimeter equation in p- system (8.3).