The altimeters are rigorously checked for their accuracy before take off and during flight. This is a must check for an aircraft that is bound to fly in RVSM (Reduced vertical separation minima), where the vertical separation between aircraft is reduced to 1000 ft from the normal 2000 ft. The RVSM air space begins at 29000 ft and ends at 41000 ft. RVSM allows more aircraft to be accommodated into given airspace. This was done to cater for the ever increasing air traffic.
The first altimeter check we do before an RVSM flight is to check each altimeter against a known ground elevation. This is done during the pre-flight. Most of the airliners have three independent altitude measurements (one for captain, one for the first officer and one on standby). In accordance with the RVSM regulations, the altimeter should not read more than 75 ft of the elevation. This means, if the airport elevation is 0, the altimeter should read less than 75 ft, for it to be within the RVSM tolerances. Both the captain’s and first officer’s altimeters should be within these limits.
In flight, and once in RVSM airspace, the two independent altimeter read out must again meet a certain level of tolerance. The pilots are required to cross check the altimeters once every hour when flying in RVSM airspace to ensure that the altimeters are giving the indications within the tolerances. This tolerance is set at 200 ft. That is, the captain’s and first officer’s altimeter read out should be within 200 ft. For instance, if the first officer’s altimeter reads 34000 ft, the captain’s altimeter should read a maximum of 34200 ft or 33800 ft. If this criteria is not met, the pilots should immediately inform the Air traffic control (ATC). The call should be ‘Unable RVSM due equipment’. Most aircraft manufacturers have much stricter RVSM limits. For example, in the Airbus A320 above 30000 ft, the two altimeters must remain within 130 ft and not 200 ft.
It is not only equipment failure that can make the altimeters go beyond the set limits. Turbulence can also make it difficult to do so. In that case, the pilots should make a call to the ATC as follows: Unable RVSM due turbulence
Author – Anas Maaz
