A Complete Manual for Conducting International Flight Operations

Altimetry Requirements

Altimetry Accuracy

Prior to operating an aircraft in an oceanic environment, it is important to evaluate the altimetry system's accuracy and performance. While operating over the ocean, the normal protection provided by ground-based radar is not available. Therefore the altimeter becomes the only means of ensuring vertical separation with other aircraft. Operators should have their own altimetry log which is completed prior to each flight, and flight crews should be aware of the manufacturer’s limitations for their aircraft to determine that the system is serviceable.

Altimetry System Check

The example below is taken from a Citation XL RVSM manual. It requires the flight crew to evaluate the condition of the static ports on the exterior of the aircraft, and it states that the operational limitations for the altimetry system on the ground should indicate that both altimeters are within 50 feet of the departure field elevation and both primary altimeters shall indicate within 75 feet of each other. (Citation XL AFM Section V, supplement 19) If you have ever received a flagged altimeter miscompare in a modern aircraft, you will realize the importance of having the altimeters agree with each other. If they do not agree, you may find yourself in a situation where the autopilot and mach trim disengages and the aircraft will no longer be RVSM compliant. In such an instance the crew would use the standby flight display to determine the most accurate altimetry source and select the correct ADC source for both sides. It would also be important to couple the transponder to the correct ADC as well.

It is also interesting to note that special attention must be given to the static ports on RVSM type aircraft when painted. It is possible that aircraft which have been repainted may have altimetry accuracy issues. RVSM compliance for paint procedures around pitot / static ports is critical and skin waviness checks and RVSM continued airworthiness requirements must be provided to the paint shop by the manufacturer.


Revision date: July 29, 2015

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