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An illustration of the moving pointer (left) and moving scale (right) altimeters in use in EFIS technology aircraft.
Air Safety- Altimeter Errors
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Altimeter Errors – Still a cause for concern
In 1989, a Beechcraft Baron, ZK-SUN, was approaching Napier airport in the North Island of New Zealand on a private, IFR flight. The pilot’s previous experience on the aircraft was limited to a 45 minute check ride with a qualified instructor. As the weather deteriorated, the controller at Napier was in communication with a number of other aircraft and cleared ZK-SUN for a VOR/DME arc approach to Runway 16. Due to additional traffic, the controller requested that ZK-SUN continue to fly the arc rather than turn inbound. This request appears to have disorientated the pilot since his subsequent position reports failed to correspond to those indicated, and his altitude was at least 1000 feet lower than reported. The pilot and his two passengers were killed when the aircraft collided with a hillside, 10nm (19km) to the north of the aerodrome.
One of the key features associated with the crash of ZK-SUN was that the aircraft was fitted with twin three point altimeters (refer Figure One): instruments which are known for their potential to be misread. In this case, the pilot was conducting both high workload and relatively unfamiliar operations in an aircraft with which he had little operational experience. It appears likely therefore, that the potential to misread three point altimeters increased in proportion to the amount of stress/workload to which a pilot was exposed.
The altimeter itself plays an extremely important part in the safety of aeronautical operations. Most importantly, it displays the absolute distance above some previously established reference point such as mean sea level or elevation above the ground. This assists our ability to maintain aircraft separation and ensure adequate terrain clearance. In addition to the provision of precise information however, altimeters also provide an indication of the rate and direction of changes in altitude. Coupled with information from other instruments, it increases the pilot’s ability to accurately control and monitor the aircraft.
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