Cold-Temperature Altimeter Error at Queenstown
A King Air 200 is dispatched from Queenstown (elevation 1,171 ft) on a medevac flight. QNH is 1004 hPa, and the OAT at 6,000 ft is -12°C. The crew levels at an indicated altitude of 6,000 ft and asks ATC for the sector minimum safe altitude, which is 6,500 ft. The aircraft's radio altimeter and GPS show terrain clearance that seems lower than the altimeter suggests. The pilot asks the AME whether the altimeter is faulty because a recent ramp check showed it within 20 ft at 1,171 ft. The AME has the aircraft maintenance manual, a current altimeter and static leak check, and a cold-temperature correction table. The aircraft is not RVSM; the altimeter has a counter-drum/pointer display. The crew is IFR and terrain is rising ahead. The AME must advise quickly by radio.
1. Estimate the true altitude and decide whether the 6,500 ft minimum safe altitude is assured.
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The altimeter is probably serviceable; this is the cold-temperature error. At 6,000 ft, ISA temperature is about 15 - 1.98 x 6 = 3°C. The actual temperature is -12°C, so the ISA deviation is about -15°C. Using the standard approximation of 4 ft per 1,000 ft per °C of ISA deviation: correction = 4 x 6 x 15 = 360 ft. The altimeter overreads in cold air, so true altitude is about 6,000 - 360 = 5,640 ft. That is roughly 860 ft below the 6,500 ft MSA. The minimum safe altitude is therefore not assured. The GPS and radio altimeter do not replace the corrected pressure altitude for terrain clearance. The crew should climb immediately and apply a cold-temperature correction from the approved table, not simply chase the GPS elevation.
2. Explain why the altimeter is serviceable and what correction should be applied for the next level.
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In cold air the pressure levels are compressed toward the ground, so a given indicated altitude corresponds to a lower true altitude. The error increases with height and with the size of the negative ISA deviation. The ramp check at 1,171 ft was within tolerance because the error there was small; it does not prove the altimeter is faulty at 6,000 ft. For the next level, the crew needs an indicated altitude that corrects to 6,500 ft true. At about 7,000 ft, ISA is near 1°C, so the deviation is about -13°C and the correction is about 4 x 7 x 13 = 364 ft. An indicated altitude near 6,860 ft would correct to about 6,500 ft, but the approved cold-temperature table and any terrain margin should be used rather than this rough figure. Advise the crew to apply the table, add a safety margin, and record the advisory. The altimeter does not need bench calibration for this error, provided the static leak check and QNH setting are correct.