The Ridge at FL 060: Two Altimetry Errors Pulling in Opposite Directions
You are the commander of a twin turboprop cruising at FL 060 with the altimeter subscale set to 1013 hPa. The departure aerodrome elevation was 1,000 ft with QNH 1028 hPa. The regional QNH beneath your present position is 995 hPa, and the destination QNH is 996 hPa. A radiosonde released nearby 40 minutes ago reports 13 °C at 6,000 ft, about ISA +10. Dead ahead the chart shows a ridge with a minimum safe altitude of 5,400 ft. The ridge is 25 nm away, you are indicating 6,000 ft, and the aircraft climbs at 700 ft/min in still air. Your colleague argues there is nothing to worry about because 6,000 is well above 5,400. The aircraft has no radio altimeter and no EGPWS terrain database for this area. Visibility is 20 km in scattered cloud and the horizon is clear.
1. Using the data given, calculate the aircraft's true altitude over the ridge and state whether the 5,400 ft minimum safe altitude is actually protected.
Read the worked answer
Two separate corrections must be applied, and they have opposite signs.
1. Pressure correction. With 1013 hPa set, the altimeter shows pressure altitude above the 1013 hPa isobaric surface. The local QNH is 995 hPa, i.e. 18 hPa lower, so the 1013 hPa surface lies 18 × 30 ≈ 540 ft below mean sea level in this area. The aircraft at 6,000 ft indicated is therefore only about 6,000 − 540 = 5,460 ft above the sea.
2. Temperature correction. The layer below the aircraft is about 10 °C warmer than ISA. The standard rule of thumb is 4 ft per 1,000 ft per °C of ISA deviation: 4 × 6 × 10 = +240 ft. Warm air is less dense, so the aircraft is genuinely higher than its pressure altitude.
Net true altitude ≈ 6,000 − 540 + 240 = 5,700 ft AMSL.
Clearance over the ridge is therefore about 300 ft, not the 600 ft the indication suggests, and the rules of thumb are each only accurate to roughly ±10 %. With no radio altimeter and no terrain database, and with the aircraft 25 nm from the ridge, that is not a margin to defend. The correct answer is that 5,400 ft is not adequately protected.
2. Explain why the pressure and temperature errors act in opposite directions here, and what would happen in the reverse case (very cold air under a high).
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Pressure error - 'high to low, look out below'. The altimeter is a pressure gauge calibrated to the ISA. If you keep 1013 hPa set while flying into air whose QNH is lower, the aircraft is encountering the 6,000 ft pressure level at a genuinely lower height: the true altitude falls while the indication stays constant. Pressure error depends on the difference between 1013 and the local QNH, not on the QNH itself.
Temperature error. The altimeter assumes ISA temperature. In air warmer than ISA the column of air below the aircraft is thicker (less dense), so a given pressure surface sits higher: the aircraft is genuinely above its indicated altitude. In air colder than ISA the column contracts, the pressure surfaces bunch downwards, and the altimeter over-reads.
In this case the two errors partly cancel: −540 ft from pressure, +240 ft from temperature.
The reverse case is the dangerous one. With QNH 1030 and air 15 °C colder than ISA at the same level, the pressure correction is about +510 ft (true altitude higher) and the temperature correction about −360 ft: the errors again partly cancel. The classic killer is the opposite combination - flying from a high-pressure cold airmass into a low-pressure warm one without resetting the subscale, or descending into a deep cold pool on a QNH set far away, where both corrections push the true altitude down (or, on approach in extreme cold, the altimeter over-reads and the aircraft flies a genuinely low approach profile). Always ask which way each of the two errors points before accepting an indicated altitude as terrain clearance.
3. Recommend a specific action, including an altitude to climb to and any other considerations on this sector.
Read the worked answer
Action: climb before the ridge, and use the regional QNH, not 1013.
Practical plan:
- Set 995 hPa (the regional QNH) as soon as practicable. That removes the 540 ft pressure error at a stroke and the altimeter then reads altitude above the local pressure datum.
- Climb to 7,500 ft indicated, which gives roughly 7,150–7,200 ft true even after the +240 ft temperature over-read is allowed for, i.e. about 1,750 ft over the 5,400 ft minimum. At 700 ft/min the climb from 6,000 ft takes a little over 2 minutes, roughly 4 nm at normal cruise speed - comfortably inside the 25 nm to the ridge, but start now rather than at the ridge.
- Tell ATC the reason (terrain, MSA) and request the level; if a block is unavailable, hold the current level clear of the ridge and negotiate.
- On descent into the destination, set QNH 996 when within the prescribed range; the destination is also in the low, so the altimeter will read lower than a 1013-referenced figure for the same true height - a further reason not to carry 1013 into the terminal area.
- Remember that temperature correction is greatest at high altitude in very cold air; for a distant destination with a very low temperature on the ground, apply the cold-temperature altitude correction to the approach minima as published in the operator's procedures.
Climbing is cheap; a 300 ft obstruction margin in a non-RVSM turboprop without a radio altimeter is not.