Altimeter Left on Departure QNH
You are flying VFR from an airfield with QNH 1030 hPa to a destination 120 nm east with QNH 998 hPa. You set the altimeter to 1030 at departure and, to reduce workload, leave it set to 1030 for the whole flight. You cruise at an indicated altitude of 4,500 ft. The minimum safe altitude along the route is 4,200 ft, and there is a 3,900 ft ridge 20 nm west of destination. Cloud base is forecast 5,000 ft, visibility 30 km. On arrival, the destination ATIS gives QNH 998 hPa. Fuel is sufficient. You have a GPS with terrain awareness, but you have not used it for terrain clearance. The aircraft has a service ceiling of 14,000 ft.
1. Explain what the altimeter indicates and calculate the approximate true altitude at your indicated 4,500 ft with the subscale left at 1030 hPa while the destination QNH is 998 hPa.
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The altimeter is a pressure instrument. With the subscale set to 1030 hPa, it is calibrated to show the altitude corresponding to the standard pressure lapse from a 1030 hPa datum. As you fly east the pressure falls to 998 hPa, but the instrument does not know that. Flying from high pressure toward low pressure without resetting the subscale causes the altimeter to overread; the true altitude is lower than indicated. The pressure difference is 1030 - 998 = 32 hPa. Using the low-altitude rule of about 30 ft per hPa, the error is about 32 × 30 = 960 ft. True altitude ≈ 4,500 - 960 = 3,540 ft. The exact value varies with temperature and altitude, but the direction and approximate magnitude are reliable: the aircraft is nearly 1,000 ft lower than the altimeter says.
2. Assess terrain clearance over the 3,900 ft ridge and against the 4,200 ft minimum safe altitude. What does this mean for the flight?
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At an indicated 4,500 ft with the old setting, the true altitude is about 3,540 ft. The ridge is 3,900 ft, so the aircraft would be about 360 ft below the ridge, not above it. Even the route minimum safe altitude of 4,200 ft is not met; true altitude is about 660 ft below it. The cloud base and visibility are irrelevant if the aircraft is not above the terrain. This is a controlled-flight-into-terrain risk, not just an altimetry error. The GPS terrain awareness would likely warn, but it should not be the primary tool; the primary fix is to correct the altimeter setting and climb. If the aircraft is already near the ridge, climb immediately rather than waiting to reset the subscale; resetting alone does not change the true altitude.
3. State the corrective actions and the correct altimeter procedure for the arrival.
Read the worked answer
Climb immediately to regain terrain clearance, using the GPS terrain display and visual references as backup. Obtain the current QNH of 998 hPa at destination and reset the altimeter subscale. When you reset from 1030 to 998, the indicated altitude will fall by about 960 ft at the same true altitude; do not mistake that for a descent. After resetting, climb until the altimeter indicates the required true altitude, for example 4,500 ft, which now corresponds to about 4,500 ft true. Use the destination ATIS and the nearest station QNH en route; do not carry a departure QNH over long distances. For VFR, set the local QNH when available and maintain the minimum safe altitude plus a margin. Brief the terrain and the altimeter change before descent, and use the GPS terrain awareness as a cross-check, not as a substitute for the altimeter.