Terrain Alert After a QNH Mis-Set
You are pilot flying a night VOR/DME approach in IMC to a regional aerodrome with high ground close to the inbound track. The ATIS gives QNH 998 hPa, but both altimeters were set to 1008 hPa, the QNH from your departure aerodrome. Passing 5 DME the altimeter reads 2,600 ft, which is above the 1,900 ft MDA. The radio altimeter reads 2,180 ft, GPS altitude reads 2,290 ft, and the aerodrome elevation is 120 ft. The EGPWS suddenly announces 'TERRAIN AHEAD' and the terrain display shows amber terrain ahead turning red. ATC reports no known traffic and the weather radar shows nothing significant. The first officer's altimeter agrees exactly with yours.
1. Diagnose the discrepancy and quantify the error. Which references are independent of the QNH setting?
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The error is a QNH mis-setting of 10 hPa, about 270 ft at low altitude (1 hPa ≈ 27 ft near mean sea level; use 30 ft for a quick mental check).
Both altimeters read 2,600 ft, but independent height references disagree:
- radio altimeter 2,180 ft plus aerodrome elevation 120 ft gives a true altitude of about 2,300 ft;
- GPS altitude is 2,290 ft.
So the aircraft is roughly 300 ft lower than the barometric altimeters indicate. The two altimeters agree with each other because they share the same wrong QNH - agreeing instruments can still both be wrong.
Cross-check with ATC: the transponder's Mode C is referenced to 1013.25 hPa and is unaffected by the QNH setting, so the controller's displayed level is correct pressure altitude. Comparing a Mode C readback with your altimeter plus the set QNH exposes the error immediately. Reset both altimeters to 998 hPa; they should drop to about 2,330 ft and agree with the radio altimeter and GPS.
2. Why did the EGPWS alert, and what is the correct response at this point?
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The EGPWS look-ahead alert compares the projected flight path - from GNSS/FMS position and track - with the terrain database and the aircraft's barometric altitude to compute terrain clearance. Because the QNH was 10 hPa too high, the aircraft was presented to the EGPWS as about 270-300 ft higher than it actually was, so the computed clearance against rising ground ahead fell below the alert envelope. The amber-to-red terrain display at the real altitude is consistent with a genuine terrain threat, not a database error.
Response: treat it as real. At night in IMC with high ground, carry out the escape manoeuvre - disconnect the autopilot, apply maximum available thrust, pitch to the terrain avoidance attitude, wings level, and climb. Do not delay to reset the QNH.
Once clear and level, correct the QNH, re-check the radio altimeter and terrain display, and then decide whether to re-attempt, hold or divert. A QNH entry error is not a reason to ignore a terrain warning - the warning may be correct despite the error.
3. Beyond the terrain warning, what else did the QNH error compromise, and what cross-checks will you build into future approaches?
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A 10 hPa error of this size affects far more than the warning:
- MDA/DA. You believed you were 700 ft above the 1,900 ft MDA when you were only about 430 ft above it - the approach would have been continued below minima without the crew knowing.
- Missed approach altitude and the altitude alert are referenced to the same QNH.
- TCAS and EGPWS use barometric altitude in their algorithms, so their alerts are biased.
- Separation and level busts. Mode C is unaffected, so a controller may see a different level from the one the crew believe they are maintaining.
- Pressurisation warnings also use static pressure.
Cross-checks to adopt:
- Set the QNH from the current ATIS/ATC on each handover and cross-check both altimeters against the radio altimeter on approach.
- Use GPS altitude and the terrain display as advisory cross-checks only once the QNH is verified.
- Treat a single QNH set in both altimeters as a single point of failure; compare against Mode C and the aerodrome elevation on the ground.