Hot-Day Departure from Islamabad: The Altimeter That Read 2,060 ft
Benazir Bhutto International Airport, Islamabad (elevation 1,663 ft AMSL, runway 12/30, 3,287 m), 1430 PST in late June. The 1400 METAR gives surface temperature 44 °C, dewpoint 12 °C, QNH 1000 hPa, wind 310/08 kt, visibility 6 km in haze, no cloud below 5,000 ft. The aircraft is a 22,000 kg MTOW twin turboprop whose AFM takeoff and climb charts assume ISA. The first officer sets 1013.2 hPa on the altimeter and reads 2,060 ft while stationary on the apron. The departure procedure calls for a right turn onto a track passing 6 NM abeam a 900 ft ridge, and the company obstacle requirement is a 3.3% net climb gradient. Performance has not been recomputed for temperature. The captain, who has operated here for years, says the runway is long enough and wants to depart on schedule with the standard setting still selected.
1. Explain the 2,060 ft apron reading and its consequences for the departure and for terrain clearance.
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The altimeter is behaving correctly; the subscale setting is wrong for the purpose. With 1013.2 hPa selected, the instrument reads pressure altitude. Field QNH is 1000 hPa, i.e. 13.2 hPa below standard, and 1 hPa is worth about 30 ft in the lower atmosphere, so the altimeter over-reads by 13.2 × 30 ≈ 400 ft: 1,663 + 396 ≈ 2,060 ft, exactly the apron figure.
The consequence is that everywhere in the departure area true altitude is roughly 400 ft lower than indicated while QNH stays below 1013.2. Applied to the 900 ft ridge 6 NM abeam, any level-off or turn flown at an indicated altitude taken from the chart leaves 400 ft less real clearance than the crew believes - the classic 'high to low, look out below'. The error is also not constant: as the aircraft flies toward higher pressure it shrinks and eventually reverses, so altitude cross-checks in the SID must be flown on QNH, not on 1013.2. Correct action: set both altimeters to QNH 1000 hPa (or QFE if the procedure is QFE-based) and cross-check the field elevation before line-up.
2. Recompute the takeoff situation in density-altitude terms and recommend an operational decision.
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Pressure altitude at the field is about 2,060 ft. ISA temperature at that level is 15 − (2 × 2.06) ≈ 11 °C, so the surface is ISA +33 °C. Using DA ≈ PA + 120 × (OAT − ISA temp): DA ≈ 2,060 + 120 × 33 ≈ 6,000 ft. The aircraft behaves as if departing from a 6,000 ft airfield on a standard day - about 4,300 ft higher than the real elevation.
Takeoff ground roll and 50 ft distance typically increase about 10% per 1,000 ft of density altitude, so book figures must be increased by roughly 40%, and the net climb gradient falls in proportion. The runway is not the limiting item; the obstacle is. Nine hundred feet at 6 NM is 900/36,480 ≈ 2.5% gross gradient just to overfly the ridge, before adding the 3.3% net requirement and the turning departure. On a degraded high-DA gradient the aircraft can be runway-adequate yet climb-limited.
Recommendation: recompute takeoff weight from the charts for +33 °C and QNH 1000. If the climb-limited weight is below the planned load, delay to the evening, reduce payload or fuel, or use a longer runway. 'Plenty of runway' is not the controlling factor on a 44 °C afternoon in Islamabad.