Hot-Day Dispatch from OKBK: Fuel, Density Altitude and a Forecast Thunderstorm

Build the minimum from the bottom up, as the operator's fuel policy (ICAO Annex 6 basis) requires:

• Taxi-out: 200 kg
• Trip: 2,400 kg
• Contingency, 5% of trip: 120 kg
• Alternate OEDF: 1,050 kg
• Final reserve, 30 min at 2,400 kg/h: 1,200 kg
• Minimum take-off fuel = 4,970 kg

The plan's 5,220 kg = 4,970 + 250 kg crew extra, so the arithmetic is sound. Four points an examiner will probe:

1. Contingency is 5% of trip fuel, not of trip plus alternate. Using the larger base is a common error.
2. The 250 kg extra is discretionary. It must be justified and does not count toward the minimum; on a short sector with a good destination it is a reasonable precaution.
3. Taxi-in fuel is not part of take-off fuel, but the landing fuel must never fall below final reserve.
4. Protection check: landing at OBBI would be 5,220 − 200 − 2,400 = 2,620 kg. After a diversion of 1,050 kg, 1,570 kg remains - 370 kg above final reserve, or 490 kg counting unused contingency. That is a thin but legal margin, and it is the number that will decide the in-flight diversion question later.

Convert the conditions into what the chart uses.

• Pressure altitude = 206 ft + (1013 − 997) × 30 ft/hPa ≈ 686 ft.
• ISA temperature at 686 ft ≈ 15 − 1.98 × 0.686 ≈ 13.6 °C.
• ISA deviation = 47 − 13.6 ≈ +33.4 °C.
• Density altitude ≈ 686 + 120 × 33.4 ≈ 4,700 ft.

So the aircraft is not operating at sea level; it is effectively at about 4,700 ft. Planned TOW = 61,800 + 5,220 = 67,020 kg against a PLTOW of 70,300 kg: a margin of 3,280 kg.

That margin is not a fixed asset. It was generated with 12 kt of headwind on a 3,400 m runway; each knot is worth roughly 100 kg at this weight, so losing the wind costs about 1,200 kg. Dropping the QNH to 0995 raises pressure altitude by another ~60 ft, which adds roughly 60–70 kg of equivalent weight penalty and shifts the whole chart. In addition, on a 47 °C day the second-segment climb gradient is frequently the controlling limit for a twin, not the field length - so the field-length margin may be irrelevant if the climb-limited weight is lower.

Verdict: acceptable as planned, but only after re-entering the chart for the actual wind and QNH. Never treat a 3,280 kg margin as proof when two of its inputs have changed.

First, quantify the delay. Roughly 40 min of APU at about 120 kg/h is ~80 kg, small compared with the 250 kg extra already carried. The delay itself is not the problem; the forecast is.

Second, check legality. Once the destination forecast includes TEMPO thunderstorms reducing visibility below the planning criteria, an alternate is required - and OEDF at 1,050 kg is already in the plan. Legality is unchanged.

Third, judge the real exposure. The slack over final reserve after a diversion was about 490 kg. At 2,400 kg/h, 30 min of holding at the destination costs 1,200 kg and would break the final reserve almost immediately. So the crew has essentially no holding fuel at OERK-style weather; the plan should be a straight approach or a direct diversion, briefed before departure.

Actions. (1) Recompute take-off fuel with actual delay burn and confirm the landing fuel still protects alternate plus final reserve. (2) Either uplift extra fuel or accept a no-hold policy, briefed and recorded. (3) If extra fuel is uploaded, re-run the take-off performance: an extra 1,000 kg at 4,700 ft density altitude eats a third of the 3,280 kg weight margin and may make the departure performance-limited. (4) Brief the cabin crew on the possibility of a diversion and tell operations control.