Bandar Seri Begawan Departure - Wet Runway and a Close-In Obstacle
You are the pilot-in-command of a twin turboprop (MTOW 9,200 kg, TORA 2,450 m) departing Brunei International Runway 03 at 14:00 local. OAT 32 °C, QNH 1009 hPa, aerodrome elevation 22 ft. The runway is wet from a recent shower and the reported braking action is medium. Reported wind 030°/8 kt. The AFM take-off chart is based on a dry runway, zero wind, sea level and 15 °C, and the operator's performance manual says to add 15 % to the dry ground roll for a wet runway and to apply a 50 % factor to any reported headwind. A 12 m-high building sits 1,180 m from the start of the take-off run, 90 m left of the extended centreline. The AFM obstacle clearance gradient for the second segment is 2.4 %. Your company SOP requires a 15 % margin over the AFM net take-off distance before accepting the departure.
1. Explain how you build the actual take-off distance from the AFM chart value and determine whether the runway and obstacle case is acceptable.
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Start from the AFM chart figure for the given pressure altitude and temperature, then apply corrections in the correct order. Pressure altitude is essentially 22 ft (QNH 1009 vs 1013 is a small negative correction), so use sea level. Temperature is ISA +17 °C, so read the chart at 32 °C, not 15 °C.
Apply the wet-runway increment of 15 % to the ground roll portion only, then apply wind: the manual says use 50 % of the reported headwind, so 8 kt becomes 4 kt effective. A 4 kt headwind shortens the ground roll by roughly 4/8 of the chart's 10 kt headwind line, or about 5 %. Net effect is a modest increase.
For the obstacle, compute the required climb gradient to clear 12 m at 1,180 m horizontal distance. The gradient needed is 12/1,180 = 1.02 %, which is well below the 2.4 % second-segment capability, so the obstacle is not limiting provided the aeroplane reaches 35 ft before the obstacle. However, the building is offset 90 m from the centreline, so it only matters if your drift at that point exceeds 90 m; with 8 kt of crosswind component (030/8 vs runway 03 is nearly aligned), drift is negligible and the obstacle is essentially on the flight path.
Finally, compare the corrected take-off distance plus 15 % SOP margin against TORA 2,450 m. If the corrected figure exceeds 2,450 m, reject the departure or reduce take-off mass. The judgement is to compute, not assume: a hot, wet, heavy departure at a sea-level tropical airport is often close to the limit even though elevation is trivial.
2. The dispatcher suggests departing with a 10 kt tailwind instead to use a longer runway. Evaluate this proposal.
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A tailwind is the worst possible wind correction. The AFM tailwind penalty is typically 10–20 % per 5 kt of tailwind on the ground roll, and it also increases the airborne distance to 35 ft because the aeroplane covers more ground per second at the same airspeed. A 10 kt tailwind could add 20–40 % to the ground roll, far outweighing any benefit from a longer runway.
Also check the AFM limitation: many turboprops are certified only to 10 kt tailwind, and some to 15 kt; exceeding the demonstrated value invalidates the performance data. The wet runway and the close-in obstacle compound the problem, because the increased ground roll eats into the distance available before the 1,180 m obstacle.
The correct decision is to keep the headwind runway, reduce take-off mass if the corrected distance plus 15 % margin exceeds TORA, or delay until the shower passes and braking action improves. A tailwind departure at 32 °C on a wet runway is a classic accident sequence and should be rejected.