Descolagem no planalto: aceitar ou recusar o Manifesto de Peso
A light transport aircraft is scheduled to depart a plateau airfield at elevation 1 770 m (5 800 ft). Reported conditions: temperature 30 °C, QNH 1020 hPa, headwind component 10 kt, runway length available 1 770 m. The flight manual gives a sea-level, ISA take-off ground roll of 900 m and states that ground roll increases by about 10 % per 1 000 ft of pressure altitude and by about 10 % per 10 °C above ISA temperature; a 10 kt headwind reduces ground roll by about 10 %. Squadron orders require the take-off distance available to exceed the ground roll by 15 % to clear the upslope obstacle. The aircraft commander must sign the load sheet accepting or rejecting the departure.
1. Compute the expected ground roll and the distance required by squadron orders, then state your decision on the load sheet.
Read the worked answer
Pressure altitude = 5 800 + 30 × (1 013 − 1 020) = 5 800 − 210 ≈ 5 590 ft.
ISA temperature at 5 590 ft = 15 − 1,98 × 5,59 ≈ 3,8 °C. Deviation = 30 − 3,8 = 26,2 °C above ISA.
Factors:
- Altitude: 1 + (5,59 × 0,10) = 1,559
- Temperature: 1 + (26,2 ÷ 10 × 0,10) = 1,262
- Headwind: 0,90
Ground roll = 900 × 1,559 × 1,262 × 0,90 ≈ 1 594 m.
Required = 1 594 × 1,15 ≈ 1 833 m against 1 770 m available.
Decision: reject. The aircraft is short by about 63 m, roughly 3,5 % of the runway, before any allowance for a lazy rotation or a wind shift.
2. The operator proposes loading 300 kg less and departing at 18:00, when the forecast temperature is 22 °C. Is that sufficient?
Read the worked answer
Temperature at 22 °C: deviation = 22 − 3,8 = 18,2 °C → factor 1,182.
Ground roll = 900 × 1,559 × 1,182 × 0,90 ≈ 1 492 m. Required = 1 492 × 1,15 ≈ 1 716 m < 1 770 m → margin 54 m.
But test the headwind: with the wind calm, ground roll = 1 492 ÷ 0,90 ≈ 1 658 m, required ≈ 1 907 m - a 137 m shortfall.
The entire acceptance therefore rests on a 10 kt headwind that a forecast cannot guarantee at departure time. Judgement: accept only if the tower reports a steady headwind of at least 10 kt at the moment of lining up, and set a re-check on the windsock; if the wind is calm or variable, reject and wait. The reduced load helps, but it is not what makes the departure legal.
3. Which correction dominates - altitude or temperature? Why does that matter for the decision you must sign?
Read the worked answer
Altitude dominates by a wide margin: a factor of 1,559 against 1,262 for temperature.
The 26 °C ISA deviation is already severe, yet it accounts for roughly half the penalty that 5 590 ft of pressure altitude imposes. The practical consequence is that waiting for cool air cannot rescue this departure. Even at a perfectly ISA day at that elevation (factor 1,559 alone), ground roll would be 900 × 1,559 × 0,90 ≈ 1 263 m and required ≈ 1 452 m - legal, but only just, and with no temperature penalty at all.
The levers that actually move the answer are therefore, in order: weight, wind, and runway length - not time of day. A commander who signs on the strength of an evening departure is treating the smaller of the two corrections as though it were the larger.