Muscat Departure with an Assumed Temperature and a Contaminated Runway
You are the commander of a twin-engine turbofan aircraft (MTOW 78 000 kg) departing Muscat (OOMS), runway 26L, length 3584 m, elevation 15 m. OAT 41 °C, QNH 1004 hPa, wind 300°/12 kt. The runway is wet with standing water in patches; braking action reported medium. Performance software gives a full-thrust field-limited mass of 74 200 kg and a full-thrust screen height of 35 ft at V2. To reduce engine wear the operator's SOP permits an assumed temperature of 52 °C, which the software says yields a field-limited mass of 71 500 kg on a dry runway. Actual take-off mass is 70 800 kg. There is a 0.8 % upslope on the first 1200 m. The first obstacle is a 90 m mast 6.5 km from the brake release point, 0.3 % below runway datum.
1. Can the assumed temperature of 52 °C be used for this departure? Justify with reference to the certification logic of assumed-temperature thrust.
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No - the assumed temperature must be discarded for this departure.
Assumed-temperature (flex) thrust is certified only on the basis that the reduced thrust still satisfies all take-off requirements on the actual runway condition. The wet/contaminated runway is the disqualifier here, not the temperature itself:
- Contamination. With standing water and medium braking action, the accelerate-stop distance is governed by a mu well below the dry value. The 71 500 kg figure in the software was computed for a dry runway; it cannot be transposed to a wet runway. Most operators and the AFM prohibit flex thrust on runways where braking action is medium or worse, or where there is standing water.
- Wet runway also changes V1. V1 must be reduced so that the stop is achievable on the contaminated surface, which further constrains the field-limited mass.
- Slope. The 0.8 % upslope over the first 1200 m increases the all-engine and one-engine-inoperative ground roll; the software figure must already include it, but any manual correction re-opens the margin.
With an actual mass of 70 800 kg against a dry flex limit of 71 500 kg the margin is only 700 kg - less than the penalty from water drag and reduced mu. Take off at full rated thrust (or the highest permissible derate) and recompute V1/VR/V2 for the wet case. If the recomputed field-limited mass falls below 70 800 kg, the departure is not legal and mass must be offloaded.
2. Assuming full thrust is used and the wet-runway field limit is 71 900 kg, verify the obstacle clearance at the 90 m mast and state what climb gradient is required.
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Required gradient. The net take-off flight path must clear the obstacle by 35 ft vertically (CS/FAR 25.115 for a twin: net = gross − 0.8 % gradient). Obstacle height above the 35 ft screen point:
- Runway elevation 15 m; obstacle top 90 m → 75 m above runway datum, plus 0.3 % of 6500 m = 19.5 m terrain fall → obstacle is about 94.5 m above the brake-release datum.
- Screen height 35 ft ≈ 10.7 m at end of TODR. Net height required at the obstacle = 94.5 − 10.7 + 10.7 (35 ft clearance) ≈ 94.5 m.
- Horizontal distance from screen point to obstacle ≈ 6500 − TODR. With TODR ≈ 2600 m, distance ≈ 3900 m.
Required net gradient = 94.5 / 3900 ≈ 2.4 %. Required gross gradient = 2.4 + 0.8 = 3.2 %.
Check. At 70 800 kg, 41 °C, QNH 1004, the second-segment gross gradient for this type is around 3.0 % and the acceleration-segment gradient lower still. That is marginal. Because the obstacle is beyond the end of the second segment, the relevant gradient is the take-off flight path gradient after gear retraction, which is higher - but the operator must confirm it from the AFM graph for 41 °C and the actual mass, not from a nominal figure. If the demonstrated gross gradient at the obstacle is below 3.2 %, the departure is obstacle-limited and mass must be reduced or a closer-in obstacle analysis performed.
Conclusion: the field limit (71 900 kg) is satisfied at 70 800 kg, but obstacle clearance is the binding constraint and must be verified from the AFM before dispatch.