Cruise Fuel Penalty After a Bleed-Valve Fault at FL350
A twin-engine transport is cruising at FL350, Mach 0.78, gross weight 62 000 kg, with 9 400 kg of fuel on board. Forty minutes after top of climb the flight engineer notices that the left air-conditioning pack has been commanded off automatically; the associated bleed-valve indication shows closed while the right pack carries the full cabin heat load. Cabin altitude is stable at 6 500 ft and pressurization is normal. EGT on both engines is 15 °C below the cruise limit and N1 is 2 % above the expected value for the current weight and level. The destination is 1 100 NM ahead with an alternate 180 NM beyond it; forecast winds are as planned. Company fuel policy requires landing with 30 minutes of holding fuel plus the alternate reserve. The crew must decide whether to continue, descend, or divert, and must record the defect.
1. Explain the aerodynamic and propulsive reason for the higher N1 and assess whether continuing at FL350 is acceptable from an engine-rating standpoint.
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With one pack off, the engine that feeds it no longer bleeds air from its compressor. The other engine continues to supply bleed air for the remaining pack, so the two engines are no longer symmetrically loaded. The engine with the closed bleed valve produces more net thrust at a given N1 because no high-pressure air is being extracted; the airframe therefore needs less N1 from that engine, but the engine still carrying the bleed must run at a higher N1 to deliver the same thrust. The observed 2 % N1 increase is consistent with a single-pack bleed configuration and is not by itself an exceedance. What matters is the resulting EGT: it is 15 °C below the cruise limit, so there is margin. The correct action is to confirm the affected engine's EGT and N1 remain within the cruise rating for the actual altitude and temperature, monitor for asymmetry, and continue at FL350 provided the EGT margin is maintained. If EGT approached the limit, a step descent of 2 000–4 000 ft would restore margin because both N1 and EGT requirements fall at lower altitude.
2. Estimate the fuel consequence and decide whether the planned destination remains legal under the stated reserves.
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A single-pack configuration raises specific fuel consumption modestly, typically on the order of 1–3 % for a twin at cruise. At a rough cruise fuel flow of about 2 400 kg/h, a 2 % penalty is about 48 kg/h. The remaining distance of 1 100 NM at Mach 0.78 and roughly 460 kt ground speed takes about 2.4 h, so the penalty is about 115 kg - small against 9 400 kg on board. The binding constraint is the reserve: 30 min holding plus a 180 NM diversion. Thirty minutes at holding flow (say 1 800 kg/h) is 900 kg, and 180 NM at 460 kt is about 23 min, roughly 700 kg, giving about 1 600 kg of required reserve. Planned trip fuel for 1 100 NM is about 5 700 kg, so total required is about 7 300 kg against 9 400 kg available - about 2 100 kg of surplus. The 115 kg penalty does not threaten legality. The decision is to continue, record the defect, and re-check the fuel figure at each waypoint; diversion becomes necessary only if the surplus erodes below the reserve.
3. State what must be entered in the technical record and what operational limitation applies to the defective system.
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The defect must be entered in the aircraft technical log (or equivalent journey log) as a description of the discrepancy, the date, flight number, and the crew member's identification, together with the action taken. Because the aircraft is being dispatched with a known inoperative pack, the crew must confirm that the item is covered by the approved minimum equipment list. A single-pack operation is normally permitted under an MEL relief with conditions: typically the opposite pack must be serviceable, the bleed-valve position must be verified, and there may be an altitude restriction to guarantee adequate ventilation and smoke clearance if the remaining pack fails. The crew should record the MEL reference and any operational or maintenance (M) procedure that applies. Maintenance action is due within the interval specified in the MEL, and the aircraft may not be dispatched beyond that interval without rectification. The commander retains responsibility for deciding whether the degraded configuration is acceptable for the planned route, including terrain and drift-down considerations.