Question: If two engines on the same wing of a 747-400 fail, can the jet fly on the other two?
-- submitted by reader Jack Ryan, Ohio
Answer: It depends on the weight and the altitude. At very heavy weights and high altitude, maintaining altitude on two engines may not be possible. At lighter weights, it may be possible to maintain altitude with 2 engines.
As for where the outages occur, having two engines out on one side would make the airplane harder to control, cause more drag due to the rudder required, and would increase pilot workload significantly. If the disabled engines were on opposite wings, there would be less rudder required, thus reducing drag. So the ability to fly and maintain altitude would be a higher weight than a same-side outage.
Pilots of four-engine airplanes do train to fly them with two engines inoperative.
Q: What happens if an engine fire cannot be extinguished?
-- Jimmy, Tampa
A: Engine fires are extinguished quickly and easily by engine fire extinguishers. Before the extinguisher is discharged, the fuel is shut off at the firewall, all electrical power is shut off to the engine, the high temperature bleed air is shut off, and all hydraulic fluid is shut off at the firewall. These remove the likely sources of ignition and fuel for a fire. If the fire continues, powerful extinguishers are discharged to put out the fire. There are normally two extinguishers available to fight an engine fire.
In the very remote possibility that a fire cannot be extinguished, the firewall will protect the airplane from a burning engine until the pilot can land the airplane.
Q: What is a "cross-bleed engine start," and why would it require permission from ground control and notification of fire rescue crews?
A: Most airliners use compressed air to start engines with pneumatic starters. Normally the air is supplied by the auxiliary power unit (APU). When the APU is inoperative, the air is supplied by a ground air cart, requiring an engine to be started at the gate. Air for the second engine start can be supplied by the running engine, using bleed air (compressed air extracted from the engine before the fuel is introduced). To have sufficient pressure, it is often necessary to increase the RPM of the running engine. This is the reason that advising Ground Control is necessary. The jet blast from an engine at increased RPMs can pose a danger to other aircraft or ground vehicles.
I have done many cross-bleed starts, which required coordination with Ground Control, but airport fire facilities were never involved.
John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems.