Can a 747 Survive a Four-Engine Failure at 40,000 Feet?

Initial Scenario and Attitude Considerations

Imagine you have a crane 40,000 feet high and a Boeing 747 just sitting there at the top. Would it be possible to drop the aircraft and start the engines before it crashes? To address this, we need to understand the physics and mechanics involved.

When dropped from such a height, the aircraft is initially not moving relative to the air. If it were to be dropped in a nose-down attitude, it would start moving forward, creating an angle of attack that could initiate lift. As the airplane builds up speed, increasing lift would occur, allowing the pilots to achieve sufficient gliding capability to slow the descent.

Starting the Engines

There are two primary options to start the engines: using the Auxiliary Power Unit (APU) or a windmill relight. Starting with the APU can provide pneumatic pressure for the engine starters, while a windmill relight relies on the airspeed necessary to generate sufficient RPM for a restart.

My choice would be to start with the APU, as it provides a more controlled environment. Similarly, I would attempt to start two engines at the same time, beginning with the inboard engines, for maximum redundancy. However, starting the APU does take time, so a windmill relight is still a viable, albeit slower, option.

Windmill starts typically involve selecting continuous ignition and turning on the fuel once the N1 RPM is high enough. APU starts are usually just a matter of pushing a button. Regardless of which method is used, the question remains: is 40,000 feet enough to achieve a controlled descent and start the engines?

Real-World Example: British Airways Flight 9

The fact is, this scenario has already happened. British Airways Flight 9, a 747–200, suffered four engine failures when it inadvertently flew into a volcanic ash cloud over the Indian Ocean. With a glide ratio of approximately 15:1, the aircraft could maintain control and restart the engines after descending below the cloud.

The plane managed to make a safe emergency landing in Jakarta despite the challenging circumstances. This is a testament to the resilience of modern jet airliners and the trained responses of their crew.

Vertical Speed and Airspeed Considerations

For a 747 to regain control and start the engines, it must be in a state of forward motion. If dropped at rest, it would stall immediately. Therefore, it is crucial that the crane provides relative forward motion to ensure a positive angle of attack.

If the engines suffer a flameout at 40,000 feet during normal cruise, will the pilots be able to regain control? The answer is mostly yes, under certain conditions.

Hydraulic Systems and Control

All flights controls on a 747 are hydraulically powered, meaning that if at least one of the four hydraulic systems is functional, the pilots can regain control. The crucial aspect is the operation of these systems.

Transport aircraft are required to demonstrate in-flight restart capability during certification, both with and without the use of the APU. A windmilling restart does not use the APU, and it is less likely to succeed at such lofty altitudes due to the thin air density.

The maximum restart altitude is determined by the inflight restart envelope, a chart provided in the FAA-approved flight manual. This chart indicates the range of airspeeds and altitudes at which in-flight restart has been demonstrated during certification flight testing.

Conclusion

While the scenario of a crane dropping a 747 from 40,000 feet is hypothetical, the real-world example of British Airways Flight 9 and the technical considerations of in-flight restart provide valuable insights. The success of such maneuvers depends on various factors, including the availability of hydraulic systems and the altitude at which an engine can be restarted. Pilots undergoing certification and training are well-prepared to handle such emergencies.

Understanding these nuances is crucial for both the pilots and the maintenance engineers, ensuring the safety and reliability of large passenger aircraft.