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What is a Compressor Stall?

What is a Compressor Stall?

By Captain Dick Hyslop

Recently, there has been a great deal of news coverage surrounding the Delta Boeing-777 dumping fuel during an approach to LAX. While the headlines are all about the fuel dumping, let us take a look at what happened that led to the need for dumping that fuel.

The flight was a non-stop LAX to Shanghai which carried over 212,000 lbs. of fuel. Shortly after takeoff, the flight experienced something called a “compressor stall.”

The basic concept of how a jet engine works is fairly simple. As air enters the front of the engine, it gets compressed. Fuel and ignition are introduced which causes combustion, and then the gases are exhausted. The key to a smooth-running jet engine is getting the correct amount of compressed air to enter the combustion chamber where the fuel and ignition are introduced. If the pressure created by the ignition of the fuel and air exceeds the pressure in the compression chamber, some of the exhaust may actually be pushed forward and exit the front of the engine. This is a compressor stall. A loud bang, aircraft shudder, flames erupting out of the front and tailpipe of the engine, along with momentary engine indicator fluctuations are common. Closing of the throttle on the affected engine usually corrects the problem, depending upon the reason that the airflow was disrupted. Reducing power and leveling off (changing the AOA) will typically allow the engine to perform normally. Should the engine continue to stall, an engine shutdown is the next step that should be taken. Even though fire may be seen coming from the engine, it is unlikely that a Fire Warning will be indicated on the flight deck. Once all QRH (Quick Reference Handbook, similar to a POH) procedures are complete, the most sensible plan of action would be to return and have maintenance check out the engine.

While the decision to return is fairly straightforward, it does require the completion of several checklists and some critical decisions to be made:

  1. Can the aircraft maintain altitude and controllability on one engine, or an engine running at idle power?
  2. Is the aircraft over the Max Landing Weight?
  3. If over the MLW, is it acceptable to delay landing while dumping fuel?
  4. If the decision is to dump fuel, the aircraft will be vectored to an uninhabited area (over water) further away from the landing airport. Is that acceptable with a faulty engine?
  5. If being vectored to a dumping area is unacceptable, ATC will request all dumping to be completed above 6000 feet.
  6. If the decision is not to dump, the approach speed will be much higher, and the landing distance will be increased. Is the intended runway long enough?
  7. Will you require the AFRR (Airport Fire Rescue equipment) to be positioned on your arrival?
  8. An overweight landing inspection will be required by maintenance and, should the touchdown be hard, a much more in-depth maintenance inspection is required.

As far as the Delta Boeing-777 mentioned earlier, it took off with full wing tanks and approximately 85,000 lbs. of fuel in the center tank. (The crew reported 212,000 lbs. of fuel onboard when declaring the emergency.) Each wing tank holds 64,000 lbs. The fuel dump rate is approximately 5,000 lbs. per minute. At that rate, the entire center tank (the only tank that can be dumped) could be emptied in about 17 minutes.

At certified MTW (Maximum Takeoff Weight) of 660,000 lbs. (Boeing-777 is legal to land at MTW), the landing distance needed to stop is approximately 6000 feet. The length of the runway they landed on was 12,000 feet.

The NTSB (National Transportation Safety Board) has not published their findings as of the writing of this article. Several questions arise. Why didn’t the crew decide to dump at 8,000 feet when they declared the emergency? Why did they then decide to start dumping fuel on Final Approach at a much lower altitude?

Blue skies and soft landings.