Ask Captain Lim ~ All about Aviation - Can you explain why the ETOPS of a Boeing 777 is limited to 3 hours?

Hi Capt Lim, A great web site. I absolutely love it!

I have a few specific questions about the Boeing 777 performance capabilities:-

1. At MTOW (maximum take-off weight), what is the single engine climb gradient (feet per nautical miles) at maximum climb thrust at sea level, ISA (International Standard Atmosphere) condition?

2. If a Boeing 777 is tracking the ILS (Instrument Landing System) on an auto land and an engine fails at, say 1000ft, can the computer still compensate for the asymmetric thrust without pilot input and still land itself safely?

3. I am kind of familiar with ETOPS (Engines Turn Or Passengers Swim!), but it doesn't make any sense why the Boeing 777 is limited to 180 minutes. If you cross-feed the fuel to the good engine during an engine failure, it seems like it could fly longer than on two engines, since only one engine is now burning fuel. Therefore it seems like it could fly even longer (fuel quantity divided by the fuel flow of the operating engine). Can you explain why then, it is limited to 3 hours. I would greatly appreciate your answer.

Thank you very much for your time. The Boeing 777 is by far my favorite and my ultimate job would be to do just what you do for a living!! Andrew

Hi Andrew, 1. I do not have the specific performance charts with me to work out the answer on the single engine climb gradient. So I cannot give you a precise answer at the moment, perhaps later.

2. Engine failure at 1000 feet - no big deal my friend! A Boeing 777 can easily handle this emergency and land safely. Below 1500 feet, the airplane rudder is coupled to the autopilot during an auto landing. When the computer sense an engine failure, the asymmetric force would be handled by the TAC (Thrust Asymmetric Compensator) and so any yawing effect would be very negligible. The pilot would notice the automatic application of power on the live engine and any rudder forces would be automatically trimmed.

3. You are right! Theoretically, the plane can continue to fly on one engine for more than three hours until it runs out of fuel! The three hours limitation is imposed by FAA for certification purposes. It takes into consideration many other safety factors, such as a particular airline maintenance practices, historical engine performances, airplane cargo fire suppression capability, weather conditions and facilities at the alternate airports. All these and other safety measures are looked into before the Authorities came out with a suitable safe time frame for the Boeing 777. This limitation can be extended or reduced by a particular Aviation Authority. For instance, if a particular Airline operating Boeing 777's has poor maintenance records resulting in three engine failures in the ETOPS segment during the last one year, it may have its 180-minutes ETOPS withdrawn or downgraded to a 120-minutes instead. If that happens, it may not be allowed to operate a particular route that has a 180-minutes ETOPS segment, such as the Los Angeles-Auckland route.