Ask Captain Lim ~ All about Aviation

Hi Captain Lim,

Thanks for your very informative web site - it?s a very interesting read!

I've got three questions for you:

Firstly, what is the altitude penalty of losing an engine whilst cruising on a twin jet airliner such as the Boeing 777? Would it cause a problem traveling over mountainous areas en route such as Hong Kong to Europe? Also, does a twin-engine aircraft such as a 777 or A330 sometimes have to take a longer indirect route than a 747 or A340 because of this?

Secondly, I've noticed that some flights, even by the same airline using the same type of equipment, vary on routes such as HKG (Hong Kong) to LHR (London-Heathrow). I know that routings can vary somewhat due to the weather, but on flights between Europe and the Far East, they can vary by thousands of miles.

What is the reason for this? Is it due to political factors such as over flying rights, weather conditions or congestion? Could flight times be reduced if aircraft operates more direct routes? Also, once a flight plan is accepted, is the routing usually adhered to except for minor deviations to avoid the weather or might significant rerouting be made en route?

Lastly, I don't know the Mach number of the Boeing 777 that you fly, but how does the fuel consumption (and other operating costs) increase if a stretched version such as the 777-300 is flown? I heard that a DC8-60 carries 40% more passengers than a DC8-50 but cost only 5 % more to operate - that could be an exaggeration. How much more fuel does the 777-300 use when compared with a 777-200 on a similar length flight and is it cheaper per seat mile?

Thanks again for a very informative site.

Best wishes

Peter Gordon

Hi Peter,

1. The penalty of height loss in the event of an engine failure would depend on the weight of the aircraft. It would be more if it were heavy and vice versa. Routes from Hong Kong to Europe would be planned such that it would satisfy the performance requirement otherwise it would be illegal to proceed. It is possible that the Boeing 777 or Airbus A330 may at times be planned to fly on an indirect route due to terrain.

2. The length of the route can vary because of overlying rights. Depending on the country the plane is over-flying, the routes are normally adhered to according to the flight plan. Minor deviation is allowed due to weather. Any significant deviations are not normally allowed unless the routing is resubmitted by the company's ground operations. Yes, flight times can be reduced if more direct routes are available. In fact, FANS (Future Air Navigation System) using direct routes are already being launched in certain specific regions of the world but not between Hong Kong and Europe yet.

3. I have not gone into the specifics of comparing the fuel consumption of the Boeing 777-300 and the 777-200. Since the former has a higher gross weight, I believe the fuel consumption would be slightly more. Overall, because of the larger seating capacity on the stretched plane, it would be more economical in terms of per seat mile.