Ask Captain Lim ~ All about Aviation

Hi Capt Lim,

I have one question with reference to the basic climb performance of a typical
Airliner like a Boeing 777. If you visualize the vertical view of the climb profile of an aircraft, after takeoff and before the top of descent I see a constant altitude segment. What is the exact reason for it?

Is it because the Aircraft is under maximum gross weight and is incapable of
reaching it's cruising altitude directly or is the pilot trying to avoid a particular region so he needs to have a constant altitude segment before cruise?

If you have a choice would you prefer to climb to the cruise directly
or do you prefer to step-climb and why?

Thanks and Regards,

Srihari J

Hi Srihari,

It is not true to say a typical Boeing 777 profile has a constant altitude segment. Perhaps the profile you mentioned covers a short sector of a 3-to-4 hours flight where further step-climbs have no economical advantage. I do a lot of long haul flights from the Far East to Europe of around 12 to 13 hours duration where there are at least 3 step-climbs. With a heavy aircraft (around 286,000 kg), the computer calculates the optimum altitude of 31,000 feet initially.

As the aircraft climbs, the computers recommends a step-climb to 35,000 feet when the aircraft weight reduces as a result of fuel burnt (a Boeing 777 consumes around 6000 to 7000 kg of fuel per hour depending on the
weight).

Three quarter way through the flight when the aircraft is about 55,000 kg lighter because of the fuel consumed, the computer will recommend a further climb to 39,000 feet. Looking at this climb profile, it is not true to say that a typical Boeing 777 has a flat segment from the top of climb to the top of descent.

Pilots will rely on the Flight Management Computer to choose the optimum level to climb to. If the pilot fly at any level other than recommended, the airplane will burn more fuel and hence uneconomical to cruise at.

When you mentioned about a constant altitude before cruise, I am wondering if you are referring to the initial accelerating altitude of 1000 feet or the 3000 feet (depending on the airport), which is part of the noise abatement procedures.