Ask Captain Lim ~ All about Aviation

Dear Capt Lim,

First of all, I have to say that I have been very impressed with your website. It has been one of my favorite websites about airplane. I know that you're retired already but I have a question that I would like to ask if you do not mind.

I'm 26 and I'm a new pilot with Thai Airways, currently flying the Boeing 777. I would like to ask you how to plan a good descent and how to use the appropriate modes during the descent.

I totally understand that you might be enjoying your vacation now. Please take your time; I*ll be waiting for your e-mail.

Thank you in advance.

Sincerely,

Thanapat

Hi Thanapat,

To plan for a good descent, all the appropriate flight data must first be accurately entered into the FMC (Flight Management Computer) and the computer will do the rest of the descent for you when your FMC is coupled to the autopilot and you are on the LNAV and VNAV mode. In reality, what you have planned doesn't often work out the way you wanted it. Why? The elaborate STARS (Standard Arrival Procedures) that you have programmed assumes that you are probably the only traffic around, but it practice, in very busy airports (yes, your new Suvarnabhumi International Airport at Bangkok!), it becomes impractical. So the radar controllers will intervene and sequence you accordingly and you would either end up high or low in the descent profile.

Well, this does not mean that an accurate set up of the FMC is not important. Yes, the FMC gives you the most economical descent point based on the LEGS and Waypoints selected on the STARS. You have to properly plan your descent so that you arrive at the desired altitude in order to capture the glideslope for landing. How? Make good use of your speedbrakes when your descent on the FMC is interrupted by air traffic! At a speed of M 0.84/310 knots in clean configuration, the speed brake will increase your rate of descent from 2200 fpm (feet per minute) to 5300 fpm. Descent at a lower speed of 250 knots is not so effective as it will reduce the descent rate to only 3300 fpm.

A good rule of thumb to continually monitor your descent profile is to loose 1000 feet for every 3 nm (nautical miles). Make a cross check at 10,000 feet agl (above ground level) so that you would be 30 nm from the runway with a speed of 250 knots. Plan to arrive at the traffic pattern with your flaps up by 12 nm from the runway threshold.

Just remember, if you use anti-ice, the additional thrust would increase your descent distance and it would upset your descent profile. So, if you anticipate icing, you should enter the anti-ice altitude on the DESCENT FORECASTS page to assist the FMC to work out a more accurate descent profile.

Happy Flying!