Ask Captain Lim

Hi Capt Lim,

On a long flight, probably at 33,000ft, approaching an airport, at what point, or time in the journey, does the descent starts?

When do you start to back off on the throttles, and start to put the nose down?

Lancelot Howard Barron,
Middlesbrough, United Kingdom

Hi Lancelot,

The criteria for planning a good descent is to ensure the greatest comfort to the passengers with the least fuel burned. On a Boeing 777, all these are calculated by the Flight Management Computer (FMC). When the FMC is connected to the autopilot, the airplane would descent automatically provided there is prior clearance to leave the cruising level first. The Captain merely sets the initial level approved for the descent. As soon as the airplane reaches the computed descent point, the auto-throttle retards on its own. The pilots would monitor this automatic sequence and call out the action. Either pilot would acknowledge the call. So, even though this is an automatic action, both the pilots have to monitor that the automation is doing the right thing.

The descent is so gradually executed that, quite often, the passengers are unaware of the descent because the FMC would schedule the depressurization so that the rate of descent is very gentle to the ears.

I believe you once told me that you have done some flying before, perhaps without automation. Well, the rule of thumb for manually calculating the top of descent point for jet engine airplane is quite simple. Just multiply the height in thousands of feet by three and add about 10 nautical miles. If you were at 33,000 feet, the descent point is (3 X 33) + 10 = 109 nautical miles. So, when the DME (Distance Measuring Equipment) reads about 109 miles from the destination, the pilot would start to retard the throttles to idle power and the nose would gently lowers to commence the descent.