Descent Performance

- James Albright (a former G450 driver)

Updated: 2013-11-05

There isn't much published about descent performance for the G450, so what follows is all technique. You obviously want to stay as high as possible, as long as possible. But then what techniques will you use to determine when to start down and with what mode of the autopilot?

Everything here is from the references shown below, with a few comments in an alternate color.

 Start Descent Point

The FMS knows TOD, right? Sometimes TOD is messed up because the track miles in your arrival procedure are not the same as the track miles in your actual future. Sometimes TOD is messed up just because that's the way it is.

I've been using three times my altitude in thousands times three for years, starting with the Boeing 707, told here in story form: Top of Descent. But sometimes that doesn't work you say?

One problem with the technique occurs when descending to a high altitude airport. That's why the technique has evolved to: "three times your altitude (in thousands of feet) to lose."

For example, let's say you are at FL450 and descending to an airport with an elevation of 7,500 feet. That means you have to lose 45 - 8 = 37 thousands of feet. That means you should start down at 37 x 3 = 111 nautical miles.

This produces a good start descent point for a G450 except the airplane can't seem to make the gradient at high altitudes without running into VMO. Dividing the altitude by three results in a 3.33 degree descent. (It involves a little math, but here's an explanation of sorts: Flight Lesson / 60-to-1.)

I've tried this a few times and it looks fairly consistent. Descending from 45,000' the airplane can hold speed flying a 2.5° descent. You can steepen this angle between 35,000 and 31,000' to 3° and even more lower than that. We know starting down at three times your altitude to lose gives you a 3.33° descent. Using the same 60-to-1 methodology reveals that starting at four times your altitude to lose give you a 2.5° descent. So, if you want to descend without the boards . . .

Rule of Thumb:

When descending from above 35,000', start down at four times your altitude to lose and select a 2.5° flight path angle on the guidance panel. You can steepen the angle to 3.0° once below 35,000'. When descending from below 35,000', start down at three times your altitude to lose and select a 3.0° flight path angle.

Autopilot Mode

Several of the most oft used guidance panel modes for an en route descent are fraught with problems:

• FLCH - brings the throttles to idle and then flails through the sky in a vain attempt to smoothly keep the selected speed. For some reason the G450 doesn't FLCH down very smoothly. Don't use this mode except for climbs and in the simulator.

• VS - gives you a smooth descent but will allow you to overspeed and "underspeed" the airplane without warning. Use VS if you like, but keep an eye on the KCAS.

Try using FPA set to 2.5° through 31,000 feet at 0.80 Mach. Then increase FPA to 3.0° with speed set to 310 KCAS until 10,000 feet. Then . . . well, you know.

Fuel

If you manage to avoid using the speed brakes and avoid an early descent and the need to cruise at low altitudes, your fuel usage should be minimal once starting down:

Rule of Thumb:

It doesn't take much fuel to get down, generally around 500 lbs total from FL450 to sea level.

It takes about 20 minutes to go from FL 450 to sea level.

 References:

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.