# A/T is off by +/- 10 kts

In almost every aircraft I fly, I notice that the autothrottle is off by around 10 kts each way. This is most noticable during descent and climb, i.e. when I'm descending, if I set the speed to 260 kts, it will hold around 270 kts. I know, however that the autopilot is capable of holding 260 kts, because there is room to lower its thrust levels to reach the assigned speed. Why is it that despite the fact that the aircraft would be able to hold my required speed, it will always stay 10 kts off?

Jonathan (99jolegg) Chief Captain

Strange I've never had that problem

What is your VS when you are climbing / descending?

pilotwannabe Chief Captain

I get this sometimes but its never exactly 10 knots. When I descend for example....my speed will be higher that I select but I thought this was normal because of the force of gravity on the plane. I usually compensate it using the airbrake or a lower VS.

It never occurs on climb unless I climb at too high a rate....for example 3000ft/min at 26000+ft

😉

Well, its not exactly 10kts, but somewhere around that figure --

For example, say I'm in a 757, and passing through FL180 descending at a rate of 1900fpm. I'd have the speed set at 270kts, and it would be using 60% N1, and holding 280kts. Why doesn't it just lower the thrust level and give me 270kts? -

robbier First Officer

hmmm...that sounds weird. anyway the only thing i can suggest is to set the A/T +/- 10 kts of what you really want. Maybe that would work 😕

robbier wrote:

hmmm...that sounds weird. anyway the only thing i can suggest is to set the A/T +/- 10 kts of what you really want. Maybe that would work 😕

Generally, thats what I end up doing, but I'd like to find a solution, because it's quite unrealistic -- I'll keep looking 😉

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