slow down

[van der vart]

is it practicable for a controller to ask a pilot to reduce ground speed and descent at the same time? shouldn't he ask pilots to reduce mach number?

[Spitoon]

I'm only a controller - but I know the answer I'm likely to get if I do ask! :D

[Pilot Pete]

Spitoon

You can ask whatever you want, we don't mind;)

VdeV

It's speed through the air that pilots fly their machines at, not speed over the ground. So if high up we would be on a Mach number and when lower down an Indicated Air Speed. We are often asked to slow down whilst descending and can do it, it's just that big jets generally are quite slippery beasts and find it hard to slow down whilst maintaining anything much more than 500fpm rate of descent. We can trade off one for the other and use speedbrake to increase either. It depends on the 'urgency' that the controller makes the request with, as generally if descending we will use quite a few miles to slow significantly.

PP

[Jerricho]

P.P., you've reminded me of a quip I heard somewhere once.

ATC: "XYZ123, descend 7000, reduce speed 210kts......range 35 miles,sorry about the late call, "

XYZ123: "Ummmm......we can't do both"

ATC: "Can't you use your speed brakes?"

XYZ123: "Ummmmm.....they're for our mistakes, not yours!!!"

[van der vart]

pilot pete,


okay. mach number at high level and Indicated at low level. but it seems that controllers ask pilots to reduce ground speed when they are at high cruising level?!

[JazzyKex]

VdV. Only been at the commercial game for 5 years but I've never been asked to either state or fly to a ground speed. Occasionally if it's a breezy day and you are the first aircraft to arrive at an airfield for a couple of hours the approach controller may ask for various wind readings through the descent. They only see our speed over the ground on the radar but control us by asking for adjustments in our indicated airspeed. The knowledge of the wind direction and speed we are experiencing helps them out with that adjustment. In the cruise we try to operate at the most economical speed for the aircraft and if that doesn't suit the controllers planned seperation may be asked to adjust IAS or Mach, but never ground speed (so far)!

[Pilot Pete]

Jerricho,

I heard that one recently too.:p

VdeV

I've had the same (limited!) experience as Jazzykex, never been asked to fly at a groundspeed. Theoretically you could stall the aeroplane with a 100kt tailwind but still be doing a significant groundspeed! A bit of a far fetched example, but it's the aircraft speed through the air that is important to us, not the groundspeed. For separation purposes of a/c on the same route we are asked to fly at set Mach numbers to keep the required separation. For instance the a/c in front will be asked his Mach and if he replies M.80 then he may be told to maintain Mach.80 minimum and the a/c behind told to fly M.80 maximum. Or at least that's what has always happened to me. For converging traffic at the same level I have either been given a vector or instructed to climb/ descend to ensure separation. So far never asked to fly a groundspeed.

PP

[Jerricho]

Hmmmmmm, never ever heard (or used) a ground speed instruction. Especially for a/c in climb/descent.