250kt speed limitation on departure,Europe/Usa/Asia

[fly744]

Some EU airports have that limitation stated on their SID charts,however in China,USA,ME or other Asian countries no speed limit on departure.

Many times I hear pilots on the radio requesting "high spd climb".Is that necessary? I would like to hear mostly from the ATC guys please...Thanks.

[welliewanger]

First of all, I'm a pilot, not ATC. So I may not have all the info you're looking for. But here's my understanding of the situation.

(Almost) everywhere, there's a speed limit below 10,000 feet of 250kts. This helps things like ATC (stuff going slower with smaller turning circles) prevention of bird strike damage (hit them slower) etc. Frequently, SID / STAR charts specify this speed limit. It annoys me that they clutter the charts with this superfluous information. The rule is there everywhere unless stated otherwise.

So why do pilots want to fly faster? I'm sure impatience is one reason, but practically there are others:

Climbing
- Higher up we burn less fuel, so the sooner we get there, the sooner we're saving fuel.
- At higher speeds we get a higher rate of climb... burning less fuel per 1000 feet climbed.

Descending
- It's actually quite hard to descend slowly! You're going downhill, so the plane wants to accelerate.
- If we need to descend quickly (sometimes we're left high for too long) then it takes less distance to dive the height off (at high speed) then, once level, decelerate again.

[M609]

At higher speeds we get a higher rate of climb

??????? IMHO, the oposite is true. If you give crews the option of speeding up, the climb rate goes from say 3-4K feet/min to 1.5K-2K feet/min. (For medum jets etc)
On the other hand, if crews are told to keep speed 250/230 or what the case might be, the climb rate stays high.

It is actually a method we controllers use to have a handle on the rate of climb of a flight. I.E you dont´t give high speed in climb to a flight you need a decent rate of climb from to possibly sort out a conflict.
On the other hand, if you have a arrival crossing a departure, giving high speed in climb to the intermeditate level you need to stop the departure at, will some times give the departure no level off at all in the end.

[Surferboy]

If i'm not mistaken you are always permitted to increase the speed to your min. clean airspeed. So in the case of heavy 744's/MD11's that would be 275kts or something.

And yes in most cases the lower the forward speed, the higher the climb rate. But in my (quite limited) experience B744's tend to build up speed, and then increase the rate..wich in the end gives 'm a higher average rate than when you keep them at standard speeds.

[welliewanger]

M609,
You're right, I didn't qualify that part of my answer sufficiently, and I can only answer for my type (Global Express, i.e. more power than most). At low levels (below 10,000') 220kts will give the best rate of climb. Slower than this and the rate decreases rapidly. Higher up, 300 kts is required. When I said that we climbed faster at higher speeds, I was referring to certain SIDs with tight turns on departure which limit (for example) to 185kts. A restriction like that reduces performance significantly.

[Jetjock330]

I fly A340-600 and at 380 tons, and with flaps slats retracted, our best climb speed below 10000ft is around 275 kts, known as green dot. Our minimum selected speed possible, "VLS" (velocity lowest selectable) is around 270kts, meaning the autopliot/autothrust will not let you select/command a speed lower than this (if you select lower than VLS, autothrust will fly 270kts as minimum speed). In order to fly at speed lower than 270 kts, the flap lever has to be selected or left in flap 1, which means climbing out to 10000ft with slats out. S speed at is is known is 248 kts roughly.

http://i836.photobucket.com/albums/zz285/Jetjock330/Screenshot2010-06-30at41023PM.png

http://i836.photobucket.com/albums/zz285/Jetjock330/Screenshot2010-06-30at41307PM.png

This would guzzle gas and lowers the safety margin of turbulence. A clean aircraft has a higher allowance of g-loading in turbulence than an aircraft that has flap or leading edge devices still out. A request for high speed climb is to to exceed 250 kts and go to green dot of around 275kts to 10000ft, there after, the jet will climb out at around 310kts indicated until Mach .82/.83 for climb on transition. This is talking in rough terms for a loaded A340-600 and is dependent on cost index used for the flight.

This request for high speed, is to allow the aircraft to clean up, and climb efficiently as possible to 10000ft and does not mean 300kts in our case.

http://i836.photobucket.com/albums/zz285/Jetjock330/Screenshot2010-06-30at40313PM.png

If you look at the bottom at "0" :green dot speed is dependent by weight and altitude and is also the best lift-drag speed (efficiency)

[jackieofalltrades]

@Jetjock330
That's very interesting and useful to know. I don't see many A340s at all where I work, but was of the understanding that once they've reached 250kts then in order to get a higher rate of climb the aircraft would use that momentum to translate into climb performance and slow to around 225-230kts.
Would this be more the case with the A340-300? Or have I been totally mis-informed in the past?

[Jetjock330]

I think you are misinformed:
A340-300, our old one had a MTOW of 275 tons and best climb speed is around the same, 275 kts at max take-off weight:

We don't ever want to go below green dot, even more so in cruise, as it is difficult to say the least, to accelerate above it in the cruise if you drop below it due turbulence, therefore we have minimum cruise speed to. Green dot is usually the minimum cruise speed too.

http://i836.photobucket.com/albums/zz285/Jetjock330/Screenshot2010-06-30at41552PM.png

As you can see and interpolate, 275 tons is around 275 kts.

At a lighter weight on short sectors, this speed is reduced considerably, due to the wide operating weight range.

Again at these heavy weights, it is not practical or really possible to slow to 250 kts clean, unless the slats are out.

[jackieofalltrades]

Thanks for the info, good to know and I'm happy to be corrected from what I had as an (erroneous) understanding.

[Quality Time]

Rate vs Angle

I've flown mainly Boeing (737,757,767) ,they have best Rate of Climb higher than 250kts but the Angle of climb is greater at lower speeds.

Weight dependent but lets say Rate 280kts and Angle 230kts would be normal ballpark figures.

The rate appears to decrease when given a higher speed because the aircraft needs part of it's climbing enegy to accelerate and the rate will reduce to maybe 1000ft/min while increasing to normal climb speed. Pilots new to type when told to 'expedite' often want to go to the FMC and press 'Rate' which is completely inappropriate as the aircraft levels off somewhat while accelerating to the Best Speed for rate.

[Spitoon]

Looking at the original question, the general 250kt speed limit comes from ICAO. Summarising, there is a limitation of 250kts below 3050m amsl (or 10,000 ft or FL100 depending on how the height is defined) in airspaces where there may be VFR flights and ATC does not provide IFR flights with standard separation from them. So, in airspace classes A and B, and C for IFR flights, the speed limit does not apply. ICAO is not explicit but presumably the speed limit is intended to give a fighting chance to 'see and avoid' VFR traffic.

As wellie mentioned, there are other reasons for having a speed limit such as ensuring that aircraft stay within the assumed/protected area around defined routes (SIDs and STARs in a terminal environment, for example).

In some States, the national law may also set out additional situations where the general; speed limit applies (or doesn't). No doubt there are other factors that may affect the speed limit for a particular flight or route in some cases.

I would guess that the chart makers have been asked to put the speed limit on the plate in order to overcome any question about whether the rule applies for that route.

Speaking as a controller who used to provide services in a variety of airspace classes (A, D and G around one airport), my abillity to approve a speed above 250 kts was determined by the airspace class and the height of the aircraft.