Speed limit or not?


#1

[Sorry in advance if this topic has been discussed but I did not see it on any of the 7 pages of topics.]

Do commercial planes routinely bust the 250 kt speed limit below 10000 ft? I noticed on a couple of recent 777 flights to/from LHR that AA’s live position/airspeed display showed we had accelerated to well over 250 kt just a couple of minutes after takeoff at 3000 or 4000 ft. And in the few days since I joined FlightAware, I’ve noticed from the detailed flight log that jets routinely do this.

If this is common practice, is there a waiver issued by the ATC controller to exceed the speed limit, or is there some other way of requesting these high speeds (perhaps in the flight plan)? Or is the limit simply ignored by the speed police at ATC?


#2

Note the difference between air speed and ground speed. Controllers frequently request speeds but usually slower speed restrictions. They also request vertical speed or maximum rate of climb.


#3

Since takeoffs are generally into the wind, the ground speed should be less than the air speed in the early portions of the flight. On my flights, I saw speeds considerably higher than 250 kts on the display while we were still basically on runway heading, like on the departure to the west out of LHR.

Even after a 180 turn, during which the ground speed would become greater than the air speed, it’s hard to imagine a 50 kt wind just a few thousand feet AGL, as in this departure a few minutes ago: flightaware.com/live/flight/AAL1 … L/tracklog.

But you do raise a good point. Any airline personnel know for sure whether the onboard displays show the air speed or the ground speed?


#4

They show both.


#5

Hmmmm,

Needless to say, you must have never heard of the term wind shear?

On the contrary, it’s quite common to have 40 to 50 knot winds just a few thousand feet above the deck especially in the winter when the core of the jet stream wanders south.

Just this week, on the ground here at KMBO, winds were light and variable, but get off the deck 1,500 feet, and the winds were screaming at 45 knots (thus our severe weather outbreak). These are conditions for wind shear which doesn’t make for a pleasant ride.

You would be amazed how layered the winds are aloft and how abrupt the wind speeds change. One altitude will offer a head wind, and another will slip a tail wind on your climb to cruise. Figuring the right altitude is the trick of the trade. Do you take the bumps or the smooth air.

Allen


#6

Last week my instructor and I flew from Salina KS down to Ponca City in a 172, we had a 60 knot tail wind at 9500. Our ground speed was constant around 170, it made for a quick trip, so yeah I would say it’d be easy for an airliner to break 250 in the ground speed dept.h


#7

Wind shear? For a the entire duration of the flight?


#8

I responded to Indy 2001’s statement “it’s hard to imagine a 50 kt wind just a few thousand feet AGL”.

Nothing in the paragraph that I quoted in my original post regarded an entire flight.

Allen


#9

To answer the original question - No, airline pilots do not routinely bust the 250 knot speed limit below 10,000 feet. Remember, that it is indicated airspeed, not true airspeed or groundspeed. An airplane that is indicating 250 knots at 10,000 feet is actually going 290 knots, or 330 statute miles per hour.


#10

yes, IAS and TAS are different. aircraft are told to maintain 250 IAS under 10,000, which is a lot faster than 250 TAS. Also, don’t forget that the speed limit is 200IAS in class B


#11

Sorry, the speeed limit in Class B is 250 knots indicated. The speed limit under the class B is 200 knots. As always, though, if your minimum safe speed is greater than that, you may fly that speed. I remember when I used to fly a DC-10-30 from ORD to HNL, our minimum safe speed right after takeoff, and after the flaps and slats were retracted, was 282 knots!