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Home > Flying the Plane > Could flying at 250 knots below 10.000 feet be exceeded?
Could flying at 250 knots below 10.000 feet be exceeded?
Flying - Flying the Plane
Saturday, 29 January 2011 15:07

Hello Captain,

On a recent domestic flight here in the US, I had my TV screen turned onto the map view, showing our current position, altitude, and speed.

I was surprised to see that all the way down to 5,000 feet MSL, we were going about 330 mph (ground speed). Even crossing 10,000, we were closer to 400 mph.

After converting to knots, this is still much faster than the 250 KIAS maximum. What did the pilot or controllers need to do to make this allowable?

Thank you.

Nicholas Turner

Air Asia A340 Flying Into London Stansted

Hi Nicholas,

The general rule in most airspace - pilots must reduce the speed to below 250 knots (287 mph) below 10,000 feet. This is to facilitate the work of the air traffic controllers, that is, to enable them to space out the incoming traffic evenly.

As far as pilots are concerned, we are happy to abide by this requirement as bird strike risks are generally higher at lower altitude. Lower speed means lesser risk of impact by birds on the windscreen or engines.

Having said that, this requirement can be changed subject to aircraft performance or at the air traffic controllers request.

However, if the pilot is not able to comply with this requirement, he must advise the controllers. For instance, when I fly the Airbus A340 out of London on a long haul flight, I take off at a very heavy weight and my safety (green dot) speed is around 275 knots during the climb below 10,000 feet. I either inform the controller of my limitation or just add this word “HEAVY” after my Call Sign, it would be understood that I would be climbing at a speed higher than normal

In your case where you were descending below 10,000 feet and the speed of your plane was higher than normal – I believe the controller was requesting the pilot to maintain a higher descent speed to facilitate his spacing with another aircraft too close behind him. So this higher speed is acceptable in such a scenario.

The Airbus A340 Construction


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Its indicated not ground speed
Or the simple fact that you have a tail wind that would increase your ground speed by that much. The rule pertains to indicated airspeed not ground speed.
Bob , 08 May, 2012
Ummm, the 250 knots is INDICATED airspeed, not GROUND airspeed (airspeed of the aircraft as measured relative to the ground). Federal Aviation Regulation 91.117 states that no aircraft may operate BELOW 10,000 feet at an indicated airspeed greater than 250kts (91.117d does allow an aircraft to operated faster than 250kts if the minimum safe airspeed for that aircraft is higher than 250kts). Pilots or published procedures determine whether to slow to 250kts PRIOR TO reaching 10,000 feet, OR temporarily leveling off at 10,000 feet and THEN reducing to 250kts. Along with a tailwind component, this will often be a reason to see an aircraft's ground speed to be 300+ knots. Depending on how quickly an aircraft slows to 250kts, it might take a few moments for the ground speed to decrease as it measures distance traveled vs. time.
At 10,000 feet, 250 knots indicated airspeed, with no tailwind, standard atmospheric temperature and pressure, will often equate to almost 300kts ground speed. At 250kts, again, with standard temperature and pressure (for the altitude) AND with ZERO wind (which is near impossible), an aircraft's ground speed would measure at around 275kts.
At least in the US, it would be extremely rare for an air traffic controller to request a pilot to fly faster than 250 kts below 10,000 feet. Controllers will take into account that the aircraft has to be reduced to 250kts and will normally slow down aircraft BEHIND to assist with any spacing issues (or issue turns, "vectors", if speed reduction on a following aircraft is not effective.
From what Nicholas described I would surmise that the aircraft was performing normally, in compliance with FAR 91.117, and possibly had a strong tailwind.
gewhiteva , 07 Jul, 2015

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