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Once a commercial aircraft is in the cruise phase, who determines the speed of the aircraft? Do pilots have the authority to fly at up to the maximum permissible speed of that plane, or does ATC decide on that depending on the approximate time it will take to reach the destination? Is it stated in the flight plan that the pilot cannot go beyond some specific speed even if the aircraft is capable of it?

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  • $\begingroup$ Non-pilots often seem to have the idea that controllers are some kind of gods that boss around aircraft. That is not how it works. The controllers are there to assist the pilots, not boss them around. Pilots may follow the recommendations of a controller, but the pilot ALWAYS has the final say and power to decide. $\endgroup$ – Tyler Durden Jan 21 '15 at 18:32
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    $\begingroup$ @TylerDurden That statement is correct, but incomplete: When operating under positive control (in the US that generally means class A & B airspace, on an IFR flight plan, or on movement areas of a controlled airfield) pilots are required to obey instructions given to them by ATC, absent an emergency or some other really good reason to justify a deviation from their clearance. While a pilot CAN do whatever they feel like with the aircraft by virtue of being at the controls if they simply ignore ATC clearances they don't like it can lead to some unpleasant conversations with the FAA. $\endgroup$ – voretaq7 Jan 21 '15 at 19:58
  • $\begingroup$ @voretaq7 Agreed, if the pilot actions trigger a review and it turns out he deviated in a way that compromised safety or showed poor judgement, gone goes the license. Keep in mind though that controllers screw up all the time or make blind instructions that are just wrong for some reason, so it is very common for pilots to reject instructions, and I have done so on many occasions. $\endgroup$ – Tyler Durden Jan 21 '15 at 20:03
  • $\begingroup$ @TylerDurden It's a bit off topic, but I am reminded of bumper stickers on some cars in airport employee parking lotS in the early 1980s before the controller strike. They said "CONTROLLERS TELL PILOTS WHERE TO GO." The stickers disappeared after the strike was broken. $\endgroup$ – Terry Jan 22 '15 at 4:07
  • $\begingroup$ Yes, pilots are in command: during a DeHavilland Comet flight, Elizabeth von Battenberg was one of the passengers, the pilot wanted exhibiting the airplane's performances, and increased power to top, to gain airspeed; experience was aborted when the flying machine started uncontrollable vibrations... $\endgroup$ – Urquiola Jan 27 '15 at 22:47
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ATC can assign speeds en-route for separation or in the TMA area for sequencing aircraft onto the approach. Speeds en-route in upper sectors are assigned using Mach numbers:

R: "DLH123, maintain speed Mach .78"

In the TMA area, aircraft are sequenced for an approach and reduced to similar speeds to maintain lateral separation. These restrictions can be paired with conditions, until when the speed needs to be maintained:

R: "BER456, reduce speed 210 knots"
R: "BER456, cleared ILS approach runway 24, 
    maintain 170 knots indicated until outer marker / 5 DME"

Certain approach or arrival charts list maximum speeds for certain turns, marked with MAX 210 KIAS, indicating that this turn should not be flown with more than 210 KIAS to make the radius.

Speed restrictions imposed by ATC need to be maintained until cancelled by the ATC unit:

R: "DLH123, no speed restrictions / resume normal speed"

The pilot however is responsible for ensuring a safe flight and can report that he is unable to follow the instructions due to performance limitations.

A: "DLH123, unable to comply, indicated airspeed will be 150 knots"

There is also legal considerations when it comes to aircraft speeds, such as general rules for traffic below certain altitudes or in some jurisdictions, only for certain airspace classes (related: What is the speed limit in European Airspace).

The FAA CFR 91.117 says:

§91.117 Aircraft speed.

(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).

The German LuftVO however rules that a speed limit of 250 KIAS below FL100 only applies to VFR and IFR aircraft in airspaces class D,E,F,G and only to VFR aircraft in airspace class C. IFR aircraft in airspace class C do not have a speed limitation.


Regarding the general authority over a flight, see the related question:
Who has the higher authority, the pilot in command or ATC?

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    $\begingroup$ Standard ATC phraseology for speed does not include "knots" nor "indicated". KIAS is implied. $\endgroup$ – Steve Kuo Jan 21 '15 at 7:17
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    $\begingroup$ @SteveKuo Corrected for the speed assignment, knots is used however according to my AIP. In the advice that one is unable to maintain a speed, the word "indicated" is used. $\endgroup$ – SentryRaven Jan 21 '15 at 7:18
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    $\begingroup$ When on Oceanic crossings with no radio coverage, at the entry point to the track the pilot is given a clearance which includes the speed. The aircraft is expected, unless safety overrides, to maintain that speed since it is the only way that the controller knows where the aircraft is in relation to other aircraft on the same track. They can also estimate the aircraft position. $\endgroup$ – Simon Jan 21 '15 at 9:15
  • $\begingroup$ @Simon Should I incorporate that into my answer? Do you have an example/source for phraseology? I am using a German AIP for the most parts, which obviously omits oceanic clearances... $\endgroup$ – SentryRaven Jan 21 '15 at 9:16
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    $\begingroup$ It's worth noting that the 250 kt limit below 10,000 ft in the U.S. does not apply if the minimum safe speed for the aircraft is above that, which is the case for a heavily loaded 747-100 or -200 and probably for the later models as well. Also, if you're icing heavily below 10,000 and expect to be there for awhile, increasing speed to above 250 kts to raise the temperature to stop the icing was normal. Also, if you're below 10,000 and get 12 miles off shore (statute as I remember), the 250 kt limit did not apply prior to the year 2000. I don't know if that's still true. $\endgroup$ – Terry Jan 21 '15 at 19:25
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Pilots will mostly set the speed they want. In cruise, the planes tend to be further apart, and speed is less critical. Different planes fly at different speeds, and ATC usually uses altitude and heading as the primary means to separate them.

However, ATC will sometimes ask for different speeds as another way to separate aircraft. If a fast plane that wants to climb is underneath a slower plane, ATC may have the slower plane slow down a bit, and the faster plane speed up a bit so the faster plane can get far enough ahead to climb.

As the plane nears its destination, speed becomes more likely to be a factor. If there are delays at the destination, ATC may tell the pilots to slow down to minimize time spent in a holding pattern. This may also serve to sequence planes in for arrival, where ATC may ask for faster or slower speeds. Speed restrictions are much more common in arrival, which SentryRaven's answer describes.

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ATC is providing collision separation services only. It is the responsability of the pilot to operate the airplane and to keep it under a safe flight envelope. So, speed:pilot, Heading-altitude:ATC :)

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    $\begingroup$ This is incorrect: there are many cases where ATC can and does assign a speed (it is extremely common in the terminal environment to ensure apropriate separation on approaches). $\endgroup$ – voretaq7 Jan 21 '15 at 17:01
  • $\begingroup$ speed is commonly assigned by the atc in order to fulfill their aforementioned role :) This is mainly needed on approach, true. Thank you @voretaq7 $\endgroup$ – kounelii Feb 3 '15 at 5:50

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