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For busy airport, sequencing can be complexe. If the ATC put all aircraft in front of each other with the correct separation (which can be really small for busy airports) in final, if the airliners' approach speed vary significantly, one plane could catch up the preceding one. Can the ATC ask for a pilot to increase its approach speed up to a minimum speed?

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    $\begingroup$ Separation is not ever "really small" (unless the controller has made a serious error). I don't think it's really common for airliners (since they have pretty well known approach speeds), but if you're flying a GA plane & mixing with commercial traffic, it's pretty normal for the tower to request you to maintain a higher than normal approach speed. $\endgroup$ – jamesqf Oct 14 at 3:34
  • $\begingroup$ @jamesqf "really small" may be too subjective. It is not uncommon to have landings separated by less than 2 minutes. Too me this is kind of no much (even if this is enough) $\endgroup$ – Manu H Oct 14 at 8:56
  • $\begingroup$ "Rush hour" xD... $\endgroup$ – Cloud Oct 14 at 9:59
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    $\begingroup$ @Cloud: google for "plane movements worldwide animation" and check out the flood of aircraft arriving from North America eg. at London in the morning. Like this one: theguardian.com/world/video/2014/jul/03/…. That's the rush hour for arirports :-) $\endgroup$ – oliver Oct 14 at 10:06
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    $\begingroup$ @Cloud take a look at the NATS (the ATC for the U.K.) blog to see the ebb and flow of air traffic - nats.aero/blog/2014/11/take-guided-tour-uk-skies $\endgroup$ – Tim Oct 14 at 22:54
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ATC can not only ask for a minimum speed, but rather instruct each aircraft to (more or less) exactly fly at a particular speed. This is essential for maintaining separation. As J. Hougaard pointed out in the comments, the speed on the last 4 NM is always up to the pilot to slow down to final approach speed.

It is the duty of the pilot to evaluate if it is safe to comply with this restriction. In case the aircraft cannot fly as slow as asked for, the pilot can simply reply UNABLE and ATC will either try to make it work or instruct the aircraft to go around.

Some airports also publish speed restrictions on the approach charts. This is an example of a minimum speed from the ILS approach into 01L/R at Stockholm-Arlanda (ESSA):

Stockholm

More precise speed restrictions are for example given on the STAR charts of the (now closed) Istanbul-Atatürk airport (LTBA):

Istanbul

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    $\begingroup$ To augment this (good answer), ATC has specific guidelines for what speeds they can assign. In the U.S., FAA order 7110.65 section 5-7-3 spells it all out - for aircraft on approach, basically 170 knots or greater for jets and 150 knots or greater for turboprops and pistons (if able). Other civil aviation authorities may have similar guidelines. (It appears ICAO does not.) $\endgroup$ – TypeIA Oct 14 at 12:32
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    $\begingroup$ Might be worth adding that ATC generally speaking has no control over aircraft speed within 4NM final. At that point, speed is entirely up to the pilots, since they need to establish a stable approach. $\endgroup$ – J. Hougaard Oct 14 at 19:09
  • $\begingroup$ @J.Hougaard Thanks, added. $\endgroup$ – Bianfable Oct 15 at 6:43
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From my experience in busy airspace (Chicago, New York, San Francisco), ATC will ask you to maintain a specific speed until the final approach fix. Usually this speed is faster than what you'd ideally be doing at that position. After the FAF, you can slow to the appropriate final approach speed. It's true that approach speeds vary depending on weight and weather conditions - ATC knows this and expects it, but the spacing is adequate for aircraft with variable approach speeds the last 4 miles of the landing.

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