5
$\begingroup$

One can reasonably expect that for a normal turning instruction (such as a radar vector), ATC will expect you to make a standard rate turn, or perhaps a half rate turn in the high altitude environment.

However, when ATC gives an "expedite turn" on a radar vector or equivalent instruction such as the "Traffic Alert, Turn Left Immediately" used for PRM approach breakouts, what level of performance are they expecting the pilot to demand out of their airplane? A 1.4g (45deg bank) steep turn? A 2g (60deg bank) steep turn? Maximum sustained Gs, even if that means using afterburner in a fast jet? Get on this heading ASAP even if it means trading a bit of altitude for energy? Does it depend on whether a light aircraft, a heavy jet, or a fast jet is being vectored around?

$\endgroup$
7
$\begingroup$

The air traffic controller expects you to 1) start your turn immediately and 2) turn as quickly as can safely be done. We do not have all the facts, and do not know exactly what sort of turn you are capable of doing. Using a phrase such as "expedite turn" or "turn immediately" basically means - I've screwed up, you're in danger, watch out and get out of the way. It is then up to you as pilot in command to avoid a collision, by flying your aircraft and knowing its limitations.

When ATC separation has been lost, you should respond promptly to any instructions issued with the purpose of re-establishing separation. We will also, as far as practical, issue traffic information, and expect you to look out the window and take any avoiding action you find appropriate to prevent a collision.

Finally, as a side note, if you have a TCAS, please remember that a TCAS RA takes precedence over any instruction issued by ATC.

$\endgroup$
  • 1
    $\begingroup$ Good that you mention the precedence of TCAS over ATC instructions. However TCAS RA,s are in the vertical plane, turns are horizontal. It seems they can be combined as long as the aircraft has sufficient performance to follow the RA in a turn. $\endgroup$ – DeltaLima Jan 14 '17 at 10:40
  • 1
    $\begingroup$ @DeltaLime You are quite right. TCAS II RA's provide only vertical resolutions, and so ATC instructions to turn should not conflict with TCAS. TCAS III will probably include horizontal resolution advisories as well, but that is well into the future. $\endgroup$ – J. Hougaard Jan 14 '17 at 10:48
3
$\begingroup$

IMMEDIATELY− Used by ATC or pilots when such action compliance is required to avoid an imminent situation. - FAA Controller pilot glossary1

The meaning of the word "immediately" is fairly imprecise. It does not specifically lay out within what parameters the pilot is expected to perform an instruction. It does imply a level of urgency, but the controller's interpretation may differ from the pilot's.

The relevant ICAO documents, ICAO Doc 4444 (Chapter 12) and ICAO Doc 9432 use the word “immediately” to imply urgency. ICAO recommends using the word "immediately" with the phrase "due traffic" when instant action is needed from the pilot.2

The term may also differ between different countries.

The UK Manual of Radiotelephony CAP 413. draws a distinction between “now” and “immediately” as follows:

“Use of the word 'now' indicates that the instruction should be complied with in accordance with normal aircraft operating procedures, but without delay. Use of the word 'immediately' indicates a further degree of urgency exists (e.g. to avoid flight into terrain or restricted airspace, or for the provision of collision avoidance, see Chapter 5 Paragraph 1.6.4 Avoiding Action Phraseology). In such circumstances, the pilot should take action to comply with the instruction as soon as practicable, subject to the safety of the aircraft.”3

Another word that can be used, which indicates a higher level of urgency is "expedite." It is usually used in terms of climb and descent, but after giving a turn instruction the controller can use the phrase "expedite turn" to indicate more urgency.

EXPEDITE− Used by ATC when prompt compliance is required to avoid the development of an imminent situation. Expedite climb/descent normally indicates to a pilot that the approximate best rate of climb/descent should be used without requiring an exceptional change in aircraft handling characteristics.1

A tragic example of the ambiguity of the word "immediate" is the mid-air collision between an F-16 and a Cessna 150 in 2015. When the controller noticed the conflict she gave the F-16 a traffic advisory and when the pilot did not appear to have a visual she gave the order "if you don't have that traffic in sight turn left heading 180 immediately."

Unfortunately the controller's expectation of the word "immediately" did not match the pilot's interpretation.

In postaccident interviews, the controller stated that when she issued the command to the F-16 pilot to turn left "immediately," she expected that the F-16 pilot would perform a high performance maneuver and that she believed that fighter airplanes could "turn on a dime."4

Instead the pilot began a standard-rate turn using the autopilot. This rate ended up being just enough to bring the F-16 directly into the flight path off the Cessna causing a mid-air collision that killed the two people aboard the Cessna. The NTSB report determined that "further clarification of her expectation, such as directing the pilot to "expedite the turn," would have removed any ambiguity."4


1 FAA Pilot Controller Glossary

2 Skybrary Urgency Instructions and Clearances

3 UK Manual of Radiotelephony CAP 413.

4 NTSB probable cause report:'14 CFR Armed Forces Accident occurred Tuesday, July 07, 2015 in Moncks Corner, SC

$\endgroup$
2
$\begingroup$

Air Traffic Controllers are aware that the higher and faster you are the bigger your turn and factor that into normal vectors.

However when a controller gives a Traffic Alert, Turn Left Immediately the controller expects that the aircraft will give the best turn they can. That is because the controller said Traffic Alert, the pilot knows it is in their best interest to get on that heading as fast as possible. The immediately is there to say don't start the turn in 30 seconds, start it now, and gives a bit more emphasis on the severity of the situation.

$\endgroup$
  • 2
    $\begingroup$ Part of the question though is "does ATC really want a F-16 to pull a 9-G turn when given an expedited vector or traffic alert?" $\endgroup$ – UnrecognizedFallingObject Jan 14 '17 at 0:53
  • 1
    $\begingroup$ @UnrecognizedFallingObject A 9-G turn is probably beyond what they're accustomed to seeing, but if you can do one, have at it. An airliner probably won't exceed 30 degrees bank unless the pilot visually sees the traffic & the need to go to more (very rare). The salient point though is, start it ASAP & turn as quickly as you can... too fast/too much turn isn't the concern. Too late and/or too slow is what needs to be avoided. $\endgroup$ – Ralph J Jan 14 '17 at 15:27
  • $\begingroup$ @Ralph that is exactly the point I am trying to make. $\endgroup$ – Bullfrog Jan 15 '17 at 0:27
  • $\begingroup$ @UnrecognizedFallingObject if they want to pull 9-G turn then at least the issue would be resolved. But I doubt they would because they would probably break a squadron regulation. $\endgroup$ – Bullfrog Jan 15 '17 at 0:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.