Turn radius / Banking angle when Immediate Turn is Required

[ATC Route]

As an ATC En-Route controller , I respectfully dare to ask IFR pilots , and especially airliners , about the banking angle and turn radius that they take in the rare and undesirable eventuality that they are asked to "Turn right/left n degrees IMMEDIATELY !" . It is for a study about most efficiently choosing the turn side , depending on the relative position of two aircraft that might loose separation .I know that certain companies have a policy of 25 deg bellow fl 300 , and only 10 deg above it . 10 degrees would lead to a radius of about 13 NM, and an 11 min orbit at 450 kts .​​​​​​​ Is this true , or you're allowed to select 25 degrees banking above fl 300 when hearing this special word "Immediately" ?

[john_tullamarine]

I would hope that, with such an instruction from ATC, especially with a touch of urgency in the ATC-er's voice, old mate civil pilot would crank in sufficient aileron to achieve 30/45 degrees of bank without any hint of hesitation other than what might be necessary to keep the cabin under control. One presumes, without knowing the details at the time, that the ATC-er has an appropriate concern which he/she desires to satisfy.

The other consideration relates to higher altitude bank and load factor problems but that should be well known to the pilot for the particular Type involved. Either way, one would expect a prompt and appropriate response.

Turn rate is not something I would expect the pilot to be considering in routine civil operations.

[EatMyShorts!]

....well as a pilot, when at cruise level, you select HDG-mode (heading) and set the new heading. The flight director and autopilot will do their thing and the resulting bank-angle is whatever is programmed into the aircraft. Above a certain flight level only "half-bank" turns are permitted, e.g. 12.5 degress instead of 25 degrees. It's all about not coming too close to the critical angle of attack. At high levels the difference between a low speed stall (=exceeding the critical angle of attack) and highspeed buffet (=too fast) can be a few knots of IAS only (this is called "coffin corner"), depending on the type of aircraft.
In essence: you cannot just yank any plane around at FL400. Business jets are usually okay with it, airliners rather not.

https://de.wikipedia.org/wiki/Coffin_Corner

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/687x600/687px_coffin_corner_airplane_cockpit_view_b5a8b7a9a3410a6a76 f259e28c43250b595326a3.jpg

[ATC Route]

Thank you very much "Shorts" ! I understand that the turn radius/ banking angle may vary depending on how much the flight envelope has room for increasing either the speed or the angle of attack , for the extra-lift needed in turn . And that if such a resource would be available ( aircraft lower than the efficiency level ) , the banking is anyhow limited by the bank-angle selector . So there isn't any turn radius I could count on , for the geometrical analysis.

[nonsense]

https://de.wikipedia.org/wiki/Coffin_Corner

https://en.wikipedia.org/wiki/Coffin_corner

[eckhard]

I think that most Western operators will limit the Maximum Altitude selectable in the FMS to that which still provides a 1.3g buffet margin. This equates to a turn using the normal 25 degrees of bank, with a 15 degree overshoot to 40 degrees.

If that is the case, it should be possible to turn at 25 degrees of bank if one is at or below the FMS maximum cruise altitude. To achieve this might require moving the bank limiter out of ‘Auto’ and into the 25 degree position while using HDG (SEL) mode instead of (L)NAV

It’s also true that most auto flight systems limit the bank angle to around 15 degrees at normal cruising altitude; however, this is not normally due to worries about buffet but rather concerns about insufficient thrust to sustain the cruise speed at higher bank angles, with their associated higher drag.

Coffin Corner is really about having an excess of thrust, enabling the aircraft to reach an altitude where the only practical cruise speed is close to both high and low speed buffet. In other words, it’s a “wing limit”.

Other aircraft run out of thrust before the aircraft is able to climb to such an altitude, in which case ones maximum altitude is due to an “engine limit”.

[ATC Route]

Thank you very much for your answer !

[172510]

If you would say 'Immediately': I would set the heading on autopilot, heading mode.
If you would say 'max bank turn' or 'min radius turn' : I would hand-roll the aircraft into a 45° bank.

[EXDAC]

As an ATC En-Route controller , I respectfully dare to ask IFR pilots , and especially airliners , about the banking angle and turn radius that they take in the rare and undesirable eventuality that they are asked to "Turn right/left n degrees IMMEDIATELY !" .

"Immediately" means "do it without delay". It does not request handling of the aircraft in a way that is not normal for the current flight condition. (My opinion of course.)

[amc890]

If you would say 'max bank turn' or 'min radius turn' : I would hand-roll the aircraft into a 45° bank.
I would hope you didn’t try that at cruise altitude ’cos it probably wouldn’t end well.

[BraceBrace]

....well as a pilot, when at cruise level, you select HDG-mode (heading) and set the new heading. The flight director and autopilot will do their thing and the resulting bank-angle is whatever is programmed into the aircraft. Above a certain flight level only "half-bank" turns are permitted, e.g. 12.5 degress instead of 25 degrees. It's all about not coming too close to the critical angle of attack. At high levels the difference between a low speed stall (=exceeding the critical angle of attack) and highspeed buffet (=too fast) can be a few knots of IAS only (this is called "coffin corner"), depending on the type of aircraft.
In essence: you cannot just yank any plane around at FL400. Business jets are usually okay with it, airliners rather not.

https://de.wikipedia.org/wiki/Coffin_Corner

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/687x600/687px_coffin_corner_airplane_cockpit_view_b5a8b7a9a3410a6a76 f259e28c43250b595326a3.jpg

Even with that picture attached you can bank 25° without any danger aerodynamically. The upper top of the lower amber band is not about stall, it is about maneuvering capability: at the top of the lower amber band it is what it is: 25° + 15° is still guaranteed and available. If you bank you would see that the upper side would not move, however with increasing g the lower bottom (stick shacker activation) would move up towards the upper limit.

There is one issue, that is in these cases there is no guarantuee that the engines are capable to produce the required extra thrust due to increased drag once you start banking. In that case you would lose speed in the turn, and there it would be "smart" to limit the bank angle... or you ask to descend. In the middle of the amber band, Boeing says you better limit bank angle to half the available.

The 10-15° bank angle at higher levels is purely company specific and has only a passenger comfort purpose. Anything above 30° is a big no-no as you increase wingloading and drag, and you would probably trigger the FDM.

To get back on topic, if ATC says "immediatly" 25° should still be more than enough as in commercial aviation we don't pull g's as it is safety issue. TCAS is also based on gentle climb/descent. If ATC would expect us to bank "hard" we would have descend anyway when flying at our maneuvering limits.

[ATC Route]

I am very grateful for your detailed answers.

[ATC Route]

Would it be possible to remember how many degrees ( approximately ) did the autopilot bank the aircraft in a few turns of more than 20 degrees , at HIGH altitude ? Especially for commercial flights .