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" ?

JohnBurke

I've never heard of a FL300 altitude/bank angle relationship/restriction, but generally after takeoff in air transport operations, turns aren't initiated below 400'. Bank angle and speed determine turn radius. Bank angle is limited by stall speed restrictions or margins. Generally the maximum bank an autoflight or flight director system will allow or command is 25 or 30 degrees, depending on the aircraft. There are times when heavy, high, or slow, that bank angle may be restricted; during takeoff is such a time. There are minimum maneuvering speeds that allow up to a 30 degree bank, for a given aircraft configuration (flap setting); typically at that speed or below, a lower bank angle is permissible, and with a margin above (say, 10 knots), then the full bank is permitted; this varies with the airplane. Some operators and some aircraft also suggest or require limiting bank with an engine failure after takeoff (a bank angle limit might be applied of fifteen degrees for an early turn in a turn procedure for an engine-out situation on takeoff, for example). Again, it varies with the operator and the aircraft.

"Immediate turn" as an ATC direction doesn't imply to me anything more than start a turn now. If I'm going fast, then it's going to take a larger radius or area to cover; if I'm going slower then it will be a smaller turn radius. If I'm particlarly slow, or if I'm close to my maneuvering margins (at altitude, for example), then the bank angle may necessarily be less.

Most operations are using automation once in RVSM, and autoflight systems will set bank angle based on angle of attack (stall margins), and also based on the amount to be turned. A turn of only a few degrees will have a smaller bank angle than a larger turn. A fly-by waypoint using turn anticipation will have a different bank angle than a fly-over waypoint, too. If I'm flying through the aircraft flight control system (autopilot), I'll typically let the autopilot choose the bank angle, so if I'm directed an immediate turn to a given heading, I'll go to heading select, put in the new course, and let the airplane turn to that course. Immediate doesn't tell me to use more bank; just to start the turn now. The only way I can increase that is to disconnect the autopilot and manually fly it. That would be a TCAS resolution advisory, or something of that nature, but otherwise, I'm simply selecting the heading as directed and the airplane will set the bank angle as a function of the autopilot. The exception that might be an engine-out situation in which I may have used the bank limiter to restrict the autopilot to a maximum bank value (such as fifteen degrees). Otherwise, unless I physically take over the airplane, I can't really make the autopilot bank any more.

Excargodog

Actually, 10 degrees would give a radius of about 18NM and a 360 degree turn of 14.6 minutes at 450 KTs if I’m still doing the math right. A 25 degree bank improves that to about a 7 mile radius and a 360 degree turn of 5 and a half minutes by my calculation. But it’s been a long time since I was doing those calculations, so those might not be correct.

But if the PF is ex military, a “break right” or “break left” call may speed those turns up considerably…

RI830

Pending the aircraft. Approx 30,000 ft is where the AP while in HDG mode will reduce the bank angle.

If a traffic or terrain conflict arose…you wouldn’t be on the AP anyhow.

rickair7777

I don't think it's company policy, it's more airplane design.

In the flight levels, the AP is always on, unless there's some kind of emergency. As RI830 said, around 300-310 the AP will switch to half-bank.

Turn radius will always be a function of altitude and airspeed. It is what it is for specified conditions.

I would not be inclined to disconnect the AP and increase bank angle in the FL's for an ATC ask.

If there's a separation crisis, an RA will give you an appropriately aggressive climb or descent. IMO that's safer than aggressive bank. If TCAS gives you a climb it should be achievable with the current aircraft state (and maybe TOGA).

I'd only bank hard if I visually acquired a collision threat and it was a last resort. TCAS RA generally works much better for that sort of thing, faster response (especially for the descending aircraft).

ATC Route

Thank you very much for all these information !
May I ask if you can recall some angles at which the autopilot banked the aircraft in a few turns of more than 20 degrees , above fl 300 ?

rickair7777

I doubt it would ever do that on a transport category airplane, although you can turn off the 1/2 bank mode on some airplanes which might force it to bank more than 20 degrees. That would often put you dangerously close to stall margins though, depending on weight and altitude. Available thrust matters too, if the plane can't hold altitude with a large bank angle it will slow down, bring it even closer to stall.

Increased load factor raises the stall speed. Insufficient excess thrust will cause a loss of airspeed. If they meet in the middle you have a big problem.

bigstick

Best bet is to just turn off the autopilot and quickly bank 30 degrees over.

captjns

Boeing 737-NG above FL300 Collins MCP 10 degrees of bank would be selected. The Honeywell MCP 15 degrees of bank would be selected. The FMS would determine a solution calculating a smart turn as to not exceed the selected reduced bank angles.

Adlerdriver

If this is about an emergency solution to avoid a mid-air, isn't this whole premise a bit of a red herring? TCAS is a vertical only system. It attempts to provide pilots a solution to a impending loss of separation by commanding a climb or descent or varying the current vertical movement if already doing so. It doesn't use a turn because the most effective method of creating separation is vertically, not horizontally.
Wouldn't ATC be taught to utilize the same methods in an emergency situation rather than worrying about what bank angle the pilot might use? Maybe a controller can jump in with some perspective.