FMS Bank Angle Limits?

[Minuteman]

Speaking of flaws in pilot training for automation...

Does your aircraft use any kind of a bank angle limit when using the FMS for lateral guidance (e.g., LNAV) at low altitude?

I'm thinking of a scenario where the aircraft is attempting to fly a course reversal or an RNP arc where excessive speed or strong winds cause the bank angle needed to follow the track bump into a bank angle limit imposed by the FMS (and not a dial-selected limit). If there is a limit from the FMS, what changes it?

 

[dasleben]

I'm going through the 757/767 manuals right now, and I honestly can't find much. The best thing I can find is that it's limited to 30 degrees of bank in a hold while flying in LNAV, and that on aircraft with the most updated FMCs, LNAV limits the bank angle at low speed directly after takeoff (LNAV can be pre-armed before departure). I'm certain that type of protection continues in all phases of flight, but I can't find an exact bank angle limit for you.

I'm just finishing initial on the thing...you'd think I'd remember minor details for at least another month yet.

 

[NJA_Capt]

I'm thinking of a scenario where the aircraft is attempting to fly a course reversal or an RNP arc where excessive speed or strong winds cause the bank angle needed to follow the track bump into a bank angle limit imposed by the FMS (and not a dial-selected limit). If there is a limit from the FMS, what changes it?

FMSs have bank limits between 25-30 degrees at lower altitudes, then switch to 1/2 bank above a set FL. Our FMSs can be pilot selected to shallow bank angles at high altitude between 5-15 degrees. (Not a fan of less than 15 bank).
That said, the algorithms calculate the speed/bank angle and lead turns in order to complete the desired turn on track. Excessive speeds or rapid speed changes can affect these calculations.

 

[Orange Anchor]

And the newer FMSs (Honeywell) are blending the turns so you don't wind up with 'sticks' that can not be flown. The points are coded for fly-over and fly-by and that helps remain on course without excessive bank.

 

[seagull]

"Newer"? That's standard on the MD-11 and 777, both of which FMS's were designed more than 20 years ago!

And the newer FMSs (Honeywell) are blending the turns so you don't wind up with 'sticks' that can not be flown. The points are coded for fly-over and fly-by and that helps remain on course without excessive bank.

 

[Orange Anchor]

Not according to the new FMS that Honeywell is putting on the 787 and other machines. Not just the next turn point but ALL turnpoints are modified.

 

[seagull]

Well, they CAN'T code a charted "fly over" point that way, it would be illegal! Their marketing dept. sometimes implies things like this, but it doesn't make it true. I have never seen a "stick that cannot be flown" in the -11. It also will adjust bank angle to protect stall margin, with the bank limits really coming together up high. Of course, if you choose to try to fly a tight procedure turn at 250 kts, you will not be able to stay in the airspace constraints. That's physics, though, not the FMS.

Not according to the new FMS that Honeywell is putting on the 787 and other machines. Not just the next turn point but ALL turnpoints are modified.