Turn Pad Use (Widebodies)
01-09-2024, 10:54 PM
#1
New Hire
Thread Starter
Joined APC: Jan 2024
Posts: 1
Turn Pad Use (Widebodies)
Hello,
I'm an airport designer based in Europe. One of the designs we're working on involves new turnpads for Code Es. I note ICAO regulations for the shape of those turnpads are somewhat conflicting. They state to use a max. nose gear steering angle of 45deg but on next page show an example design where a A350X turns with >60 degrees, much closer to the maximum the aircraft is capable of. This has a big impact on infrastructure size so I want to get this right.
I wonder if I could get your opinion on how those turnpads are used in practice? (especially people experienced with widebodies)
In particular:
-what maximum turn rate do you normally use? I understand it's to minimize slipping but is the 45 degree nose gear steering rule realistic?
-is there any maximum turn rate imposed by airlines?
-What turn type is normal? do you tend to use differential thrust / braking?
-Is there any shape of turnpad you prefer? After the initial taper it can go directly into a turn or have a short section running parallel to the rwy before the turn.
-In smaller aircraft do you tend to use all available pavement or rather make a shaper turn to get around sooner?
Appreciate thoughts, thank you
I'm an airport designer based in Europe. One of the designs we're working on involves new turnpads for Code Es. I note ICAO regulations for the shape of those turnpads are somewhat conflicting. They state to use a max. nose gear steering angle of 45deg but on next page show an example design where a A350X turns with >60 degrees, much closer to the maximum the aircraft is capable of. This has a big impact on infrastructure size so I want to get this right.
I wonder if I could get your opinion on how those turnpads are used in practice? (especially people experienced with widebodies)
In particular:
-what maximum turn rate do you normally use? I understand it's to minimize slipping but is the 45 degree nose gear steering rule realistic?
-is there any maximum turn rate imposed by airlines?
-What turn type is normal? do you tend to use differential thrust / braking?
-Is there any shape of turnpad you prefer? After the initial taper it can go directly into a turn or have a short section running parallel to the rwy before the turn.
-In smaller aircraft do you tend to use all available pavement or rather make a shaper turn to get around sooner?
Appreciate thoughts, thank you
01-10-2024, 06:47 AM
#2
Line Holder
Joined APC: Dec 2021
Posts: 62
I used to be an Airport Compatibility Engineer for Boeing and currently fly the 757 and 767.
If you want to be ICAO compliant the regulations are pretty clear. Both Annex 14 section 3.3 and the Aerodrome Design Manual - Part 3 (Runways) Appendix 4 specify that a maximum 45 degree of nose wheel steering angle should be used for design, the initial intersection angle should not exceed 30 degrees, and the pavement width should provide the recommended 4.5m clearance between the edge of the landing gear and the edge of the pavement. The 45 degree maximum steering angle is used because for the design criteria the turn should be able to be accomplished while maintaining the cockpit over the centerline marking without the use of any additional techniques.
In the real world I will probably use some differential thrust, a bit more steering angle, and oversteer the marking a little bit in the last portion of the turn to align myself earlier and use all available runway, but those are all techniques and cannot be used as a justification in the design and certification process. In the case of a nonstandard turn pad I may have to use all the techniques you mentioned, including differential braking, although I personally try to avoid differential braking in a tight turn because it puts a lot of stress on the inboard main gear bogey. But if we're trying to design to standard than these techniqes should not be accounted for and assume the turn will be completed with a simple cockpit over centerline maneuver, symmetrical thrust, and no differential braking.
The ideal design in my opinion would be the one in Figure A4-7 in the ADM Part 3, Appendix 4, with a little bit of a straight section after initially opening up to the left before the final 180 degree turn. This is ideal for passenger comfort and to reduce wear on tires and landing gear structure.
If you want to be ICAO compliant the regulations are pretty clear. Both Annex 14 section 3.3 and the Aerodrome Design Manual - Part 3 (Runways) Appendix 4 specify that a maximum 45 degree of nose wheel steering angle should be used for design, the initial intersection angle should not exceed 30 degrees, and the pavement width should provide the recommended 4.5m clearance between the edge of the landing gear and the edge of the pavement. The 45 degree maximum steering angle is used because for the design criteria the turn should be able to be accomplished while maintaining the cockpit over the centerline marking without the use of any additional techniques.
In the real world I will probably use some differential thrust, a bit more steering angle, and oversteer the marking a little bit in the last portion of the turn to align myself earlier and use all available runway, but those are all techniques and cannot be used as a justification in the design and certification process. In the case of a nonstandard turn pad I may have to use all the techniques you mentioned, including differential braking, although I personally try to avoid differential braking in a tight turn because it puts a lot of stress on the inboard main gear bogey. But if we're trying to design to standard than these techniqes should not be accounted for and assume the turn will be completed with a simple cockpit over centerline maneuver, symmetrical thrust, and no differential braking.
The ideal design in my opinion would be the one in Figure A4-7 in the ADM Part 3, Appendix 4, with a little bit of a straight section after initially opening up to the left before the final 180 degree turn. This is ideal for passenger comfort and to reduce wear on tires and landing gear structure.
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