A Pegasus A320NEO operating flight PC939 from SAW to BSL landed at Basel with the nosegear turned 90 degrees. This is the same type of incident that happened on a Jetblue A320 at LAX in 2005, A Vueling A320 in 2011 and an Asiana A320 in 2019.



https://aviation-safety.net/wikibase/24 ... OeGD-HNb3g

I’d say Airbus needs to reassess their nose landing gear struts on their A320 fleets and sub fleets. This seems to be to coincidental. But that’s just my thoughts.

FLY

Thought they fixed this ages ago.

I assume the pilots knew beforehand? Is there an indicator?

toobz wrote:

I assume the pilots knew beforehand? Is there an indicator?

They will get a gear unlocked light, but they won't know the actual position of the gear.

Terrifying. Thanks!

Varsity1 wrote:

toobz wrote:

I assume the pilots knew beforehand? Is there an indicator?

They will get a gear unlocked light, but they won't know the actual position of the gear.

They will know. A32x nose gears are specifically designed to fail in a 90 degree off center if there is an issue with steering or loss of associated Hyd pressure to the NLG. This is to prevent the aircraft from skidding off the runway upon landing.

A damage significant enough to move the wheel from this position will have implications of likely greater importance than this in any case.

ikolkyo wrote:

Thought they fixed this ages ago.

It is a failure mode, not a specific failure type. Things like loss of Hyd pressure or damage to NLG components can cause this. Without having read up on the other incidents, it is not possible to know whether or not they are the same root cause, but a loss of pressure will turn the wheel sideways if it is not lined up with the nose or on the ground already.

If we are going by statistics, however, it would likely be a BSCU failure. Those do turn periodically in flight, and if a bad seal causes a jam, this will happen.

toobz wrote:

I assume the pilots knew beforehand? Is there an indicator?

There is not direct indication of this failure mode. However this failure will trigger both the WHEEL NW STRG FAULT and the L/G SHOCK ABSORBER FAULT, so the combination of these two failures might indicate this fault. The A320 FCOM has the following note in the L/G SHOCK ABSORBER FAULT procedure: "If WHEEL N.W. STEER FAULT is also displayed, then the nose wheels may be at maximum deflection. (Turned 90 ° from center.) During landing, delay nose wheel touchdown for as long as possible."

You engineering types, please forgive my ignorance. Why would one want to fill your mode to be with the wheels turn 90°? It seems to me that upon landing once those wheels hit the ground it would put a very high stress on the airframe itself. Second, why not a failure mode with the wheel straightahead and locked? To me it makes more sense. Thank you.

WesternDC6B wrote:

You engineering types, please forgive my ignorance. Why would one want to fill your mode to be with the wheels turn 90°? It seems to me that upon landing once those wheels hit the ground it would put a very high stress on the airframe itself. Second, why not a failure mode with the wheel straightahead and locked? To me it makes more sense. Thank you.

in a 90 degree position this is simply a rubber pin
it does not harm the direction the plane moves because there is no wheel turning
so it will stay pretty much @ the center line

in a 0 degree position the frontwheel turns
it will deflect the planes direction because the wheel never ever steers completely straigh ahead
a turning but fixed frontwheel would be more difficult to handle

This seems like an Airbus trademark at this point

The pilots did a good job keeping it on center line. It's also interesting to see that the wheel isn't completely destroyed, only the tire. Probably had a much lower landing weight than the JetBlue incident. That's not to say that the wheel doesn't need to be replaced, though.

WesternDC6B wrote:

You engineering types, please forgive my ignorance. Why would one want to fill your mode to be with the wheels turn 90°? It seems to me that upon landing once those wheels hit the ground it would put a very high stress on the airframe itself. Second, why not a failure mode with the wheel straightahead and locked? To me it makes more sense. Thank you.

The steering on the A320 is hydraulic, in order for it to be straight there must be hydraulic pressure to the steering. However the failure modes can be hydraulic or mechanical so you are not always guaranteed to have steering.

The 90 degree position gives certainty, the aircraft will not turn. There will be no runway excursion. The outcome is safe and predictable.

Does this happen on any plane other than the A320?

zeke wrote:

The steering on the A320 is hydraulic, in order for it to be straight there must be hydraulic pressure to the steering. However the failure modes can be hydraulic or mechanical so you are not always guaranteed to have steering.

The 90 degree position gives certainty, the aircraft will not turn. There will be no runway excursion. The outcome is safe and predictable.

I’m still wondering: How much stress does this put on the airframe itself? It seems to me the wheel sideways is going to create an awful lot of upward and backward stress on parts. Or, are airframes tougher than I thought they were? To everyone who’s answering my questions, thank you very much.

WesternDC6B wrote:

zeke wrote:

The steering on the A320 is hydraulic, in order for it to be straight there must be hydraulic pressure to the steering. However the failure modes can be hydraulic or mechanical so you are not always guaranteed to have steering.

The 90 degree position gives certainty, the aircraft will not turn. There will be no runway excursion. The outcome is safe and predictable.

I’m still wondering: How much stress does this put on the airframe itself? It seems to me the wheel sideways is going to create an awful lot of upward and backward stress on parts. Or, are airframes tougher than I thought they were? To everyone who’s answering my questions, thank you very much.

It is going to stress the strut in a way that isn’t conventional. I don’t know what side loads the strut is designed for. If this is an anticipated failure, it wouldn’t be that big of a deal. If it exceeds the design limit loads, then cracks can start to form that will ultimately affect the fatigue life of the gear. In general the gear structure is designed for fatigue due to the landing stresses so it is quite resilient. The gear is probably the strongest structure on the airplane, although it is the main gear that take the heavier loads.

WesternDC6B wrote:

I’m still wondering: How much stress does this put on the airframe itself? It seems to me the wheel sideways is going to create an awful lot of upward and backward stress on parts. Or, are airframes tougher than I thought they were? To everyone who’s answering my questions, thank you very much.

I would suggest the drag from the wheel in such situations is less than the load a pushback tug would place on it at maximum weight.

Need to remember that the nose gear will have less than its normal load in it after initial touchdown as lift is still being generated.

asdf wrote:

WesternDC6B wrote:

You engineering types, please forgive my ignorance. Why would one want to fill your mode to be with the wheels turn 90°? It seems to me that upon landing once those wheels hit the ground it would put a very high stress on the airframe itself. Second, why not a failure mode with the wheel straightahead and locked? To me it makes more sense. Thank you.

in a 90 degree position this is simply a rubber pin
it does not harm the direction the plane moves because there is no wheel turning
so it will stay pretty much @ the center line

in a 0 degree position the frontwheel turns
it will deflect the planes direction because the wheel never ever steers completely straigh ahead
a turning but fixed frontwheel would be more difficult to handle

A turning nosewheel is nothing a rudder or assymetric braking can't counteract.
It makes more sense to fail straight and correct with brake and rudder inputs as required.
As to why those fail, probably a failed actuator or low hydraulic pressure due to a leak. I've seen some of those seals leak prematurely and apparently it's a part that many MRO's don't keep in stores as we once had an A320 go AOG over this. I laughed at a maintenance manager who was seriously trying to convince an engineer to rob a similar looking seal off a BAe146. That's how you end up with this kind of ... problem.
I wonder if the nose gear needs to be replaced after this, there could be enough force to bend the whole strut.

DarkSnowyNight wrote:

A32x nose gears are specifically designed to fail in a 90 degree off center if there is *snip* loss of associated Hyd pressure to the NLG. This is to prevent the aircraft from skidding off the runway upon landing.

No. Just no. If you lose the green (or yellow on later aircraft) system, the gear does not turn 90 degrees. It stays straight and you use the rudder and differential braking to steer then a tug comes and gets the aircraft.

toobz wrote:

Terrifying. Thanks!

What's so terrifying about it if proper procedure is followed?

This has nothing to do with any sort of Hydraulic leak that many on here seem to think.

One theory is the A320 series has a notoriously weak nose gear when it comes to pushback issues, and could have been caused by a prior pushback issue. It might have been damaged due to a pushback being commenced with the brakes on, or the nose wheel went out of the pushback turn radius limits. Either way, it could have been caused by damage to the nose wheel prior to this flight. I know at my company pushback malfunctions are a hot topic item and we had a 319s nose wheel go sideways on a pushback due to pushback malfunction on a flight a few days prior.

Kudos to the crew who kept it on centerline and kept the damage to a minimum.

Wacker1000 wrote:

DarkSnowyNight wrote:

A32x nose gears are specifically designed to fail in a 90 degree off center if there is *snip* loss of associated Hyd pressure to the NLG. This is to prevent the aircraft from skidding off the runway upon landing.

No. Just no. If you lose the green (or yellow on later aircraft) system, the gear does not turn 90 degrees. It stays straight and you use the rudder and differential braking to steer then a tug comes and gets the aircraft.

False, as is clearly indicated by these incidents, for a start. As well, and as previously mentioned, the BSCU does turn the NLG upon extension to ensure operability. This turn is initiated by pressurizing Hyd Pwr and unlocking a cam. This is needed to pressurize, but primarily only to turn it back to center. That is what is being verified. If that fails, there is not enough Hyd pwr to overcome aerodynamic loads and the wheel stays in the 90 degree offset position. As well, the centering cam rotates to a flat area. Again, this is purposeful as the NLG now acts the same as a rubber stopper in front of a roller skate would. Both what is taught in Level II & III systems classes from Airbus and experience say I am right. Unless you have a very specific reference that shows otherwise —I already know what the answer to that is—, you may assume you have just learned something.

See NTSB report NYC99IA062 excerpt below:

Additionally, during landing gear extension, the brake and
steering control unit (BSCU) would have been energized and hydraulic pressure would have been
directed toward the steering servo valve. The BSCU would have then commanded a small rotation of
the nose wheel to check for proper movement. Any disagreement between the commanded position and
actual position of the nose wheel would have deactivated the nose wheel steering. However, if
hydraulic pressure had bypassed the steering control valve, there would have been continued
pressurization to the servo valve, and because of the servo valve's inherent offset, in-flight
rotation of the nose wheels.

Procedures existed for removal of hydraulic pressure from the steering control module. However,
once the nosewheel strut had deflected 90 degrees, the centering cam would have been rotated to a
flat area, and would have been incapable of overriding the 3,000 PSI hydraulic system, and
returning the nose wheels to a centered position.



Emphasis mine.

cbphoto wrote:

This has nothing to do with any sort of Hydraulic leak that many on here seem to think.

One theory is the A320 series has a notoriously weak nose gear when it comes to pushback issues, and could have been caused by a prior pushback issue. It might have been damaged due to a pushback being commenced with the brakes on, or the nose wheel went out of the pushback turn radius limits. Either way, it could have been caused by damage to the nose wheel prior to this flight. I know at my company pushback malfunctions are a hot topic item and we had a 319s nose wheel go sideways on a pushback due to pushback malfunction on a flight a few days prior.

Not the case in the past and likely not here. The BSCU is somewhat susceptible to this under certain circumstances; namely ageing or installation of incorrect seals. See more from the same report below:


Examination of the steering control modules
on two of the airplanes revealed extrusion of the selector valve's external seals similar to that
found on N628AW. Airbus had attributed the extrusion failures to the lack of a backup seal or the
effects of aging on the seals. As a result of these incidents, Airbus issued Service Bulletin (SB)
A320-32-1197 on October 8, 1998, to recommend replacement of the external seals on the steering
control module's selector valve on A320 and A321 airplanes within 18 months of the SB's issuance.



This is not to say damage cannot happen during a tow or push; it does. But incidents like these are the general result of the above more than anything else.

Link to full report here, if you are interested

zeke wrote:

The steering on the A320 is hydraulic, in order for it to be straight there must be hydraulic pressure to the steering. However the failure modes can be hydraulic or mechanical so you are not always guaranteed to have steering.

The 90 degree position gives certainty, the aircraft will not turn. There will be no runway excursion. The outcome is safe and predictable.

CaptainHaresh wrote:

I've seen some of those seals leak prematurely and apparently it's a part that many MRO's don't keep in stores as we once had an A320 go AOG over this. I laughed at a maintenance manager who was seriously trying to convince an engineer to rob a similar looking seal off a BAe146. That's how you end up with this kind of ... problem.
I wonder if the nose gear needs to be replaced after this, there could be enough force to bend the whole strut.

MROs can be problematic with things like this, which is part of why I am always in favor of increased liability standards there vs Line MX. Unfortunately, your story does not surprise me.

DarkSnowyNight wrote:

False, as is clearly indicated by these incidents, for a start. As well, and as previously mentioned, the BSCU does turn the NLG upon extension to ensure operability. This turn is initiated by pressurizing Hyd Pwr and unlocking a cam. This is needed to pressurize, but primarily only to turn it back to center.

I don’t know the maintenance manuals but this can’t be exactly true. I agree that there is are failure modes that will result in the nose wheel being 90 degrees from the center position, but most hydraulic or nose wheel steering failures won’t result in these failure modes. If you loose the green hydraulic system and perform a gravity gear extension (or perform a gravity gear extension for other failures) there is no hydraulic pressure present in the landing gear. Still in these failure modes, the nose wheel will be centred and directional control will be ensured by the rudder and if needed differential braking. However nose wheel steering will not be available after a gear gravity extension.

Flow2706 wrote:

DarkSnowyNight wrote:

False, as is clearly indicated by these incidents, for a start. As well, and as previously mentioned, the BSCU does turn the NLG upon extension to ensure operability. This turn is initiated by pressurizing Hyd Pwr and unlocking a cam. This is needed to pressurize, but primarily only to turn it back to center.

I don’t know the maintenance manuals but this can’t be exactly true. I agree that there is are failure modes that will result in the nose wheel being 90 degrees from the center position, but most hydraulic or nose wheel steering failures won’t result in these failure modes. If you loose the green hydraulic system and perform a gravity gear extension (or perform a gravity gear extension for other failures) there is no hydraulic pressure present in the landing gear. Still in these failure modes, the nose wheel will be centred and directional control will be ensured by the rudder and if needed differential braking. However nose wheel steering will not be available after a gear gravity extension.

That is because in that situation the system was never pressurized in the first place. This is actually far less likely to occur than a seal leak or BSCU failure, as losing the entirety of the Green System would indicate a PTU or Yellow System failure as well. And even then, there is no 'will be' when it comes to centered. If it is off center, even very slightly, aerodynamic loading will push to 90 degrees. This is owing largely to the shape of the cam plate and where the flat spots are, but full deflection is considerably more likely to happen even then.

It is true that you will be unable to turn the NLG after a stop, without regard to its position.

This incident is not a common occurrence, but it does happen enough to be well known, regarding the causes and failure modes.

Something happened at my local airport... and luckily, nothing really happend...

BSL is also home to a few maintenance companies, the most important one being Jet Aviation. They convert peasant jets into government and rich people jets.

DarkSnowyNight wrote:

Wacker1000 wrote:

DarkSnowyNight wrote:

A32x nose gears are specifically designed to fail in a 90 degree off center if there is *snip* loss of associated Hyd pressure to the NLG. This is to prevent the aircraft from skidding off the runway upon landing.

No. Just no. If you lose the green (or yellow on later aircraft) system, the gear does not turn 90 degrees. It stays straight and you use the rudder and differential braking to steer then a tug comes and gets the aircraft.

False, as is clearly indicated by these incidents, for a start. As well, and as previously mentioned, the BSCU does turn the NLG upon extension to ensure operability. This turn is initiated by pressurizing Hyd Pwr and unlocking a cam. This is needed to pressurize, but primarily only to turn it back to center. That is what is being verified. If that fails, there is not enough Hyd pwr to overcome aerodynamic loads and the wheel stays in the 90 degree offset position. As well, the centering cam rotates to a flat area. Again, this is purposeful as the NLG now acts the same as a rubber stopper in front of a roller skate would. Both what is taught in Level II & III systems classes from Airbus and experience say I am right. Unless you have a very specific reference that shows otherwise —I already know what the answer to that is—, you may assume you have just learned something.

See NTSB report NYC99IA062 excerpt below:

Additionally, during landing gear extension, the brake and
steering control unit (BSCU) would have been energized and hydraulic pressure would have been
directed toward the steering servo valve. The BSCU would have then commanded a small rotation of
the nose wheel to check for proper movement. Any disagreement between the commanded position and
actual position of the nose wheel would have deactivated the nose wheel steering. However, if
hydraulic pressure had bypassed the steering control valve, there would have been continued
pressurization to the servo valve, and because of the servo valve's inherent offset, in-flight
rotation of the nose wheels.

Procedures existed for removal of hydraulic pressure from the steering control module. However,
once the nosewheel strut had deflected 90 degrees, the centering cam would have been rotated to a
flat area, and would have been incapable of overriding the 3,000 PSI hydraulic system, and
returning the nose wheels to a centered position.

What you quoted is completely different than "losing hydraulic pressure to the landing gear" as you originally stated. Loss of a green system on a non-enhanced aircraft will cause loss of manual extension and nose wheel steering. If there is no pressure to extend the gear, there is no pressure to release the cam. Enhanced use the yellow system for steering but again if there is no pressure to steer, there is no pressure to unlock the cam.

We would be seeing these incidents weekly if what you said is true.

Wacker1000 wrote:

Loss of a green system on a non-enhanced aircraft will cause loss of manual extension and nose wheel steering.

Again, not true. Loss of Green and Yellow, and/or PTU. This is also venturing into territory that is not statistically relevant, and clearly nowhere near what has happened here or in previous related incidents. You are speaking about something who's failure mode is so different that it cannot be properly related to what happened here. Or with JetBlue292. Or with AW2811, or with any of the others where this occurred. In fact, for that sort of total loss, I am not aware of any incidents that were not also collisions or bombings...

I am going to ask you to read what I actually said in the first place, as I am identifying the possibility that this misunderstanding may be a communication issue on your end. When I say A32x nose gears are specifically designed to fail in a 90 degree off center if there is loss of associated Hyd pressure to the NLG., that is not, and specifically not, a Green and Yellow system total loss. BSCU seals, for example, will cause a loss of associated Hyd Pwr to the NLG. The rest is what we see here.

And this is the correct failure mode for this and the other like situations. I am not sure what else you are looking for, but this is well covered here and elsewhere.

DarkSnowyNight wrote:

Wacker1000 wrote:

Loss of a green system on a non-enhanced aircraft will cause loss of manual extension and nose wheel steering.

Again, not true. Loss of Green and Yellow, and/or PTU. This is also venturing into territory that is not statistically relevant, and clearly nowhere near what has happened here or in previous related incidents. You are speaking about something who's failure mode is so different that it cannot be properly related to what happened here. Or with JetBlue292. Or with AW2811, or with any of the others where this occurred. In fact, for that sort of total loss, I am not aware of any incidents that were not also collisions or bombings...

Sorry I mistyped - meant normal extension. What I said is correct for normal extension regardless of what you incorrectly understood from a training class. Loss of a hydraulic system will not cause what happened here.