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Old 3rd Apr 2012, 22:06   #61 (permalink)
 
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SAS, show of force sometimes works, sometimes doesn't.

Depends.

I was in the Mideast in 2004, and Show of Force was a typical mission requirement, thanks to the RoE. A lot of times, the powers that be wanted weapons release, like a GBU 12 or GBU 31 or whatever was loaded, to be a last resort. I think that frustrated the ground guys more than a little bit, but at other times, it was the ground commander who was very strict on there not being bombs dropped since he had some political realities on the ground with locals that would be screwed if bombs started dropping in that local area ... yet again.

Nothing is simple in COIN, nor in post-conflict operations stuff.
It's all a bit of a mess.

Now and again, when our ground guys were engaged with folks in houses (in Iraq) and the go fasters (F-16 and F-15 mostly, and Tornados) made that low speed pass, without weapons release, they'd get some of the folks shooting at them to surrender, stop fighting.

And more often, it either got their heads down or had no effect.

As I said, it depends.

There was a story going around about some high speed passes in Afghanistan by B-1's at supersonic speeds that apparently scared some Taliban, or Muj of some sort, into moving, which got them hit by small arms fire from local Coalition forces.

That's another possible outcome of a "show of force" by air assets.

Show of force frustrated quite a few fighter jocks, since making that low speed "scare them" pass also put the jet into MANPAD envelope. One never really knew how many MANPADS were in anybody's hands ...

My further comments in re the RoE and some of the political crap the operating forces have to mitigate are censored to keep my blood pressure down.

ADDED LATER IN EDIT:

For John D et all VRS flying things ...

The 40/800 guideline, which we used as a rule of thumb when teaching steep approaches in the SH-60B, back when I was an instructor, was used to prompt when to wave off a steep approach so that YOU DON"T PROCEED TO GET NEAR OR INTO VRS.

That close to the ground, below 500 feet, usually somewhere near 300 feet AGL, if you actually enter into VRS and got the RoD and handling changes/problems John describes, your chances of waving off successfully aren't good.

I note that when checking this out, John took the bird up to 6-8000 feet.

There's a very good reason for that.
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Old 4th Apr 2012, 05:32   #62 (permalink)
 
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I was always taught that power made VRS worse. That's been from my first days in the TH-57 (B206) until now. If you can power out of it, then by definition, it wasn't VRS. The only exception, I would think, is if the additional power aggravated the VRS to such an extent that one continued to descend through the bad air and managed to get into better air below.

Even in helos, we were taught to freeze the collective and get forward airspeed.

In the V-22 one rolls nacelles forward, which immediately changes the thrust vector.

Now, the V-22's induced velocity is so high that one has to have a truly sick 2000fpm + on the descent to get VRS. If you have that on short final with less than 40KIAS, your approach is going to end in failure long before encountering VRS.
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Old 4th Apr 2012, 15:29   #63 (permalink)
 
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OD,

What is the Natops approved procedure for dealing with asymmetrical VRS in the MV-22? What criteria is used to define "A-VRS" as compared to "Incipient A-VRS"?
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Old 6th Apr 2012, 15:26   #64 (permalink)
 
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The Dreaded 40/800 Boundary

It used to be that Army flight school included, in the second half of the curriculum at Ft Rucker, a demonstration of " settling with power ", which, when one listened to the verbal description, was their understanding of VRS. The demonstrations were usually unsuccessful: only a few of the students in my class had the eye opening experience. I wasn't one, as my instructor, who was an ace in all other things, set the H-19 up in a vertical descent and I received one of those: " do you feel that....there it is.....we are on the edge of it...." recitations, while in the back of my mind I was thinking, " feel what ? ". Two months later I was out of flight school at the Test Board at Rucker in a "lead the fleet " UH-1D with 44' main blades, by myself, and after a few tries I finally got that vehicle into a real VRS event.

The point of the story, is that it is very, very hard to get a single rotored machine ( my only experience in this subject area ) into this aerodynamic situation.

Rotary Wing history seems to prove that to be the case.

  • Does the logging community have a history of VRS induced accidents?
  • Does the military heavy lift and artillery haulers have a history of VRS incidents?
  • Does the special ops community have a history of VRS accidents?
  • In all of the military H/V testing ( less the V-22 ), and FAA H/V and Cat A testing done all over the world by various manufacturers, has the VRS situation been encountered and reported?
The list could go on, and one would expect, based upon the protestations of the V-22 community that ALL helicopters are subject to VRS if within the dreaded 40/800 boundary, that there would have been accident fields of aluminum shards all over the world, accompanied by the type of FAA and military safety symposiums associated with the current EMS accident situation.



That hasn't been the case, as the entire current VRS discussion has been initiated by the V-22 community following the 2000 Marana accident. Well it should, as far as the V-22 is concerned.If the V-22 manufacturers and operators, based upon their testing, choose to impose a flight limitation, they are correct in doing so.



On the other hand, I have not seen nor read any evidence that justifies applying the V-22 flight limitation to other flying machines.


Two other related observations:



1. One aspect of the V-22 vs single rotored helicopters in VRS to ponder is why the V-22 apparently "rolls off" and cannot be controlled, whereas the single rotored vehicle remains controllable in VRS. I know the latter to be true, and assume the former based upon the accident details that were in the press.



2. Osprey Driver wrote"



"If you can power out of it, then by definition, it wasn't VRS. The only exception, I would think, is if the additional power aggravated the VRS to such an extent that one continued to descend through the bad air and managed to get into better air below."


Never having attempted to exit VRS in that manner, I can't comment from experience, but in the well-known Ray Prouty book, if one looks at figure 2.7 ( page 105 ), there is an analytical argument that says one can, with a slight "predicted" additional power( 10-12 % ) required to overcome what he refers to as a "settling with power" region.


Thanks,
John Dixson
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Old 7th Apr 2012, 00:21   #65 (permalink)
 
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John D. wrote:

Quote:
One aspect of the V-22 vs single rotored helicopters in VRS to ponder is why the V-22 apparently "rolls off" and cannot be controlled, whereas the single rotored vehicle remains controllable in VRS.
Apologies if this has been posted already, but here is Prouty's take:

Quote:
For a tilt-rotor aircraft in the helicopter mode, we could make the same scenario for descent in the vortex ring state if both rotors were acting the same. It is probable, however, that they will not act the same because of thrust fluctuations that are always associated with the vortex ring state. If, for some reason, a small roll to the right starts, the right rotor drops, its descent angle increases and it loses thrust in the process. The left rotor, on the other hand, rises and regains some of the thrust that it had lost. This is another unstable situation in which the roll rate will just keep increasing in a divergent manner unless the pilot stops it with his roll control. But since the situation is unstable, he might go into a divergent left roll. It can be represented by a one-degree-of-freedom system with negative damping — a very difficult situation to control!
Rotor & Wing Magazine :: Ask Ray Prouty
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Old 7th Apr 2012, 20:24   #66 (permalink)
 
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Prouty and 40/800

Good post, Matari. It brings up other writings by Ray Prouty:

In the R/W article ( could not get to the associated figures, unfortunately ), Mr. prouty concentrates on the V-22 aerodynamics, whereas in the 1986 ( 1990 revision ) text, his section on VRS has to do with single and tandem rotor machines.

In the R/W article, the main discussion centers on the 40 kt VRS data for the V-22, whereas in his 1990 text ( writing about a smaller tandem helicopter, not the V-22 ), he writes: " The results showed that for the test helicopter, forward speeds above 10 knots were sufficient to avoid vortex ring vibration at all rates of descent." (The cited reference was a 1958 paper and how they could attest to a 10 knot speed in those flying conditions is not stated ).

The other issue I had raised, and which the Prouty article skirts around is one of controllability. I mean controllability in the technical sense: does the pilot have the rotor control moment capability to offset a disturbance ( or "roll-off" to use the term favored by some in the V-22 community ). It is clear from personal experience that the single rotor machines I have flown have no issue there, but the Prouty article leaves the V-22 situation in doubt. Since a Chinook is in one sense a sideways V-22, rotor-wise, isn't an apt analogy a SAS-OFF Chinook hovering in gusty weather, nose into the wind? Is the hands off dynamic stability aperiodic/unstable? You bet. Does the pilot have the control power ( controllability ) to manage the disturbances? Certainly does. All I am suggesting is that the Prouty article does not go very far in explaining the details behind the Marana accident. Doubly so given the experience of the crew.

Oh, I did want to respond to one thing Lonewolf wrote:
"I note that when checking this out, John took the bird up to 6-8000 feet."

Two reasons. First is that it isn't easy to find a fully developed VRS situation, and the second is that once found, a respectable length of recorded data is appreciated by all except the flight crew being bounced around.

Thanks,
John Dixson
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Old 8th Apr 2012, 13:26   #67 (permalink)
 
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John,

In discussions about the Chinook...we are usually seeing the Aft Rotor System being affected by airflow coming from the Forward Rotor system....and I see the V-22 and A-VRS as being a different situation as the V-22 wants to roll off.

In the Chinook....if anything....the aircraft would want to pitch up (actually pitch down on the aft head.....resulting in a felt pitch up movement.).

During aggressive landings at high MAUW's....there was a thing we called "Falling Through" where the Aft Rotor system was less efficient and the aircraft would pitch up....requiring a nose down cyclic input....which now began to match the two heads up lift-wise....and the net result was a Chinook "falling". If there was adequate height....as the aircraft leveled....and if there was sufficient lift in the Rotor System....the landing could be salvaged. If not....the landing could be anywhere from merely embarrassing to really disastrous.

But that was not VRS or A-VRS as a lightly loaded aircraft could not be made to respond the same way.
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Old 8th Apr 2012, 16:05   #68 (permalink)
 
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Chinook Handling

SAS,

I only used the Chinook as a handling qualities analogy with regard to the Prouty comment.

With regard to your first paragraph, it reminded me of comments made by Jim Campbell, Boeing test pilot who was at Ft Rucker 1963-4 and who signed me off in that aircraft: he had done some of the CH-47 qualification testing and related that when flown into straight ahead blade stall, the aft rotor, operating at higher angles of attack than the forward rotor ( your observation ), would stall first, the aircraft would react nose up, and the situation was, to some extent, partially self correcting.
Sorry for the off-subject diversion.

Thanks,
John
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Old 8th Apr 2012, 20:09   #69 (permalink)
 
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Ray Prouty's Bottom Line...

Anybody who doesn't agree that it takes a higher rate of descent for a tiltrotor to get into VRS than a helicopter will have to argue with Ray Prouty (backed up by the empirical data gathered post-Marana accident by Tom Macdonald and team). Nobody will deny if that higher rate is exceeded in a tiltrotor than A-VRS can result in devastating consequences that a single rotor helicopter would not be vulnerable to.

Ray Prouty's comments after the accident and before it was known that the Marana V-22 entered VRS at 285ft agl at 2500+fpm ROD:
Quote:
"The vortex ring explanation never made any sense to me," adds noted aerodynamicist and R&W columnist Ray Prouty. "By my calculations, the Osprey should have been coming down a lot faster than it was to get into a classic vortex ring state." The doomed MV-22, he concludes, "must have been hit by a meteorite."

It’s all part and parcel of the many "unknown unknowns" that afflict helicopters—especially revolutionary new rotorcraft like the Osprey, Prouty says. Or, as military aviators bluntly put it: the flight manuals are written in blood.

"I would say the V-22 has had some really bad luck," Prouty says.
Prouty says in the previously referenced article:
Quote:
Test pilots have found that by tilting the engine nacelles forward, they can get out of this interesting situation, but the main result of the tests was to define a safe flight envelope within which they never would get into trouble.
Rotor & Wing Magazine :: Ask Ray Prouty

However, at 2500fpm ROD and 285ft agl I think few of us would disagree that it would be too late for either a tiltrotor or a helicopter to recover.


JD says:
Quote:
Oh, I did want to respond to one thing Lonewolf wrote:
"I note that when checking this out, John took the bird up to 6-8000 feet."
Two reasons. First is that it isn't easy to find a fully developed VRS situation, and the second is that once found, a respectable length of recorded data is appreciated by all except the flight crew being bounced around.
John,
Would you be willing to give that a try at 2500+fpm ROD at 285ft agl?

Just curious...

thanks,
21stC
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Old 9th Apr 2012, 02:33   #70 (permalink)
 
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Quote:
On the other hand, I have not seen nor read any evidence that justifies applying the V-22 flight limitation to other flying machines.
Actually it's the other way around. The 40/800 rule was in several NATOPS manuals well before the V-22 came along and it was automatically put into the V-22 NATOPS, before Marana. As recent flight test data shows, the V-22 is less susceptible to VRS than are most single rotor systems therefore those limits shouldn't be applied to it. Why are they? Because those limits are perfectly acceptable for both commercial and combat operations so it doesn't really matter. Fact is that if both types are flown within their limits theyre pretty darn safe from VRS.
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Old 9th Apr 2012, 03:43   #71 (permalink)
 
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What is magic about 2500 fpm rate of descent....most autorotations can provide that ROD....it is staying in the downward moving column of air that is being accelerated by the rotor system that trips you up. 300feet AGL and 60 Knots used to be our approach gate in the Chinook with the Thrust Lever full down....then a transition to full power getting the old girl stopped at a hover. It idd not matter if it was a 105mm Howitzer and A-22 bag of ammo or an internal load. The object of the exercise was to spend as little time between 2500 feet AGL and the Landing site. I would suggest airspeed and power setting is far more critical to the onset of VRS than Rate of Descent. One does not turn into a Frog at 800fpm ROD and 40Kts IAS.....there is far more to it.
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Old 9th Apr 2012, 07:05   #72 (permalink)
 
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Quote:
I would suggest airspeed and power setting is far more critical to the onset of VRS than Rate of Descent.
Sas,
Those are of course the other two critical elements in the establishment of VRS that go without saying. It has been discussed ad infinitum on this thread that in the Marana situation the aircraft was slowing rapidly with a tailwind and max power was being pulled in at 285ft agl with a 2500+fpm ROD putting it in an unrecoverable condition whether it be a tiltrotor or a helicopter -- at that point both of them would turn into a 'Frog.'
21stC
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Old 9th Apr 2012, 14:55   #73 (permalink)
 
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Flight Manuals etc.

JeffG,

What flight manuals ( excepting the V-22 ) do you refer to? See my previous post re Sikorsky models.

21st,

At 285 ft AGL and -2500 fpm, one had best be either, getting ready to flare, if in autorotation, or starting to apply collective/power if not.

Is everyone aware that for civil machines, the FAA certifies an envelope that one tests, and no more? Thus for example the S-76 and 92 were flown thru the vertical rate of descent envelope to include autorotation, and at the extremes of the NR range. One can assume that absent flight manual limitations to the contrary, other civil certified machines were tested similarly.

Thanks,
John Dixson
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Old 9th Apr 2012, 15:08   #74 (permalink)
 
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Jeff,

I would not fully agree with that statement.....as in a helicopter....there would not have been the roll off resulting in a loss of control. In the helicopter at 285-300 feet....there is still time and height to do something....as simple as a slight turn, lowering the nose a bit, easing the collective down....any number of things.

If my memory serves me right....Nick Lappos...once posted a very good discussion on VRS and the 22....and as I recall suggested it would take a much higher ROD than 2500 fpm to cause a problem.

My main issue is the two Pilots are being left to carry the Can as a result of a PR Statement that ignores the Accident Investigators statements about the cause of the crash.

I accept the Pilot's were steep, hot, and downwind. I will even accept there existed an 800/40 caution in their NATOPs data.

I also note they were not the flight lead....Lead put them into the situation they were in. The full dangers of A-VRS were not known at the time. It was a new aircraft to them and they were working to prove the aircraft. When you stack up all the chain in this tragedy.....picking just one link and then forever and again calling it a "Pilot Error" caused crash is just being unfair to two Dead Men who cannot stand and defend themselves.

When we add all the politics and the reasons why the Marines, Bell-Boeing, and way too many Congressmen were loathe to consider the crash to be anything but "Pilot Errror"....it easy to see why all and sundry grabbed onto the idea of blaming the Pilots.

If the proximate cause of the accident was Pilot Error....the Accident Investigators would have said so....yet three have made written statements they did not consider Pilot Error to be the proximate cause of the accident. The Pilots contributed to the accident by not doing an early Go Around.....but that does not mean they were the proximate cause of the crash.

Since when does a Press Release take precedence over investigations for matter of fact.
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Old 9th Apr 2012, 15:31   #75 (permalink)
 
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JD,

40/800 has been in the UH/AH for a very long time, well before the V-22. It's also in the H-57 manual so every Naval Aviator should be familiar with it. I also believe but can not confirm that it is in the CH-46 manual.

I know its not in the 53 NATOPS. I know this because the morning after Marana our 53 project pilot at NRWATS came over to our section asking what this '40kt 800 fpm rule was'. He was surprised there was such a thing and we were equally surprised he didn't know about it. A difference between communities.
While it may not be in the RFMs I know the Navy, Army and FAA teach different versions of this rule as basic piloting technique.

SAS
"One does not turn into a Frog at 800fpm ROD and 40Kts IAS.....there is far more to it."
True. In fact the V-22 won't encounter VRS roll off at 40 its until at 3000 fpm ROD, at 15kts it will be about 2000 fpm. But it's different for every helicopter. Heavier disk loading equals less susceptibility to VRS, therefore JDs experience in the 53 doesn't surprise me wrt having a hard time finding it and maybe that's why it's not in the Sikorsky manuals? I believe 40/800 is a rule of thumb taken from early VRS testing done well over 40 years ago. That data showed that a single rotor system could get into VRS at less than 800 fpm ROD.
By the way, staying within the limits of 40/800 you can shoot up to a 15 degree glide slope approach.
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Old 9th Apr 2012, 15:55   #76 (permalink)
 
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Bell Flight test

JeffG,

Did Bell Flight test document the REAL VRS envelope for those machines*, or did the flight manual entry arise due to " other considerations "? For instance, the legal department reviewers playing a " what if " scenario around the HV diagram?

* and not that other, very high N/Rev vibration aerodynamic phenomena that some have misconstrued for VRS and which I described in an earlier post.

Thanks,
John Dixson
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Old 9th Apr 2012, 17:05   #77 (permalink)
 
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JD
I don't recall seeing the 40/800 in a bell civil RFM so that would lead me to believe it might have more to do with NAVAIR then Bell. I could be wrong. If so why no carry over to the 53 is beyond me.
As far as VRS goes, I know some pilots who got into it in the AH (while testing at Pax, not for VRS though) and they have no desire to encounter it again as it scared the crap of them. For the record they encountered it much higher than 285 agl (800 agl) and recovered at less than 100 agl. So to SAS; at 285 agl, in a 2500 fpm near vertical (less than 30 kts with a tailwind) descent, no matter what you were flying, I would submit bad things were going to happen.
Remember, the aircraft was commanded to that point in space, 3X beyond what the flight manual stated. I think it's a little unfair to blame the aircraft as many want to do.
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Old 9th Apr 2012, 17:17   #78 (permalink)
 
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JD says:
Quote:
At 285 ft AGL and -2500 fpm, one had best be either, getting ready to flare, if in autorotation, or starting to apply collective/power if not.
John,
You mentioned to Lonewolf that the reasons you performed a zero airspeed vertical descent VRS demo at 6-8,000 feet was because:
Quote:
Two reasons. First is that it isn't easy to find a fully developed VRS situation, and the second is that once found, a respectable length of recorded data is appreciated by all except the flight crew being bounced around.
Is it possible that not putting the a/c in an unrecoverable position was a factor also? Do you believe if the CH-53 was in the same scenario as the Marana V-22 a CH-53 could have recovered? (none of the other CH-53 drivers I have spoken to believe it would have been survivable, but perhaps there is some empirical data we are not aware of?)

Sas mentions in those conditions (slowing rapidly with a tailwind and max power being pulled in at 285ft agl with a 2500+fpm ROD) it might be possible to recover with a "slight turn, lowering the nose a bit, easing the collective down." Do you believe the CH-53 could recover using these techniques in that scenario?

thanks,
21stC
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Old 9th Apr 2012, 19:30   #79 (permalink)
 
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Quote:
Is it possible that not putting the a/c in an unrecoverable position was a factor also? Do you believe if the CH-53 was in the same scenario as the Marana V-22 a CH-53 could have recovered? (none of the other CH-53 drivers I have spoken to believe it would have been survivable, but perhaps there is some empirical data we are not aware of?)
At least a '53 would have hit upright (like the Lead V-22, Nighthawk 71 did), not inverted and ensuring that it would be unsurvivable. Below 800 feet or so, A-VRS is unsurvivable. Marana proved that.
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Old 9th Apr 2012, 19:30   #80 (permalink)
 
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Quote:
I think it's a little unfair to blame the aircraft as many want to do.
If it is "unfair" to blame the aircraft....why is it "fair" to blame the Pilots when we all know there were a whole raft full of causes to this crash? The old "chain" concept is well proven here.

If A-VRS was well understood prior to the crash....might the pilots not been better trained and educated about such risks....and perhaps might have waved off prior to losing control effectiveness?

As this is unique to the tilt rotor design....why can we not assign some fault to the aircraft?

Some of us quite happily blame the Pilots....why not the aircraft?
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