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MCAS ALTITUDE V ATTITUDE ??

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MCAS ALTITUDE V ATTITUDE ??

Old 15th Mar 2019, 19:38
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Question MCAS ALTITUDE V ATTITUDE ??

I've started this thread since the other long threads are getting impossible to follow. In previous thread(s) severl have claimed that MCAS has an ALTITUDE input of some sort. Gums and others have not been able to find such a constraint. I think I've found where the confusion started and IMHO its probably a typo !!

OK here is the basic link that may have started

737 MAX - MCAS

And if one scrolls down one finds

" Technical Description of Maneuvering Characteristics Augmentation System (MCAS)
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.
The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees
BUT further on is this official document ( pasted part of image since could not copy and paste )


and the term is ATTITUDE !
RUllaWaY Stabilizer
Required by AD 2018-23-51
Disengage autopilot and control airplane pitch attitude with control column
and main electric trim as required.
Attached Files
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Old 15th Mar 2019, 19:51
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Which also IMHO brings me to another related IMHO probable screwup by newstypes

The mention of ' critical' or " overspeed" or max speed in various accounts re Ethopian crash

I suggest the confusion is that 300 or similar knots at less than 1000 AGL for a commercial airliner is definitelty too fast under both rules and common sense, thus for the unwashed, they use critical or overspeed

That meme gets picked up and repeated - and without full context makes ' sense' ?

Whereas 300 knots at any reasonable altitude is a non event and not close to VNE
Just this SLF .00002 cents
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Old 15th Mar 2019, 19:55
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Salute Conso !!

Yep, although we had a great discussion and got into the technical stuff about MCAS and such about 4 months ago, it may be time to refer newbies here or back to the original thread that arose due to JT610.

Before logging, the ALTITUDE, as well as temperature, is part of the mach value used by MCAS. So in a roundabout way altitude is there for the MCAS magnitude of down trim component ccording to available info.
Tough to analyze something that is a kludge to begin with, and then the inputs to the kludge may have "back doors" the sftwe folks had not thot of. The stability problem was an aerodynamic one that Boeing tried to correct with a "simple software"doofer. But the fix used the aerodynamics of the hozontal stabilizer controlled by a new sftwe module in the FCC, and the fault tree analysis was "faulty"!!!!

I can tellya that having the stick/wheel feel get light as you approach a stall AoA is not good. And then, if you pull back just a tad longer the nose continues to rise all by itself.

Gums sends...
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Old 15th Mar 2019, 20:47
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Originally Posted by CONSO View Post

BUT further on is this official document ( pasted part of image since could not copy and paste )

There is no "but." The sections before and after say two unrelated things that are not in contradiction. ALTitude is one of the inputs that determines at what AOA the MCAS will make a stab input. ATTitude is what the runaway stab procedure wants you to control with the column and manual trim.
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Old 15th Mar 2019, 21:10
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Originally Posted by Vessbot View Post
There is no "but." The sections before and after say two unrelated things that are not in contradiction. ALTitude is one of the inputs that determines at what AOA the MCAS will make a stab input. ATTitude is what the runaway stab procedure wants you to control with the column and manual trim.
Good idea to pull aside and have a level setting. I concur with everything added to the thread so far. Let me point to one possible source for the confusion about the role of ALTitude in the activation of MCAS. I recall that in the ET accident thread at one point there were several submissions wondering if MCAS could have played a role based on the data suggesting that the airplane did not gain much ALTitude and thus speculating that the flaps would have still been extended. I believe that at least one entry suggested that on climbing out of Addis Ababa it would not be abnormal to be cleaned up to flaps up by the point where the airplane was 1000' above the runway ALTitude. This may have been seen by some as indication that MCAS has logic to prevent activation below 1000'. I am not aware of any direct use of ALTitude in the activation of MCAS. I am checking with a source and will report back once this is confirmed or denied.
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Old 15th Mar 2019, 21:16
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Yeah, according to something reliable-looking that someone posted in the other thread, the real threshold determiner is Mach. Of course that's a function of temperature, which roughly translates to altitude and airspeed, as the first section quoted by CONSO says. So strictly speaking that's wrong too, but good enough for the purpose of whatever that document is. I was only clearing up the CONSO's perceived contradiction between that and the ATTitude referenced in the second passage.

I also agree that 1000 feet is likely meaningless, and was a result of 1) MCAS activates when the flaps go up, 2) the flaps go up at 1000 feet, and yada yada Chinese Whispers, 3) MCAS activates at 1000 feet. Oops! New wrong fact introduced into the collective consciousness.
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Old 15th Mar 2019, 21:48
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Salute!

O.K., Vessbot. Show us the source code that uses altitude to determine the AoA and not the Mach value/gain for the application of trim value, rate and such.[/SPOILER]
Our post crossed, and I am glad FC clarified my original understanding of the activation criteria. No hard feelings. None, nada, no-way dot com.

The available documents show AoA as the main driver once flaps up, and Mach determines the gains that the FCC uses to move the stab. FCeng84 has explained this before, as has Bjorn. Being the all around good guy that I am, I can stand to be corrected if I misinterpreted our documentation.. Heh heh.

The problem is aerodynamic, and then a kludge sfwe fix by Boeing that is poorly understood , even by pilots flying the plane, will now open the door for billions of $$$$ that Boeing will have to payout. Hell,if I was flying that plane and having over a hundred folks depending upon me to visit Aunt Clara, I might file a suit myself.

Gums bitches.....


Last edited by gums; 16th Mar 2019 at 02:49. Reason: redaction due to cross posting and expert testimony
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Old 15th Mar 2019, 21:58
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I'm sure we all remember the graph showing angle of attack against lift with the stall occurring at about 15 degrees. Fairly true for a straight winged light aircraft and lowish speeds and maybe not too far out for a 737 at 2000'. The same graph for a (slightly) swept wing aircraft flying at altitude above 0.7M would look completely different.

So presumably MCAS has a set of stored parameters as to what is an acceptable AOA in both regimes. It might allow say 14 degrees at 2000' but only 6 degrees at FL 390. Hence the need for inputs other than AOA.
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Old 15th Mar 2019, 22:04
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feng said
I am not aware of any direct use of ALTitude in the activation of MCAS. I am checking with a source and will report back once this is confirmed or denied.
Granted I did a poor job in my comparison/confusion between altitude and attitude. And gums rightfully explained the difference - BUT that leaves myself and perhaps a few other much more qualified to guess just what sensors are involved in MCAS.

It ** seems** to me that the so called official descriptions and documents released so far leave MUCH to ' can you spot the problem within the next few minutes or die ' complex

MY confusion comes from the **apparent** assumption *** that IF the amount of stab trim is a function of speed and altitude- that both speed and altitude should/must have some sort of inout to when AOA triggers AND hows much movement per tweak results.
IF in general the trim movement is a function of speed - then at low speeds them trim/second would logically be larger . OK
Since speed is usually lower at low altitudes - re takeoff- in the simplified world-- then a logical question is how is altitude measured ? AGL or the standard baro altitude set at airport So what happens when airport alt is say 7000 above sea level - how is this compensated for re how much trim movement per second/ tweak?

While one can assume the whiz kids who programmed this kluge did take such issues into account- the Ethopian accident makes one wonder.- especially when one knows that mach number ( actual speed ) varies with actual temperature NOT altitude per standard tables. Which suggests that somehow a local ( to airplane ) temperature or at least at takeoff must somehow be fed into MCAS to prevent errors in stab movement.
Which then drives into the REAL effects of airspeed, altitude, temperature, airport altitude, inputs to MCAS and how derived/measured.

Seems to me one could legitimately question just how/where/when the above inputs are accommodated in a reliable fashion re MCAS activation. ? Dont expect to get answers here- but hopefully the code sausage making will be fully questioned.
Of course none of the above applies to the microsquish simulator crowd- and maybe not even the real simulator inputs ???

For the above - other than raise what I believe are real questions that need real answers by the experts on MCAS- I'll consider it my .0003 worth
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Old 15th Mar 2019, 22:10
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Originally Posted by gums View Post
Salute!

O.K., Vessbot. Show us the source code that uses altitude to determine the AoA and not the Mach value/gain for the application of trim value, rate and such.

The available documents show AoA as the main driver once flaps up, and Mach determines the gains that the FCC uses to move the stab. FCeng84 has explained this before, as has Bjorn.
You misunderstood my post. The threshold is determined by Mach, which is a function of airspeed and temperature. This can also be said to be airspeed and altitude. As altitude does not strictly relate to temperature this is not strictly correct, but it's a good enough approximation for a general description for a lay reader, which is why it's written on B737.org.uk; and I'm clearing up CONSO's confusion between that use of "ALTitude" and his perceived contradiction between that and pitch ATTitude which is referenced in a completely different document that he quoted.
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Old 15th Mar 2019, 22:53
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Salute Vess !

No misunderstanding , I really DID misunderstand! and I am very reasonably knowledgeable about all the thermodynamic relations.
My position is simple, and FC eng and others can refute:
The primary activation for the MCAS nose down trim is AoA
- MCAS inhibited with A/p engaged ( maybe)
- MCAS inhibited if flaps are not up ( demonstrated on two or more flights, .one fatal)
- MCAS can be defeated with a manual trim switch on the stick/yoke ( at least two flights in Indonesia, one fatal)
- The amount of trim rate applied by MCAS, and possibly the total amount has a mach function as has been explained by FCeng and other sources. [deleted using CONSO's explanation and review of books, heh heh] But mach is not what triggers MCAS nose down trim, it tailors it

That's my story, and I am sticking to it.

Gums sends...

Last edited by gums; 16th Mar 2019 at 03:05. Reason: apology and redacted verbiage
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Old 15th Mar 2019, 23:05
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Originally Posted by gums View Post
Salute Vess !

No misunderstanding, and I am very knowledgeable about all the thermodynamic relations.
My position is simple, and FC eng and others can refute:
The primary activation for the MCAS nose down trim is AoA
- MCAS inhibited with A/p engaged ( maybe)
- MCAS inhibited if flaps are not up ( demonstrated on two or more flights.one fatal)
- MCAS can be defeated with a manual trim switch on the stick/yoke ( at least two flights in Indonesia, one fatal)
- The amount of trim rate applied by MCAS, and possible the total amount has a mach function as has been explained by FCeng and other sources. And mach has temperature and air density and less factors. But mcah and its contributing components are not what trigger MCAS nose down trim.

That's my story, and I am sticking to it.

Gums sends...
I didn't say you misunderstood MCAS or thermodynamics, but rather that you misunderstood my post. This is spurred by your challenging me to produce proof that MCAS "uses altitude to determine the AoA and not the Mach," while that's not a position that I take.

As I clarified in my last post, my position is that the AOA activation threshold is determined by Mach, and my invocation of altitude is not to say that MCAS uses it, but to relate CONSO's quote of "altitude," from the beginning of this thread, to the real Mach-based usage.
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Old 16th Mar 2019, 00:40
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Since speed is usually lower at low altitudes - re takeoff- in the simplified world-- then a logical question is how is altitude measured ? AGL or the standard baro altitude set at airport So what happens when airport alt is say 7000 above sea level - how is this compensated for re how much trim movement per second/ tweak?

While one can assume the whiz kids who programmed this kluge did take such issues into account- the Ethopian accident makes one wonder.- especially when one knows that mach number ( actual speed ) varies with actual temperature NOT altitude per standard tables. Which suggests that somehow a local ( to airplane ) temperature or at least at takeoff must somehow be fed into MCAS to prevent errors in stab movement.
Which then drives into the REAL effects of airspeed, altitude, temperature, airport altitude, inputs to MCAS and how derived/measured.
My recollection from hydrodynamics courses taken as part of a maths degree almost 60 years ago is that mach number is airspeed relative to the local speed of sound. The speed of sound depends on temperature and pressure (assuming constant air composition), so MCAS activation and operation should depend on air pressure, temperature and airspeed. It should be independent of altimeter setting and height above the surface (unless it is considered useful to involve the terrain clearance system in the picture). Perhaps if MCAS was overridden by TCAS we would not be having this discussion.
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Old 16th Mar 2019, 01:07
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The speed of sound depends on temperature and pressure . . .
Nope - speed of sound in AIR in a 500 psi tank is only dependant on temperature. In the extreme ( vacuum ) with no air molecules speed of sound is of course zero. But we are not talking the extreme here but within the normal range of flight- and as such its strictly temperature

basic calculation speed of sound (m/s) = 331.5 + 0.60 T(C) note only a relation to temperature - not ' pressure' or altitude withing the normal range. Check any table for speed of sound vs altitude and note that it is the temperature standard at altitude which controls

Which gets back to my oversimplified question - what other inputs control MCAS -- and how measured or compared to what standard versus actual ??
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Old 16th Mar 2019, 01:12
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Originally Posted by CONSO View Post
Which gets back to my oversimplified question - what other inputs control MCAS -- and how measured or compared to what standard versus actual ??
This is total supposition, but I would bet that it's fed Mach from the ADC, where it's already calculated and fed to probably a dozen other systems.
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Old 16th Mar 2019, 01:30
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Unhappy

Originally Posted by Vessbot View Post
This is total supposition, but I would bet that it's fed Mach from the ADC, where it's already calculated and fed to probably a dozen other systems.
IMHO probably right - more sensors to impact calculations - but at least some sort of visible aural warnings if mismatch gets us to ???

Hmmm-
Nowadays, ADCs are digital. The transducers called Analogue to Digital Interface Units are present near the static and pitot source. The readings are converted to digital signals before the ADC receives the information. The Total Air Temperature has always been an electrical reading as it uses a temperature sensitive resistor which gives out a Voltage.
OK - so at a high temperature near takeoff , the mach calculation would give a LOWER mach number meaning a greater stab deflection ? And if ANY other sensor ( static v dynamic , temperature ) than AOA was incorrect on the same computer as AOA - the old standby match the curve game goes bonkers and ....
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Old 16th Mar 2019, 01:49
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per conso's req:


25.672 Stability augmentation and automatic and power-operated systems

If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with 25.671 and the following:

(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.

(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failuresof the type specified in 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.

(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system -

(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;

(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplaneconfigurations) which is described in the Airplane Flight Manual; and

(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.

[Amdt. 25-23, 35 FR 5675 Apr. 8, 1970]
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Old 16th Mar 2019, 02:19
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Originally Posted by fdr View Post
per conso's req:


25.672 Stability augmentation and automatic and power-operated systems

If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with 25.671 and the following:

(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.

(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failuresof the type specified in 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.

(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system -

(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;

(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplaneconfigurations) which is described in the Airplane Flight Manual; and

(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.

[Amdt. 25-23, [url=https://www.law.cornell.edu/rio/citation/35_FR_5675]35 FR 5675 Apr. 8, 1970]
And adding ( my emphasis added )
25.671 General.
(a) Each control and control system must operate with the ease, smoothness, and positiveness appropriate to its function

.(b) Each element of each flight control system must be designed, or distinctively and permanently marked, to minimize the probability of incorrect assembly that could result in the malfunctioning of the system.

(c) The airplane must be shown by analysis, tests, or both, to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems), within the normal flight envelope, without requiring exceptional piloting skill or strength. Probable malfunctions must have only minor effects on control system operation and must be capable of being readily counteracted by the pilot

.(1) Any single failure, excluding jamming (for example, disconnection or failure of mechanical elements, or structural failure of hydraulic components, such as actuators, control spool housing, and valves).

(2) Any combination of failures not shown to be extremely improbable, excluding jamming (for example, dual electrical or hydraulic system failures, or any single failure in combination with any probable hydraulic or electrical failure).

(3) Any jam in a control position normally encountered during takeoff, climb, cruise, normal turns, descent, and landing unless the jam is shown to be extremely improbable, or can be alleviated. A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable

.(d) The airplane must be designed so that it is controllable if all engines fail. Compliance with this requirement may be shown by analysis where that method has been shown to be reliable.
[Doc. No. 5066, [url=https://www.law.cornell.edu/rio/citation/29_FR_18291]29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]


Last edited by CONSO; 16th Mar 2019 at 02:24. Reason: more readable and easier to quote parts
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Old 16th Mar 2019, 03:27
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Salute!

I have corrected the misunderstanding(s) that existed a few post back ( Vess and Gums). Not surprising when dealing with the not so simple implementation and explanation of MCAS from various sources.

@FDR thanks for the references. I had to look them up myself back in November because I couldn't believe the plane got certified with a crappy pitch momentum plot at high alpha.

It still amazes me that many folks posting lately think MCAS is a "stall prevention" or "stall warning" feature. Oh well, being the old instructor pilot, I explained how it works to my wife using a steering wheel analogy where the wheel moved all by itself as she tried to turn harder near the stops. "Oh yeah, I get it"

Thanks for putting up with my rants, but this whole thing bugs me. If we prevent just one accident then it is worth it in spades, huh?

Gums sends...
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Old 16th Mar 2019, 03:40
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CURRENT - 833 PM SEATTLE TIME
BA plans to provide software fix within about 10 days and mandate installation. Then after a test flight- plane can be returned to service ! meanwhile the FAA and Boeing are going rounds as to type and kind and level of ' training ' on the ' new' software/system/ effects shall be required. !

The above is a paraphrase of the news release by a few ' sources'

https://www.thestreet.com/investing/...e-fix-14897876

[QUOTE]Boeing Co. (BA
- Get Report) shares rose Friday following a report that that planemaker is set to roll out a software upgrade to address safety issues linked to the grounded 737 MAX series.France's AFP news agency said the rollout, which will come in the next ten days, would focus on the MAX's automatic stall-prevention system that was linked to the Lion Air crash in Indonesia, but has not yet been proven to have caused Sunday's Ethiopian Airlines ET 302 disaster. Reuters reported Friday, however, that the timeline for the software fix "has not changed" and will be installed "in the coming weeks".Boeing shares were marked 1.52% higher Friday to close at $378.99 each.

[QUOTE]

I'm simply reporting- I'm sure within a few hours or less, more details will be available-

IMHO- while FAA may allow this in the U.S- I doubt anyone else will . but I'm just a poor SLF retired mechanical injun ear from BA and not an aeronautical whiz bang at all- Not even a microsoft simulator player .
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