B737. Can it go supersonic in steep dive?

[Hudson]

Rumours and News contains info on the Silk Air B737 crash court case. Radar indications seem to indicate that the rate of descent was phenomenal from an initial 32,000 fpm from 35,000 to 19500 and a very likely much faster rate of descent (in region of 50,000 fpm) until impact. The stab trim was found at 2.5 units (full electrical fwd), the evidence revealed engines at high power and speed brakes retracted. The FDR circuit breakers appear to have been deliberately pulled one minute prior to push over with the last reading being normal stab trim cruise setting of 4.6 units. Thus the dive was with full forward stick, full fwd stab trim, and high power.

Can anyone advise me under those conditions, would it be possible for the aircraft to have reached or exceeded the speed of sound during the dive, and how can this be proved?

[Manflex55]

Well my (basic) understanding of aerodynamics is that if U exceed an a/c's MMO (eg by reaching Mach 1 in a 737), which is a structural limitation, chances are it will fall into pieces. In this case the a/c was still in one piece before hitting the ground, so I would say it didn't go supersonic.
Even @ 50,000fpm (about 474 kt) in an absolute vertical descent, we would B far from the spd of sound @ FL200 (613kt) & obviously even further from the spd of sound @ lower levels.

MF

[twistedenginestarter]

I would have thought you could easily get it supersonic. The flight recorder would prove that for you. I guess you are saying 'How can I work out if it went supersonic?'

Quite honestly, if it's pointed downwards with power on, I can't see how it could avoid it.

It wouldn't break up at MMO which is not a structural limit. Control would deteriorate but I guess the pilot wasn't bothered by that.

[alosaurus]

Years ago a 146 had both crew shot by a psychopathic/disgruntled employee.The subsequent investigation revealed
1 The a/c had exceeded mach 1
2 It hit the ground in one piece
3 The maniac had gone back to his seat and strapped himself in

[Iceman49]

A C141 from my squadron had a yaw damper problem at 410. Did a split s down to 12000. Flight recorder showed it had exceeded mach 1 in the dive.

[Les Bee'un]

Manflex55

Without wishing to sound too harsh, I'd say that you have no (let alone a basic) knowledge of aerodynamics.

Aircraft regularly exceed MMO without breaking up and/or falling out of the sky.

MMO is a factored figure that provides a safety margin to protect the first part of the entire airplane from exceeding the "local speed of sound".

To generalise, with modern "airliners" the most usual limiting factor is not the airframe but the airflow through the engines.

If MMO was the "break up" figure then all UK Boeing 757's would have crashed.

Why?

The FAA certify the airframe to MMO 0.86 but the CAA to 0.84, for what ever reason.

So, day one I fly B757 airframe number 123 at M.85 as a "G" registered airplane. Day two I fly at M.85 and it's now "N" registered. Why didn't I crash on day one?

[pterodactyl]

Hudson,
With full power I would be very surprised if a B737 could not easily exceed the speed of sound even in a 30 degree nose down attitude. As stated before VMO/MMO is not breakup speed which would be well in excess of VMO?MMO. Boeing should have a good idea of that and it would vary depending on whether G loading was applied or whether excess speed in an "unloaded" profile was the reason for the ultimate event.

Maxflex55 says that 50000 feet per minute in a vertical descent is less than the speed of sound but that only represents the VERTICAL component of the descent. Shallower dive angles will need a higher speed to produce a VERTICAL component of 50000 feet per minute.

Taking the case of a 30 degree dive angle a speed of 986kts would be required to give a VERTICAL component of 50000feet per minute. Well in excess of M 1.0 at whatever the local temperature was.

Likewise a 45 degree dive angle needs a speed of 697kts to get the same result and at 60 degrees dive angle speed required would be 569kts.

So it was possible but the dive angle may not have been constant and only a FDR trace of appropriate parameters could yield accurate data.

PS Forgot to add:

Using Maxflex's M1.0 of 613kts at 20000feet and assuming M1.2 which is 737kts a 42 degree dive angle would suffice. (Just a coincidence the 737 bit...cheated by the odd knot!)

[ 20 July 2001: Message edited by: pterodactyl ]

[Manflex55]

Thanks guys, lots of things I obviously didn't consider - or improperly stated - in my 1st post. But I'm still not comfortable with (intentionally or not) exceeding Mmo, even if I can "rely" on this safety margin. U could actually claim the "safety margin" as an excuse to exceed any other parameter as well (eg MZFW, V1, "g" tolerance, C of G, flap extension/retraction, etc). These speed - or weight - limits have been established for a good reason (same story on the road, actually) & I'm not the one who'll put them to test. Better to have the law on your side.

MF

[pterodactyl]

Maxflex55,
No one is suggesting that any sane person would want to exceed VMO/MMO but rather whether it was possible for a B737 to exceed the speed of sound as has been suggested at the Silkair court hearing. All kinds of consequences can arise beyond the limit speeds including control reversal/flutter and structural failure but they would not all necessarily occur one knot beyond VMO/MMO. As well, whether the engines would continue to produce thrust and at what amount, would be a pertinent question.

[ 21 July 2001: Message edited by: pterodactyl ]

[joema]

I think in the 4-Apr-79 Hoot Gibson TWA 727-100 incident, JFK-MSP, possible uncommanded slat deployment, supersonic dive, 6g pullout, landed safely in DTW, that it was pretty well established he was supersonic. Incredible how strong those planes are, at least in the direction of intended stress.

BTW this was well told in the excellent book, Emergency! Crisis in the Cockpit by Stanley Stewart. See http://www.amazon.com/exec/obidos/ASIN/0830634991/qid=995674714/sr=1-1/ref=sc_b_1/002-6587801-6869603

[Hudson]

Iceman49. Thanks. I have emailed you requesting further info re the C141 dive.
For those who have so kindly replied to my post so far, thank you very much indeed. It has been priceless information towards getting at the truth in the Silk Air 737 crash. When an emminent British Professor of Aeronautics stated in court that there is no way a 737 can ever get supersonic clean at full power in a dive, the judge nodded wisely and believed him...

I didn't.

[Jim lovell]

alosaurus i saw that case on a Black Box documentary. It was a PSA BA-146-captain and co-pilot both fatally shot and then the a/c put into a vertical dive at full power. They seem to think it exceeded Mach 1 on descent and hit the ground at about 650 mph(didn't break up)!

[Expedite your backtrack!]

Didn't break up?

What, even after it hit the ground at 600+ kts?

:eek:

No wonder the 146 manages to put up with Aer Lingus landings at London City every day!

ps, I thought the problem of pulling out of a +mach1 situation was elevator flutter. Correct me if I'm wrong, but a B727 doesn't have a moving tailplane, but normal elevators. How did the pilot of the B727 manage to recover when so many of our spitfires died due to this problem?

[joema]

Expedite said: "...a B727 doesn't have a moving tailplane, but normal elevators. How did the pilot of the B727 manage to recover when so many of our spitfires died due to this problem?"

I know this sounds incredible, but after exhausting all other methods to escape the supersonic vertical dive, Gibson deployed the landing gear. The gear doors immediately departed, causing tail plane damage, and the right main gear strut bent backwards several degrees. However it slowed the plane enough to regain control. He did a no flaps landing (they were damaged in the dive), and the bent right main stayed in place.

-- joema

[pterodactyl]

Expedite.......k
Not quite correct about the B727 tailplane. It is moveable by means of the trimming mechanism and does have a separate elevator. You may have been confusing this with a STABILATOR or "all flying " tailplane.

[ 21 July 2001: Message edited by: pterodactyl ]

[john_tullamarine]

Hudson,

Don't have your ISP email to hand and the Pprune email is blocked for some reason. Can you email me to my Melbourne ISP, please. Things are going well with the team over here ....

[pterodactyl]

Good to see that you are "on the job" John. Hopefully you will be able to add valuable input on this matter. :D

[New Bloke]

I will try to be a bit sensitive here as I know this thread will attract people who have lost loved ones on the Silk Air flight and that is why I have avoided this question on the other silk air threads.

As this is a tech thread here goes.

If the stab trim was fully nose down, and the CC fully forward, why didn't the aircraft do an outside loop? In other words, it would go in the vertical plane, then through the vertical plane.

Surely to achieve the aim of hitting the ground, once the nose was pointing downwards, the stab trim would have been wound back and the CC centralised?

[innuendo]

Did not Douglas exceed Mach one in a test flight in a DC-8. I believe the aircraft was subsequently delivered to CP Air.

[quid]

Yes, a DC-8 was flown supersonic. I've got the details here someplace. Now, where did I file that.........? :)