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Old 6th Jul 2007, 01:36   #1 (permalink)

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Is VMO an absolute aerodynamic limit ?

Why don't jets have VNE ?

Previously under CARs all jets had published VMO and VNE figures for example:

Type: VMO / VNE (kts)

B747-200: 375 / 445

DC-8-73: 352 / 406

DC-9-30: 350 / 425

DC-10-30: 356 / 400

The B757-200 however has a VMO below 10,000 ft of just 250 knots because the windshield isn't certified to withstand birdstrike above 313 knots. Above 10,000 ft the 757 is VMO limited to 350 knots.

The son of Bill Lear, John lear and I are debating the point in relation to his 9/11 conspiracy theories. 9/11 conspiracists maintain that novice pilots could not have flown a 757 into the Pentagon because it was VMO limited and thus the dive at 350 knots was not achievable as it exceeded VMO.

I say this is balderdash. The VMO limit is a windscreen issue only. John Lear maintains that VMO is an absolute aerodynamic limit on the airframe above which flutter would prevent novice pilots performing the manouveres.

I say it is not an absolute speed limit. I say there is a higher limit VNE at which, then loss of control is possible. Trouble is FAR Part 25 no longer requires demonstration of VNE.

John Lear and I have also crossed swords over United 175 which flew into WTC2 at 466 knots (according to radar tape analysis). John Lear maintains a novice pilot could not have done this.

In fact I am not quite sure what John Lear is saying because in some instances he seems to suggest that the 767 could not have flown at that speed at all because it exceeds the 767 VMO (which Lear says is impossible).

At other times he simply says the hijackers lacked the skill to do so. (case of conveniently shifting the goalposts).

Comments ?
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Old 6th Jul 2007, 02:09   #2 (permalink)
 
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Having had a very few 'discussions' with JL before, his points of view nearly always were at odds with others...although he most certainly did not want to discuss with yours truly the technicalities of the Lockheed L1011 (and especially the FMS installed therein) as clearly he was at a severe disadvantage, figuring that perhaps I had forgotten more that he would ever know about the subject...which I suspect was a true conclusion.

I cannot address the merits of the B757/767 as I have not flown these, however the L1011 was flown well in excess of Vmo during flight test, and indeed, considering Mmo, which for the TriStar is M.90, flight test was performed, according to Lockheed test pilots that I know...at M.98.

So, excess speed certainly is possible, but only a fool would go there....unless you were a properly trained test pilot.

Of course, exceeding Vmo/Mmo will sound the overspeed clacker, but simply tripping the appropriate circuit breaker would 'fix' this inconvenience.
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Old 6th Jul 2007, 02:37   #3 (permalink)
 
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Quote:
John Lear maintains that VMO is an absolute aerodynamic limit on the airframe above which flutter would prevent novice pilots performing the manouveres.
Thus proving quite conclusively that engineering skill is not an inherited trait, because that statement is complete and utter garbage.

VMO (and MMO) are exceeded in flight test on every single aircraft. Even VDF/MDF (or VNE) are not ABSOLUTE limits - they are the limits beyond which it is no longer known to be safe - not the limits where it is known to be dangerous. Margins between VMO and VDF are typically of the order of 50-75 kts (just like the old VMO/VNE splits); margins between MMO and MDF are typically of the order of 0.05M-0.07M.

In fact, its a requirement that VDF/MDF be DEMONSTRATED in basic certification.
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Old 6th Jul 2007, 02:56   #4 (permalink)


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Speaking of John Lear, here's a trivia question:

True or False. He has a sister named Crystal Shanda Lear, as in crystal chandellier.

Well, it's false, her name is just Shanda Lear. No kidding.
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Old 6th Jul 2007, 04:44   #5 (permalink)

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Some choice quotes from a conversation recently with John at Abovetopsecret's website:

Quote:
It [VMO] is an aerodynamic limit which will activate the overspeed warning horn which is very loud and very annoying and its made that way so that there is no misunderstanding about what it means. It means 'you are going too fast." To suppose that an inexperienced hijacker was flying a profile with incredible skill at 787 feet per second with the overspeed warning horm blowing is to ignore reality. It could not happen and it did not happen.
Quote:
No they don't. That statement is complete false. VNE applies to propeller powered airplanes or those certifcated under the old CAR 4b. VMO is velocity max operating, MMO is mach maximimum operating. And those limitations are all that are given to the pilot, printed on the airspeed indicator and published in the limitations manual. VD and MD are certification limits and are much higher but not to be used or even known about by other than certification test pilots.
Quote:
The VMO is a speed limit for descent from altitude such as towards an airport and is normally intended to avoid structurally harmful wing bending. This is not the VMO limiting factor for the 757/767. It is not the maximum speed at which the aircraft may be flown.


This statement is incorrect and absolute poppycock. It is most definately the maximum speed at which the aircraft may be flown.
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Old 6th Jul 2007, 04:49   #6 (permalink)
 
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I'm no freaking aero pro, but even I can tell you that "wing bending" is not likely to be the boundary condition that VMO is predicated on.
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Old 6th Jul 2007, 14:01   #7 (permalink)

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So back to the topic, is the above just semantics ?

I mean is it just that instead of VNE I should call it MNE ?

and likewise VMO is MMO ?

Can anybody tell me if the 757 or 767 have VNE or MNE numbers published anywhere ?
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Old 6th Jul 2007, 14:52   #8 (permalink)
 
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1. There's no such thing as "MNE" - the "never exceed" and "normal operating" concepts behind VNE and VNO apply only to aircraft where Mach effects are not explicitly listed in defining speeds (even though good engineering should account for Mach beffects, the resulting limitations strictly shouldn't be expressed as Mach number limits)

2. VMO is not MMO. Rather VMO and MMO are just ways of saying "the maximum operating limit where airspeed effects dominate" andn "the maximum operating limit where Mach effects dominate" - taken together they JOINTLY define the maximum operating speed limit.

3. 757 and 767, being Part 25 aircraft, do not have VNE defined; VNE is a Part 23 concept. Even if you somehow tried to certify a 757 as a Part 23 aircraft (!) 23.1505(c) would mean that it STILL had a VMO/MMO and VD/MD, not a VNE.

However....

Part 23, per 23.1505, uses both VNE and VMO/MMO nomenclature - depending on whether the aircraft are "turbine airplanes or to airplanes for which a design diving speed V D /M Dis established under §23.335(b)(4)" or not. Therefore, one can attempt to crudely relate the terms.

Assuming that you have defined the structural design speeds VC and VD for your aircraft.

If you now follow 23.1505(a) and (b) you get:

VNE=max(0.9VD or 0.9 max speed demonstrated per 23.251)
and
VNO=max(0.89VNE or VC)

If you follow 23.1505(c) instead you get:
VMO =max (VC, safe margin to VD)
where the safe margin is determined by various speed/gust upset/recovery tests.

In practice, what usually happens is that both VNO and VMO end up being equal to VC. So that would imply that
VNE=1.1(VNO,VMO)
and
VD=1.1VNE

So if your VMO/VNE were 300kts, say, that would imply a VNE of the order of 330-340kts and a VD of about 370-380kts. Basically, VNE ends up being about half way between VMO and VD. Part 25 has such a speed for certification - it's called VFC, and is the maximum speed for demonstration of "flight characteristics". But this is all based on somewhat abusing the certification speed definitions (many of the relations are not "equals" but "not more than" or "not less than" so there's no real way to reverse engineer the speeds for a real case.

Note also that even for aircraft where VNE is the certified speed used, the rules state:
Quote:
§ 23.1505 Airspeed limitations.
(a) The never-exceed speed VNE must be established so that it is—

(1) Not less than 0.9 times the minimum value of V Dallowed under §23.335; and

(2) Not more than the lesser of—

(i) 0.9 V Destablished under §23.335; or

(ii) 0.9 times the maximum speed shown under §23.251.
23.251 is a FLIGHT TEST DEMONSTRATION. Therefore any aircraft with a posted VNE MUST have been flown at least 10% faster during flight test. Doesn't sound like VNE is a speed where the aircraft is going to be falling out of the sky, does it?
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Old 6th Jul 2007, 15:43   #9 (permalink)
 
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Quote:
AFAIK, the only Part 25 turbojet aircraft that has such limitation
Any Part 25 aircraft with a Vmo "cut back" below 8000ft/10000ft almost certainly has a similar design constraint. There's nothing else in the design requirements which would drive an abrupt altitude/speed change. (The regs assume a maximum altitude for birds). I can think of a fair few aircraft with such a cutback.
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Old 6th Jul 2007, 22:57   #10 (permalink)

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Quote:
We have turbocharged condors here, probably fitted with O2 masks on their beaks, flying above 20,000 feet...
Next you'll be telling me they're IFR rated ?

No actually I read somewhere that the 757's windshield is rated to only 313 knots for birdstrike. That didn't come from John.

As mad scientist notes many aircraft are VMO limited by windscreens.

VD, I understand is the dive speed, but in the case of the 757 this is often quoted as the same as VMO.

I understood that in propellor aircraft VNE was set at 0.9 of VD. Why in a 757 would they be cited as the same ?

Is there an online source to check out V numbers for 757 and 767.

In particular can anyone comment on JL's assertions about wing flutter precluding the B767 attack on WCT2 at 466 knots ?

Anybody care to comment on the alleged "impossibility" of novice pilots diving a 757 at the Pentagon.

I am quite fed up hearing pimple faced kids citing JL as an authority that it couldn't have happened.

I would like to invite the knowledgeable folks here to put these rubbish theories to bed once and for all.
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Old 6th Jul 2007, 23:30   #11 (permalink)
 
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Were the terrorists flying all the time ? Is it that hard to hit a building?

Quote:
John Lear maintains that VMO is an absolute aerodynamic limit on the airframe above which flutter would prevent novice pilots performing the manouveres.
Want to point out that the plane was flying low, 100 ft. and less at 530MPH, cruise speed for the 752, no excessive anything but stupidity. It seems like it dived and levelled some then crashed.
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Old 6th Jul 2007, 23:51   #12 (permalink)
 
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Interesting about the 313 knots as a windshield issue.

The L1011's that were on the UK civil register had this limitation, below 8000 feet, as I recall....however it was not an FAA requirement.
Of course, the Brits required other changes as well...scarfed pitot tubes, lower Mmo (M.88) among others, and most distressing of all, you could not select CWS with an autopilot on the ground for takeoff, quite unlike the FAA certified aeroplanes.

Also, further to Vmo and Vne, under present FAA certification rules, if a piston powered aeroplane was re-engined with turbopropellor engines, the previous Vne would go away, and the old piston Vno would then become Vmo on the 'new' turbine aeroplane.
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Old 7th Jul 2007, 00:08   #13 (permalink)
 
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I always thought that many speed limits were the limits at which significant damage to the structure could start to occur - clearly structural failure rarely happens immediately at any operational speed, and flying up to published limits would not break the aircraft at that point in time, but result in many structural repairs to prevent eventual failure at a later date under normal operating conditions??

nano404 - 530mph at 100' will result in an excessive indicated airspeed, well above the limit: the cruise speed is, of course, at altitude where the air is far thinner and less stressful on the airframe.
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Old 7th Jul 2007, 04:26   #14 (permalink)
 
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Quote:
nano404 - 530mph at 100' will result in an excessive indicated airspeed, well above the limit: the cruise speed is, of course, at altitude where the air is far thinner and less stressful on the airframe.
Meh you think if everything don't ya? Thanks for the pointing that out. Though I guess broken in 3 or all together damage will still occur if a plane hits a building.
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Old 7th Jul 2007, 18:55   #15 (permalink)
 
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Quote:
Is there an online source to check out V numbers for 757 and 767.
The main design speeds can be found on the Type Certificate Data Sheet, which is a publicly available document.

Go to http://www.faa.gov/, select the 'Licences and Certificates' tab, then under 'Aircraft' there is "Type Certificate Data Sheets (TCDS)" which links you to here

You can then look up a specific TCDS by make/model etc.

For example, this is the B767-200 TCDS.

Quote:
Airspeed Limits: VD = 420 KCAS to 17,854 ft/.91M above 23,000 ft, linear variation between these points.
VFC = 390 KCAS to 17,600 ft/382 KCAS at 23,000 ft/.87M above 26,000 ft, linear variation between these points.
VMO = 360 KCAS/.86M
VLE = 270 KCAS/.82M
VLO = 270 KCAS/.82M
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Old 7th Jul 2007, 20:54   #16 (permalink)
 
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Isn't it just the case that jet a/c cruising speeds are much closer to the "maximum" airspeed/mach no whereas piston engine aircraft cruise at speeds well below VNE? I always understood that the concept of VMO/MMO was introduced at accomodate cruise speeds on jet a/c.
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Old 8th Jul 2007, 01:22   #17 (permalink)
 
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MfS: While engineering talent may not be hereditary, the ability to sling BS most certainly IS, in this case. Having listened to LR pilots over the last 30 years, I would not be surprised that the ability was included in the purchase price.
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Old 16th Jul 2007, 14:12   #18 (permalink)
 
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Thumbs up

Quote:
Originally Posted by Mad (Flt) Scientist
The main design speeds can be found on the Type Certificate Data Sheet, which is a publicly available document.
Go to http://www.faa.gov/, select the 'Licences and Certificates' tab, then under 'Aircraft' there is "Type Certificate Data Sheets (TCDS)" which links you to here
You can then look up a specific TCDS by make/model etc.
For example, this is the B767-200 TCDS.
Thanks a lot for the link! Very useful.

I was looking for VD (VDF) / MD (MDF) figures for the B737-300, -400 and -800 and for the B747 myself; however the TCDS documents refer to "the appropriate FAA Approved Airplane Flight Manual", which I can't find online. Would you happen to know an online source for those figures too?
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Old 16th Jul 2007, 15:31   #19 (permalink)
 
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Quote:
I was looking for VD (VDF) / MD (MDF) figures for the B737-300, -400 and -800 and for the B747 myself; however the TCDS documents refer to "the appropriate FAA Approved Airplane Flight Manual", which I can't find online. Would you happen to know an online source for those figures too?
You generally won't find this information in the TCDS, as these speeds are not certificate limits.
Many (most) FAA approved flight manuals don't have this information either, as it does not concern the normal line pilot.

It is only the possible concern of the test pilot, who are far more experienced in this rhelm of operation.
I have known pilots in the past who routinely operate right at the barber pole during descents, the overspeed clacker sounding repeatedly and continuously.

These folks are generally classified as being...not too bright.
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Old 17th Jul 2007, 06:06   #20 (permalink)
 
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Just to clarify: I do have the FCOMs for these Boeings, and indeed you won't find VD/MD or VDF/MDF figures there. But I think what the FAA is referring to in the certification documents comprises of more than just the FCOM.
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