Commercial airliner at speed > VNE: what's likely to fail?

[Machine_Elf]

One of the planes used in the 9/11 attacks reportedly struck its target at 510 knots (586 MPH). This is pretty much cruising speed - except the Boeing 767 is supposed to cruise at 30,000+ feet, where the air is nice and thin. Down near sea level, that’s surely way faster than Boeing recommends.

The aircraft held together until impact in that particular case. which leads me to my question: if we do this (i.e. a wretched excess of speed at low altitude) with a whole bunch of 767s (or other commercial aircraft), what sorts of failures are we likely to encounter?

[Broomstick]

Based on smaller airplanes (but not personal experience)…

There’s some buffer factor built in - a failure is not expected at *exactly *Vne. Once you get far enough above it, though, you start to do things like pop rivets and other fasteners, bending/deforming of flight surfaces, and the like. Controls can potentially lock up. Exceed it by enough and yes, bits can start to come off the airplane.

How and what exactly would fail will vary from airplane to airplane depending on what the weak points are, how exactly it’s stressed, and so forth.

It would not surprise me if someone comes along with more or better information.

[Morgenstern]

There is a YouTube video about a 747 that exceeded Vne (the redline speed) in an unintentional dive and suffered damage to the horizontal and vertical stabilizers as well as the main gear doors. Control was reestablished and the plane made it down safely.

I’d say, based on that evidence, we know know the first structures to fail on a 747 when it over speeds.

I don’t remember the airline, but the event occurred over the Pacific IIRC.

[Johnny_L.A]

@Machine_Elf

One of the planes used in the 9/11 attacks reportedly struck its target at 510 knots (586 MPH). This is pretty much cruising speed - except the Boeing 767 is supposed to cruise at 30,000+ feet, where the air is nice and thin. Down near sea level, that’s surely way faster than Boeing recommends.

IANA ‘heavy’ pilot.

Airspeed can be indicated airspeed (IAS), true airspeed (TAS), or calibrated airspeed (CAS). First you have IAS. That’s what your airspeed indicator says. Next there’s CAS. Calibrated airspeed corrects for errors that can’t be designed out of the system. There’s also another factor that kicks in above 20,000 feet and 200 knots, but I don’t know about that. Taking in the factors, you arrive at TAS.

If you’re indicating 510 knots, you need 510 knots of air entering the pitot-static system. If you’re at 30,000 feet, you need to have a higher ground speed to get 510 knots worth of air than if you’re near the surface. Off the top of my head, I can’t think of any aerodynamic reason it should make a difference. A 767 is more efficient at altitude, but terrorists aren’t really concerned about efficiency.

[GusNSpot]

Last time I was in the front of a heavy was a DC-10 West bound from London to Dallas in 1985.
Ground speed was around 500
Indicated airspeed was 168. ( clean & heavy, that is not a lot above the stall speed I was told by the pilot.

The main wings do not really care about anything but air flow. They don’t know much about altitude.

What are you thinking of that may be different above 20k that affects big birds Johnny?

Some aircraft get cranky at a small % above VNE, such as a 7A Champ but a Swift can go a lot more of a % above but they accelerate so fast because they are a clean design that you are going too fast quickly.

There are many aircraft that are considered to be in the class of : “You can slow down or come down but you can’t do both at the same time.”

How big a problem that is for big new jets I don’t know.

IMO, any sudden movement of the control surfaces, especially on the vertical stabilizer could break things near VNE & above…

[LSLGuy]

Jets don’t actually have a V[sub]NE[/sub]. What they have instead is two numbers called V[sub]MO[/sub] meaning Airspeed - Max Operating, and a Mach limit, M[sub]MO[/sub] meaning Mach - Max Operating. At low altitudes the V[sub]MO[/sub] is more limiting whereas at high altitudes M[sub]MO[/sub] is more limiting. The crossover altitude varies by type but is typically around 27,000 ft. V[sub]MO[/sub] in a 767 is 340 knots indicated airspeed (IAS) whereas M[sub]MO[/sub] is M0.86.

At very low altitude, flight at V[sub]MO[/sub] = 340 knots IAS will correspond very closely to 340 knots actual speed through the air (TAS) or over the ground (GS), as ably explained by **Johnny **in post #4. If we take the OP’s 9/11 speed estimates at face value, 510 knots GS ~= 510 knots IAS is 1.5 times V[sub]MO[/sub] but is still only around M 0.75, well below M[sub]MO[/sub].

With all that introduction out of the way …

There are two main risks with flight above the limits: flutter above V[sub]MO[/sub] and local supersonic flow above M[sub]MO[/sub].

Flutter is chaotic motion of moving parts. Think like a sports team pennant attached to a car at 70mph whipping madly in the breeze. Usually it affects control surfaces or small fixed protuberances like antennas. Essentially the aero forces get stronger than the stiffness of the part and its actuators or supports can withstand & it begins to oscillate. They won’t survive but a few seconds once flutter sets in.
Local supersonic flow is more subtle. Once the overall aircraft is traveling above about 0.9 Mach there will be localized areas where the flow exceeds 1.0 Mach as it rushes around curves or sharp edges. The top of the wing inboard and the bottom of the horizontal tail are typical locations to first see supersonic flow as the speed increases.

If the area of supersonic flow gets big enough on the horizontal tail, the tail loses effectiveness & the nose drops uncontrollably. Which in turn means you pick up speed going downhill. This is obviously a positive feedback loop that can potentially run away and quickly lead to ludicrous speed going more or less straight down.
Compared to the two main risks above, a third comparatively minor risk factor is gear doors and fairings blowing off. These things are held closed or held in position by mechanical locks. Which are sized for the expected forces. If those forces are exceeded by enough, the lock fails and the door starts to open or the fairing starts to peel off. Once the relative wind gets under there, the part is ripped off almost instantly. Which may cause cascading damage if it hits something further aft such as rear-mounted engines or the tail. Big flat doors are more at risk from V[sub]MO[/sub] exceedances, whereas small highly curved fairings are more at risk from M[sub]MO[/sub] exceedances.

Overall, getting even a good margin above V[sub]MO[/sub] doesn’t scare us. Not that we treat the limit casually, but we recognize the airplane won’t turn into a pumpkin if some kind of upset recovery finds us temporarily over V[sub]MO[/sub]. The bigger danger in a V[sub]MO[/sub] exceedance is getting too antsy & exceeding a G-limit trying to get the nose back up to level too quickly. That way lies structural failure and catastrophic inflight breakup.

Conversely, getting even a smidgen above M[sub]MO[/sub] is a whole 'nother kettle of fish. Things get bad in a real non-linear fashion not too far over the limit. Thar be dragons. I’ve said this before, but IMO most attempts by lightplane or non-pilots to John Wayne a jet will quickly devolve into an M[sub]MO[/sub] exceedance & a runaway irrecoverable dive.

[Johnny_L.A]

@GusNSpot

What are you thinking of that may be different above 20k that affects big birds Johnny?

It’s one of those things that I picked up in passing, but never looked into. I did just find this:

In high-speed airplanes (generally more than 200 KTAS), equivalent airspeed (EAS) is a factor. It can be ignored in small training airplanes, but for completeness: EAS corrects CAS for adiabatic (no heat transfer) compression flow. At 20,000 feet and higher or more than 200 kt, air is compressed in front of the pitot tube, resulting in substantially higher indicated airspeeds. But for our purposes, let’s forget EAS and high-speed, high-altitude airplanes and jump directly to true airspeed.

And this:

Equivalent airspeed (EAS) is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed (TAS) and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an airspeed indicator with zero error It is useful for predicting aircraft handling, aerodynamic loads, stalling etc.

It looks like I misremembered ‘20,000 feet and more than 200 knots’. According to the first link I found, it’s ‘or’.

[GusNSpot]

OK, cool, you & LSLGuy cleared things up for me.