A 767's VMO is 360 knots, VD 420 knots. Would it be possible for a 767 to momentarily reach higher speeds than VD at low altitude without breaking up, and if so, how high would those speeds be? In other words, is a safety margin built in beyond VD?
Reason I'm asking is because, according to the NIST report, on 9/11 UA flight 175 was going 471 knots when it impacted the tower. Conspiracy theorists conclude from that that the planes flew at an 'impossible speed' and therefore, you know, 'it's a conspiracy'. Since those theorists are usually conveniently forgetting a couple of things in reaching their conclusions, I'm trying to get some learned opinions on this.
Moderators: richierich, ua900, PanAm_DC10, hOMSaR
RE: 767 VMO / VD Question
The 471 kts would be ground speed. Vmo, Vd, etc are airspeeds (IAS, CAS, or whatever).
[Edited 2014-02-26 09:04:28]
[Edited 2014-02-26 09:04:28]
"You're sitting. In a chair. In the SKY!!" ~ Louis C.K.
RE: 767 VMO / VD Question
Vd is the maximum demonstrated airspeed. The aircraft will be structurally safe at that speed. Exceeding that speed does not mean the aircraft will breakup. There will be quite a margin between this safe speed and one where the structure starts to break up.
471 kts represents about 25% more dynamic pressure than 420 kts and therefore 25% higher forces. The civil aircraft structural limit factor is 1.5, representing a 50% excess load before ultimate failure. Therefore it's quite likely the 767 structure would be intact at 471 kts. The conspiracy theorists cannot say with any certainty that this is impossibly high for a 767.
Conspiracy theorists take published data and draw the wrong conclusions to suit their own ends.
471 kts represents about 25% more dynamic pressure than 420 kts and therefore 25% higher forces. The civil aircraft structural limit factor is 1.5, representing a 50% excess load before ultimate failure. Therefore it's quite likely the 767 structure would be intact at 471 kts. The conspiracy theorists cannot say with any certainty that this is impossibly high for a 767.
Conspiracy theorists take published data and draw the wrong conclusions to suit their own ends.
Last edited by KarelXWB on Fri Jul 29, 2016 7:09 am, edited 1 time in total.
Reason: Fixed font
Reason: Fixed font
The glass isn't half empty, or half full, it's twice as big as it needs to be.
RE: 767 VMO / VD Question
| Quoting Jetlagged (Reply 2): VD is the maximum demonstrated airspeed. The aircraft will be structurally safe at that speed. |
I believe Vd usually refers to the maximum design dive speed. See this:
http://theflyingengineer.com/2012/03.../diving-into-the-a320-dive-speeds/
"The dive speed is the absolute maximum speed above which the aircraft must not fly. Typically, to achieve this speed, the aircraft must enter a dive (steep descent), as the engines cannot produce sufficient thrust to overcome aerodynamic drag in level flight. At the dive speed, excessive aircraft vibrations develop which put the aircraft structural integrity at stake."
"You're sitting. In a chair. In the SKY!!" ~ Louis C.K.
RE: 767 VMO / VD Question
| Quoting hivue (Reply 3): I believe Vd usually refers to the maximum design dive speed. |
Yes you're right, I thought they were different definitions of the same term, but they aren't. The demonstrated maximum speed is Vdf, which will not be greater than Vd.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
Re: 767 VMO / VD Question
"For much of its final descent, UAL 175 maintained a decent rate between 4000ft per minute to 8000ft per minute. During the decent from 12000ft to 6000ft, the aircraft groundspeed remained between 500-520 knots. As the aircraft made its final descent to 1000 ft, it accelerated and impacted World Trade Center tower #2 at approximately 510 knots groundspeed"________NATIONAL TRANSPORTATION SAFETY BOARD.
According to radar data study on 9/11, the alleged aircraft which impacted the south tower maintained a speed above 500 knots for few minutes, while lining up with its target. The aircraft was doing 150 knots above VMO and 90 knots above VD. Please give me a professional opinion on whether a standard 767 could perform at the speeds reported.
According to radar data study on 9/11, the alleged aircraft which impacted the south tower maintained a speed above 500 knots for few minutes, while lining up with its target. The aircraft was doing 150 knots above VMO and 90 knots above VD. Please give me a professional opinion on whether a standard 767 could perform at the speeds reported.
-
- Starlionblue
- Posts: 20565
- Joined:
Re: 767 VMO / VD Question
Using load limits as an example, the limit load for an airliner is +2.5G/-1G. Beyond that you may see permanent deformation of the structure, but you won't see structural failure. The safety factor to ultimate load is 1.5, so that means +3.75G/-1.5G. Once you go beyond the ultimate load, you risk structural breakup. It is not a certainty.
Having said that, speed is not a load limit. You can be going Mach 0.98 and still be below the load limits. You'd have quite some buffet going, which stresses the wing structure, but breakup is not a given at a certain speed.
There are also "degrees of breakup". It's not like you reach a certain speed and you have an instant structural failure of major components. Airliners are millions of parts flying in close formation, not one solid item. I'm guessing you'd start seeing things like control surfaces and antennas coming off first, long before you have the wings coming off.
Having said that, speed is not a load limit. You can be going Mach 0.98 and still be below the load limits. You'd have quite some buffet going, which stresses the wing structure, but breakup is not a given at a certain speed.
There are also "degrees of breakup". It's not like you reach a certain speed and you have an instant structural failure of major components. Airliners are millions of parts flying in close formation, not one solid item. I'm guessing you'd start seeing things like control surfaces and antennas coming off first, long before you have the wings coming off.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
Re: 767 VMO / VD Question
WillSmith wrote:According to radar data study on 9/11, the alleged aircraft which impacted the south tower maintained a speed above 500 knots for few minutes, while lining up with its target. The aircraft was doing 150 knots above VMO and 90 knots above VD. Please give me a professional opinion on whether a standard 767 could perform at the speeds reported.
The speeds referenced there are ground speeds. Depending on wind and altitude, the actual airspeed would typically have been lower.
-
- Starlionblue
- Posts: 20565
- Joined:
Re: 767 VMO / VD Question
rslifkin wrote:WillSmith wrote:According to radar data study on 9/11, the alleged aircraft which impacted the south tower maintained a speed above 500 knots for few minutes, while lining up with its target. The aircraft was doing 150 knots above VMO and 90 knots above VD. Please give me a professional opinion on whether a standard 767 could perform at the speeds reported.
The speeds referenced there are ground speeds. Depending on wind and altitude, the actual airspeed would typically have been lower.
Ground speeds are lower or higher depending on wind direction. "Typically" would depend on prevailing wind direction.
Also, winds at low level aren't normally over 30-40 knots due to ground friction.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
-
- Florianopolis
- Posts: 353
- Joined:
Re: 767 VMO / VD Question
WillSmith wrote:Please give me a professional opinion on whether a standard 767 could perform at the speeds reported.
Nobody has ever test flown a 767 that fast to see what would happen. The Vd airspeed on a Vn/Vg diagram is a "design dive" airspeed, which the engineers use to spec out the airplane's parts (to some factor of safety above the dynamic pressures at that airspeed). And then they go flight test it. They test it to that speed, and call it good. They do not go faster and faster until something breaks to see when that happens.
All we can say for sure is that at airspeeds outside the design and test envelope, the airplane's performance is no longer guaranteed. That does not mean that a 767 will explode at Vd + 1 knot, merely that the engineers can no longer promise that everything will stay together up to their specs. We can speculate that parts will probably come off as we go faster and faster, but we have absolutely no idea at what airspeed that happens. 9/11 conspiracy theories like this are literally unfalsifiable, because we have no data to indicate if a 767 is able to stay together at such high airspeeds. But we also have no data to indicate that a 767 cannot stay together at very high speeds. All we know for sure is that a 767 will be okay at a lower airspeed.
In some way, your opinion of what happens to a 767 at high airspeed depends on your opinion of 9/11. If you're like me, 9/11 shows that a 767 can be flown at some speed faster than it was ever tested. If you're a truther, a 767 doing so is evidence of a vast conspiracy.
Who is online
Users browsing this forum: F9LASDEN and 11 guests