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I read an article that mentioned "VMO" speeds of a 747 and that the plane, when it exceeds its VMO, shakes uncontrollably and destroys itself.

I've never heard of planes doing this and the article explained VMO in a way that is beyond my understanding.

For someone that doesn't know much about aviation:

What does VMO stand for and how does it affect planes when exceeded? If it does cause the plane to shake to the point that it destroys itself, why is that?

kevin
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Memj
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    related: http://aviation.stackexchange.com/questions/3373 http://aviation.stackexchange.com/questions/3088 – Federico Jul 21 '15 at 04:35

4 Answers4

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$V_{MO}$ is a “maximum operating speed”. It is one of many “V-speeds” defined for an aircraft that limit operation at various flight phases. All V-speeds are indicated airspeeds¹.

The $V_{MO}$ is the maximum permitted speed for the aircraft. This includes a safety margin, so pilots can reasonably fly near that speed. Exceeding it is not immediately dangerous, but as it reduces safety margins, is an incident that should be investigated.

There is another speed, $V_{NE}$, the “never exceed speed”, which is somewhat higher and which marks the point where things start to get dangerous. When the aircraft exceeds $V_{NE}$, aeroelastic flutter may develop that will destroy the aircraft and/or important parts may break off as the dynamic pressure becomes higher than what they are designed for.

Also these speeds have their Mach number counterparts², $M_{MO}$ and $M_{NE}$. At high altitude, the same indicated airspeed (IAS) corresponds to much higher true air-speed, since indicated airspeed decreases with pressure, while the same Mach number corresponds to slightly lower true air-speed, since speed of sound only depends on temperature. So at high altitudes, the Mach limits are reached first while at low altitudes the IAS limits are.

Exceeding the $M_{NE}$ has different effect, though also dangerous. Above that speed flow separation occurs above the wing due to supersonic speed of the flow. This leads to loss of lift and significant down-pitching moment (a.k.a mach tuck), which aircraft not designed for supersonic flight may not have enough elevator authority to compensate.

¹ The airspeeds are given as IAS, the speed measured by pitot probes, so they can be directly compared to the indication in cockpit (aircraft with EFIS (“glass cockpit”) usually correct those measurement errors that can be estimated, showing so called calibrated airspeed (CAS)). However it may not be just a simple number but may be tabulated depending on other parameters.

In particular $V_{NE}$ and $V_{MO}$ may depend on altitude, because flutter depends on true airspeed (TAS) and at lower density (due to lower pressure at altitude) the same TAS corresponds to lower EAS/CAS/IAS.

² Mach number is true airspeed as fraction of speed of sound. In this case the limiting factor is formation of shockwaves due to the flow speed locally exceeding speed of sound around some parts of the aircraft.

Jan Hudec
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  • I was warned that VNE (Never Exceed) is actually a true air speed, not indicated. It reduces with altitude. A small distinction but maybe an important one. Apparently most of the others are indicated airspeeds as you say. (It's just a little unclear that VNE can actually be variable.) – Andy Jul 21 '15 at 09:26
  • @Andy: Be carefull: that's calibrated (CAS) not true (TAS) air speed. – Radu094 Jul 21 '15 at 10:35
  • Thanks for the explanation, I'm content now. (My VNE case was for gliders by the way, which tend to suffer from particularly low limits. Doubt it matters as much for most types.) – Andy Jul 21 '15 at 13:42
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    @Andy: Hm, thinking about it again, I am not actually sure whether aeroelastic flutter and divergence depend on EAS or TAS. They may in fact depend on TAS, in which case $V_{NE}$ would indeed decrease with altitude (it would still be expressed as IAS, just not as single value, but as table depending on altitude; the other V-speeds also depend on parameters, though for most of them it is weight) – Jan Hudec Jul 21 '15 at 13:54
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    Im 99% sure it is depending on CAS. Well, EAS if you want to be pedantic, but the compression factor is very small (and constant) for the speed ranges we are talking about, so I suppose it makes more sense to display them as CAS in the manual. Let me find a source on the net about this... – Radu094 Jul 21 '15 at 16:10
  • Also, I'm just going to be obnoxious and say it, but having read the answer only now: Mixing Vmo and Vne makes no sense. Vne and Vno are part of one certification, while Vmo and Vdf part of another. An aeroplane will either have one or the other. – Radu094 Jul 21 '15 at 16:25
  • Vne varies with TRUE airspeed. This is especially critical for gliders which have a low Vne and which fly in wave at high altitude. Many glider POHs and placards have Vne tables for altitudes of 10,000ft and above. – rbp Jul 21 '15 at 19:11
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    @rbp: Well, the Vne, as IAS value, varies with pressure altitude, because there is a TAS limit. But yes, that confirms that flutter depends on TAS rather than EAS. – Jan Hudec Jul 21 '15 at 19:24
  • (actually, it varies with density altitude as dynamic pressure depends on density, not static pressure) – Jan Hudec Jul 21 '15 at 19:35
  • IAS corrected for pressure (and, less important, temperature) altitude is called??? – rbp Jul 21 '15 at 19:36
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    Well, a glider is not really a fair example, since they don't have a proper service ceiling, so all they can do is give you this Vne vs alt table. I'm willing to bet those Vne values for 10,000ft and above are IAS though.... Here is the only proper source I could find on the net about this ( nice read anyway) : https://www.scribd.com/doc/84074210/Print-VNE-Revisited-Aviation-Safety-Article – Radu094 Jul 22 '15 at 15:33
  • @Radu094, gliders are important example, because they are primarily limited by flutter due to their long wings. There are other limiting factors that depend on IAS and transsonic aircraft are limited by Mach number, so for different classes of aircraft the $V_{NE}$ dependency on altitude looks differently. – Jan Hudec Jul 22 '15 at 15:46
  • Well, that was my point aswell: that gliders are a special case. I guess (like in the article I linked) you could say that Vne is sometimes IAS and sometimes TAS (if flutter is limiting) ? – Radu094 Jul 22 '15 at 15:51
  • Vne is in True Airspeed. I have added an answer with the table from the ASK-21 – rbp Jul 23 '15 at 12:47
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Vne is defined in terms of True Airspeed (TAS), while all other V speeds are Indicated airspeed (IAS).

Gliders are particularly susceptible to the aerodynamic issues that define Vne because they fly at high altitudes and have long wings.

Since determining true airspeed in the cockpit would require a calculation, gliders have tables in their flight manuals and placards in the cockpit that show Vne in indicated airspeed at various altitudes.

Vne chart for ASK-21 sailplane

Vne chart for LET L-33 solo

Vne chart for Discus B

rbp
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    The table (which is barely visible) lists values of $V_{NE}$ depending on altitude. These values are indicated speeds, intended to be directly compared with the altimeter reading. The actual limit is a true airspeed, but $V_{NE}$ is the listed value and that is still indicated speed. Also, the limit is a true airspeed if the aircraft is flutter-limited. There are other possible limits with different characteristics. – Jan Hudec Jul 23 '15 at 13:03
  • Yes that's what I wrote. If it's not clear make a suggestion – rbp Jul 23 '15 at 13:04
  • Ok I tried to make it clearer and added two more tables. Some of these POH PDFs are hard to read. – rbp Jul 23 '15 at 13:15
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There have been several incidents where a Boeing 747-SP, B747-100, B747-200, and B747-400 have exceeded not only Vmo, Mmo, and the Vne speeds and still survived, not “shaking itself to pieces.” These aircraft were damaged, of course, but there hasn’t been a case of the B747 aircraft ever being lost due to a catastrophic in-flight breakup due to the excursion of exceeding the Vmo, Mmo, and/or Vne speed/Mach numbers.

Whaledriver
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I think we need to study more! To understand these speed, we need to refer to Part 25 for transport category airplanes and Part 23 for Normal category airplanes. Such as Cessna, Piper or Diamond Et.al. Regs. such as 25.335 Et.al. and for Normal category 23.2160 of 23.2165 Et.al. Opinions without proper understand of Part 25 or 23 regarding each airplane category, are just that "Opinions" and lead to a misunderstanding of the subject.

John
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  • would you like to address such points? or should we take your answer as being nothing more than a mini rant? – Federico Nov 29 '22 at 05:51
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    Hello John, welcome to aviation.stackexchange.com. The format of this site is Questions & Answers. If you would address the questions with an answer that explains Vmo in the context of Part 23 and Part 25, that would be valuable. Your current post does not answer the question and will likely be removed by the community. – DeltaLima Nov 29 '22 at 07:34