# What exactly is VMO and how does it affect a plane when it is exceeded?

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?

• Commented Jul 21, 2015 at 4:35

$V_{MO}$ 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.

• 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
Commented Jul 21, 2015 at 9:26
• @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) Commented Jul 21, 2015 at 13:54
• 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... Commented Jul 21, 2015 at 16:10
• @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. Commented Jul 21, 2015 at 19:24
• 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) : scribd.com/doc/84074210/… Commented Jul 22, 2015 at 15:33

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.

• 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. Commented Jul 23, 2015 at 13:03
• Yes that's what I wrote. If it's not clear make a suggestion
– rbp
Commented Jul 23, 2015 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
Commented Jul 23, 2015 at 13:15

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.

• This answer only addresses half of the question, and does not talk about the definition of Vmo. Commented Dec 18, 2018 at 20:40

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.

• would you like to address such points? or should we take your answer as being nothing more than a mini rant? Commented Nov 29, 2022 at 5:51
• 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. Commented Nov 29, 2022 at 7:34