# How can an aircraft’s maximum design operating speed be greater than its never-exceed speed?

According to this AAIB report on a June 2009 mid-air collision between an RAF Grob Tutor and a glider, the Tutor’s maximum design operating speed (VDO) is greater than its never-exceed speed (VNE):

(5) In the case of aerobatic category aeroplanes, allow each occupant to abandon the aeroplane at any speed between VSO and VDO10; [Quoting the applicable certification requirements for aircraft emergency exits; footnote in source.]

[...]

10 VDO is the maximum design operating speed which for the Tutor is 1.1 VNE or 205 kt. [The report’s explanation of the aforementioned footnote. The bolding is mine. Everything I have quoted from the report comes from page 62 (numbered as page 52).]

The point of the never-exceed speed is, as indicated by its name, that this speed should never be exceeded in this aircraft for any reason whatsoever; as such, aircraft are generally designed and certified with maximum operating speeds well below VNE. How can the Tutor’s design maximum operating speed be greater than its never-exceed speed?

• I don't understand the confusion here. Any other expectation is insane: The aircraft is designed to be able to fly up to a maximum of V<sub>DO</sub> and presumably may self destruct above that speed, so you don't want to even fly near that speed! It's pretty basic stuff: The only sane thing is to make V<sub>NE</sub> less than V<sub>DO</sub>, otherwise the expectation is that aircraft would routinely destroy themselves. – Bohemian Feb 27 '20 at 23:51

Maximum design operating speed means "we calculated the aircraft will break apart at that speed". It's obviously higher than the never-exceed speed, which means "you can safely operate the aircraft up to that speed".

Key here is the word "design". It always means something along the lines of "we calculated it to break at that load, please never try". The quotient of design load and maximum expected/allowed load is called safety factor. In this case, the safety factor is 1.1.

• Hmm, do you have a source for that definition? I would have expected maximum design operating speed to be more like "long term operation at this speed will not exceed the fatigue limits of the airframe". Calculated to break apart at 1.1 times the Vne is cutting it quite close. – jpa Feb 27 '20 at 15:53
• @jpa - If you fly 10-20% below flutter speed, everything is safe even in the long term. For "G"-loads, a safety factor of 1.5 must be demonstrated. – Rainer P. Feb 27 '20 at 19:23
• I'm no aviator, but shouldn't a definition of VDO be closer to "According to our schematics, calculations, and initial testing, the plane should technically be capable of withstanding this velocity, but we make no guarantees above it"? – Arthur Feb 27 '20 at 20:09
• There seems to be a mistake in the report. The glossary contradicts the footnote as to what Vdo is: Vdo Design Diving speed. The same thing which in the U.S. is called Vd: en.wikipedia.org/wiki/V_speeds#Regulatory_V-speeds – Jeff Y Feb 27 '20 at 20:13
• @Arthur As an Engineer, I'd agree with your definition. – Mast Feb 28 '20 at 6:54

VDO is the maximum speed, demonstrated in testing, that the structure is designed to handle (usually for flutter).

VNE is an operating limitation for the pilot to observe that includes a bit of safety margin below VDO