Vne is true airspeed...

[shumway76]

Recently read an article in Aviation Safety (Jan 2011 issue I think) with regards to ASI markings. It mentions that all the markings are based on IAS, EXCEPT for Vne. Vne is actually based on true airspeed, therefore, if flying at higher altitudes, your Vne will be reached even before your ASI shows you are near Vne.

If exceeding Vne, the effect is the airspeed can cause surface flutter. However, I can't understand how Vne is based on true airspeed? If the air density is lower, there would be less molecules hitting the surfaces, so how can the flutter occur at that lower IAS?

[KKoran]

Vne is a required marking on the airspeed indicator and airspeed indicator markings show indicted airspeed. Besides, the things Vne is based on (flutter, etc.) are dependent on indicated airspeed.

US FAA aircraft certification guidance (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=9b06546343b9c534cd8cbe093366e8a1&rgn=div8&view=text&node=14:1.0.1.3.10.7.105.16&idno=14)
§ 23.1545 Airspeed indicator.

(a) Each airspeed indicator must be marked as specified in paragraph (b) of this section, with the marks located at the corresponding indicated airspeeds.

[Pilot DAR]

Yes, Vne, which corresponds to Vmo (speed for maximum operation) will always be in IAS, not TAS. The report you refer to was perhaps referring to an aircraft which also had a Mach airspeed limitation (Mmo). This will still be indicated as an IAS with a moving "barber pole" pointer on the airspeed indicator which tells the pilot where the maximum permitted speed is in those atmospheric conditions.

Vne for GA aircraft is derrived from a number of factors, flutter margin being one. This speed will have a margin of safety of 1.1. For several very good reasons, you should not be deliberately flying an aircraft at these high speeds, flutter is one, upset and overloading due to gusts are others. Below Va you're safe from both of these, above Va, your piloting skill is required to maintian structural and overspeed safety (not to mention what such speeds can do to the engine and prop with carelessness).

That said, 110% of Vne is tested for every certified aircraft. It is something I have to do during every test program - very carefully!

http://i381.photobucket.com/albums/oo252/PilotDAR/Jims%20DAR%20Testing/172Divetest.jpg

[Denti]

In our glider planes we had an IAS marking for VNE, however in the POH was a table of pressure altitude vs VNE which showed a marked decrease of VNE with rising altitude. Flying the marked VNE or close to it at higher altitude can very easily lead to flutter and partial or total destruction of the airplane. Probably more of a problem with gliders than with most SEPs as gliders have no natural altitude limit apart from pressure breathing limitations, flying into the 300s to 400s is very much possible given the right weather conditions.

[peterh337]

As Pilot-DAR says, Vne is marked as IAS - just like everything else on the airspeed indicator. All the speeds on the ASI are indicated airspeeds - obviously :)

It is some of the factors which give rise to Vne that are related to TAS and not IAS - control surface flutter I think being the main one.

The amount by which TAS exceeds IAS is altitude dependent, so the ASI marking for Vne takes into account the aircraft operating ceiling - because you could legitimately climb to that altitude and then immediately commence a dive at Vne.

This causes problems if you fit a more powerful engine (e.g. as in turboprop conversions) because you can now fly higher, where the TAS gain is greater that the figure used for the original ASI marking for Vne.

So, on straight TP conversions like e.g. the Jetprop, the certification forces Vne to be marked at a lower IAS figure than on the original aircraft (piston Malibu).

At least that is my understanding :)

[englishal]

Flutter is caused by the SPEED of the air molecules over the control surface not the number of molecules. So yes, Vne decreases with altitude, but as Peter points out, in your design configuration you won't be anywhere near those speed in normal operations.

The problem comes if you boy race your aeroplane, bung in an extra 200 HP with turbo and super charger, so now you can cruise at IAS-Vne at 30,000'. You will actually be over Vne-TAS.

Vans has a very good article on this on their website.

[India Four Two]

That said, 110% of Vne is tested for every certified aircraft. It is something I have to do during every test program - very carefully!

Pilot DAR,

That's a very impressive photo. Something I hope never to see in MY flying career. I'm surprised the altimeter pointer is not blurred. ;)

I note there is no evidence of "white knuckles", but as you said, you do it very carefully.

[peterh337]

Notice the pitch attitude and the rate of descent :)

[BackPacker]

That said, 110% of Vne is tested for every certified aircraft. It is something I have to do during every test program - very carefully!

The Vne marking on the ASI is set so that the "true" Vne is higher than the "indicated" Vne, except at the service ceiling, as said above.

But shouldn't that mean that a Vne+10% test should be done either at the service ceiling (using the ASI Vne+10%) or at a lower altitude, but then for a re-calculated TAS Vne, which is higher than the IAS Vne?

4400' doesn't seem like the service ceiling of the C172 at all...

(Am I making sense?)

[englishal]

Yes complete sense. If you did the same as in the above pic at 18,000'.....good luck! At 4400' TRUE Vne might actually be above the red line!

[peterh337]

Yes, testing indicated Vne at say 5000ft is nothing like as thorough as testing indicated Vne at the certified ceiling.

It's an interesting point.

[Rod1]

In the UK all LAA Permit aircraft have to be test flown once a year. This includes flying them up to VNE. This is a bit of a pain as it has to be done in smooth air so can delay things. I am not aware of any accidents caused as a result of this practice, but there have been a small number of flutter incidents due to rigging / finishing issues.

Rod1

[chrisN]

For those who have not seen it before, here is a glider being test flown and getting flutter:

How to break a glider´s wing - YouTube

If anyone wants to challenge the wisdom of respecting VNE, I hope you are as lucky as you feel.

Chris N.

[patowalker]

In the UK all LAA Permit aircraft have to be test flown once a year. This includes flying them up to VNE. This is a bit of a pain as it has to be done in smooth air so can delay things.It is permitted to conduct the Vne dive solo, which is welcomed by my usual ballast. The replacement sand bags remain equally untalkative during proceedings. :)

[vihai]

All the few gliders I flew have and additional placard listing the Vne changes at various altitudes. The ASI mark is at the Vne at sea level.

Mine goes from 275 km/h at sea level to 210 km/h at 8000 m.

The Vne calculation is quite complex and involves both IAS and TAS, thus the need for a table of Vne at various altitudes arises if you don't want to use the lowest one.

[Genghis the Engineer]

The LAA test schedule requires some stalls as well, and I for one won't do those in any significant turbulence either!

The test should normally be done with minimum essential crew - so if testing close to MTOW, a few sandbags are generally inevitable, although usually for an annual PtF air test, testing at an "as found" weight is usually acceptable.

G

[patowalker]

although usually for an annual PtF air test, testing at an "as found" weight is usually acceptable.

I once accepted an invitation to join a check pilot on the annual air test of my Quantum. It was the most exciting ride I even had in it. Saw La Baule from the most unexpected angles, but it turned out OK and that nice fellow at Deddington issued the PfT.

[shumway76]

So Vne is IAS, not TAS? Something wrong with the article I read (Aviation Safety, January 2011)?

[peterh337]

Vne has to be IAS because it is marked on the Airspeed Indicator (ASI) :)

In flight, if you want to check you are below Vne, you look on the ASI.

You don't look on the ASI, get your IAS, convert it (using altitude and temperature) into TAS and then check if the resulting value is below the ASI Vne marking. That would be completely useless.

What exactly does that article say?

[Denti]

The real VNE is a TAS, however the marking on the ASI is an IAS. In small piston engine airplanes it is apparently normal to take the lowest IAS corresponding to the real VNE and mark that on the ASI which should usually keep one save, however if you climb higher than the normal service ceiling (using updrafts, mountain waves for example) you might be in for a rude surprise if you try the marked VNE.

Since gliders do not have a service ceiling they simply mark the VNE for sea level and give the pilot information how it changes with altitude in the POH and/or a placard on the panel.

[Genghis the Engineer]

No the real Vne is in EAS, which is converted (via CAS) to IAS.

However, particularly in aeroplanes with a flutter issue (most high performance gliders) it can vary altitude, which in reality is an extra TAS limit being applied, built on flutter avoidance.

G

[rapidshot]

Print - VNE Revisited - Aviation Safety Article (http://www.scribd.com/doc/84074210/Print-VNE-Revisited-Aviation-Safety-Article)

[Pilot DAR]

So Vne is IAS, not TAS?

Well Shumway, I was all set to smugly answer "Yes" to this very simple question (in respect of GA powerplanes anyway), but I decided to do some fact checking first.

I reviewed about a dozen random Cessna and Piper type certificate data sheets across the history of their production. I found to my surprise, that the airspeeds on those sheets are listed in any of CAS, TIAS, or IAS. In some cases, it differs through vintage of the same aircraft.

As Peter correctly points out, putting a "placarded" limit in anything other than IAS, when the only information presented to the pilot is in IAS is silly. Having the pilot make the conversion from IAS to TAS or CAS, while hurtling toward earth at Vne, or balancing on the buffet of the stall seems silly. When I present limiting airspeeds for an aircraft in a placard or FMS, I present those speeds in IAS.

The older CAR 3 cerification basis introduces a rather confusing term for the marking of airspeed indicators: "True indicated airspeed", where the later FAR 23 does not use this term. FAR 23 does make a reference to providing the pilot with a means to determine Vne, if it varies with altitude.

So, after all that, I can imagine how an article might delve beyond simple IAS with reference to Vne, and how this could be confusing (I'm certainly more confused now, than I was before I tried to offer an answer to your question!). For your more basic flying practice, until you are trained otherwise, assume that the red line on the ASI (which is IAS, unless it moves) is Vne in IAS. (Two weeks ago I was test flying a Piper Cheyenne II, and it does have a moving red line on the ASI).

You will find many instances where details in aviation are self contradictory - just get used to it. To some degree, this is evidence of growth and learning in the industry, over it's entire history. Unlike cars, and other equipment with shorter lives (and less written), aircraft's service spans a comparibly long vintage. What was written at the time of entry into service about a very old aircraft, and is still valid for that aircraft in service today, would not be written the same way, or be valid for a newer aircraft.

But shouldn't that mean that a Vne+10% test should be done either at the service ceiling (using the ASI Vne+10%) or at a lower altitude, but then for a re-calculated TAS Vne, which is higher than the IAS Vne?

4400' doesn't seem like the service ceiling of the C172 at all...

(Am I making sense?)

Yup, makes sense.

The dive in the 172 started a little closer than 4400 feet to the service ceiling. There is a little "give" in the application of the rules when testing. The 10% margin would cover this.

For the 172, things are not that critical. When mach speeds begin to become a factor, it all changes. I once had to do a dive to mach 0.57 (at a very high altitude), in a King Air B200. There was a lot more involved in the planning and conduct of that test.

[Denti]

Thanks for clearing it up Genghis, of course, as usual, i forgot about EAS.

Nice summary Pilot DAR. Yes, sometimes things are not as clear as one thinks initially, i guess it is a good habit to keep an open mind and try to never stop learning about flying and probably the rest of it all as well.

Anyway, lately flew an aircraft where the VNE was not limited by any direct structual limit but rather by the maximum deploy speed of the parachute system. So called Ultra Lights over here (ULs can weigh up to 472kgs) have to have a parachute system for the whole aircraft and that quite often defines the VNE as structually much more is possible and indeed certified in other countries without that restriction.

[vihai]

So Vne is IAS, not TAS? Something wrong with the article I read (Aviation Safety, January 2011)?

The Vne mark on the ASI or on the glider's placard is specified as an IAS so you can easily compare it with the ASI reading.

The difference is that while most limits (Vs0, Va, Vm, ...) have a constant indicated value (and a variable true speed), Vne MAY be more constant as a true value but it actually is a bit more complicated.