What does the red line on your ASI mean? It seems to say bad things might happen if the pointer goes past that mark. Jim Davis disagrees. Here he explains why things can sometimes go seriously wrong way before the needle gets near the red.

In April 1 2010 a young charter pilot was descending from FL95 towards Swakopmund, in Namibia, in a Cessna 210. Apparently, without warning the aircraft suddenly came apart and scattered itself over a large chunk of territory. Two months later a pilot and his navigator were descending their Flamingo towards an air race checkpoint near Bella-Bella, north of Pretoria, when it also broke up in flight.

We can’t expect official findings on these accidents for a while, but from pilot reports no significant turbulence existed in either case. It’s reasonable to suspect that aerodynamic flutter might have been the culprit in both cases.

Both pilots seem to have been doing what we all do – using the descent to make up for speed lost in the climb. We like to think we are safe as long as we keep below Vne – the red line. Sorry folks, it’s not that easy.

If you want to let the needle move towards the top of the dial you had better understand exactly what’s going on. It’s a bit of a minefield, so let me walk you through it, and you decide what’s safe and what isn’t. First, let’s look at what ‘flutter’ is, and what causes it.

Flutter is what a flag, or your washing, does in the wind. It oscillates, at a fairly regular frequency, which increases with the wind speed. Almost any part of the aircraft can flutter, and the flutter mechanism can take many forms, but let’s look at aileron flutter because it’s one of the most common, and it’s easy to understand.

Have a look at the diagram. Now imagine that you hit a bit of turbulence and the wing deflects up. It won’t go far before it starts to spring back down towards its normal position.

Now look at what happens to the aileron – it gets left behind. So as the wing comes down the aileron goes up. It does this because its C of G (centre of gravity, the heavy bit which is marked with the little BMW sign) is behind the hinge point.

And when the aileron is up it forces the wing down so it sails straight through its normal position, reaches a maximum and then springs up again. But once more this causes the aileron’s C of G to swing past the hinge point and now it deflects the aileron down. This naturally forces the wing up again past its normal position and the whole thing starts again. Given the right combination of forces, each up and down stroke will be larger than the previous one until the wing thrashes itself to pieces. That, ladies and gentlemen, is flutter. Well, one form of it.

It can be anything from an almost unnoticeable buzz when a small area, such as a trim tab, flutters as it reacts with the main control surface in the same way that our aileron reacted with the wing. Or it can be a major terrifying event which shakes the whole aircraft so violently that it rips it apart. It may last for several seconds or for only a fraction of a second before it causes mayhem.

When the conditions are right the slightest thing may start it. It could be minor turbulence, or a twitch of the control column, or almost nothing.

It’s aggravated by any looseness in the structure, the hinges the control cables, the ball joints and the actuating rods.

Let’s look at the five things that influence it. Then we’ll see what you, as a pilot, should know to make sure it never happens to you.

1. Aerodynamic inputs
The first is the interaction of aerodynamic inputs. We saw how the initial trigger, in this case a gust, caused lift which, through mechanical action quickly finds the wing being forced to bend up and down by alternating aerodynamic forces. The only thing a pilot can do about that is to avoid any turbulence, however slight, when the ASI creeps towards the business end of the scale.

2. Structure’s elasticity
Your shirt flapping on the washing line is very elastic and flutters at the slightest provocation. Obviously a strong, stiff structure is less so prone to flutter. Nothing you can do about that – it’s all up to the designers.

3. Mass distribution
The weight (mass) distribution of the elements of the structure is critical. Our example had the aileron’s C of G behind the hinge point. This is obviously not good so the designer often mounts a lead weight ahead of the hinge line in order to balance the controls. Although this is initially a design feature, you as the pilot, can become very involved – so sit up straight and harken.

Some time ago a Cherokee in the US fluttered itself into destruction at circuit speed because the little arm with a lead weight on it, at the outboard end of the aileron, corroded and broke. The pilot failed to spot this during his pre-flight.

In April 2005 two pilots died at Stellenbosch, near Cape Town, when the wings were ripped off their Interavia, a tough aerobatic machine. It seems the spades had been removed from the ailerons, which altered their C of G and caused flutter way below the red line.

When we speak of the mass and C of G, remember we are thinking of critically small limits. For instance, simply painting a control surface can alter its mass and C of G enough to cause flutter. Folks who protect the leading edges of their full flying tail planes with chopper blade-tape or rubber strips are playing with fire.

4. Air density
Thick, dense air damps oscillations and delays the onset of flutter. It’s like oil in the shock-absorbers of your car. Obviously you have direct control over the density of the air in which you fly. High altitudes and hot temps mean less damping and a greater chance of flutter.

5. True airspeed
Finally, the one over which you have almost total control – the airspeed. Unfortunately this is not as simple as it appears. Although the airspeed indicator has a good solid red line on it, that is not necessarily the maximum speed you can expect to fly safely at, even in calm air.
Remember, as you climb into thin air you travel faster because you have less drag. But your ASI actually says you are going slower because its reading is influenced by fewer air molecules at the pitot.

So now four things are ganging up against us as we go higher:
1. We go faster.
2. There is less anti-flutter damping.
3. The ASI needle lies – it under-reads.
4. For flutter protection we need to know our TAS, but there is no gauge to tell us that.

Okay, don’t panic about the IAS and TAS – all will become plain in a moment. First you must know that there are different rules for normal aircraft, gliders and NTCs (Non Type Certified aircraft). Here’s how it works.

Normal aircraft
A bloke with a slide-rule works out a thing called design dive speed (Vd) which must, according to FAR23, be at least 40 per cent above design cruise speed. So Vd is a theoretical speed at which the aeroplane must be strong enough to withstand the aerodynamic forces. The designer must also be sure that it will not flutter. In fact, predicted flutter must be at least 1.2 of Vd.

The way they predict flutter is mathematical up to a point, after which it is by plotting actual test results on a graph with gradual airspeed increments.

Out of interest, the modelling analysis for the Ravin 500 predicted that it would be free from flutter up to around 390kts. But the aircraft’s red line is 243kts, presumably due to some other structural limitation.

So a test pilot climbs in, with his parachute, gets some altitude under his bum and goes for flight test dive speed (Vdf). This has to be Vd or less. We can’t expect the poor guy to fly the aircraft faster than the maximum speed for which it is designed. In reality, of course, this is not done in one flight – Vdf is only reached after a lengthy build up of test flights, each with its own data analysis.

Then the FAA guy comes along with his red paint brush and makes a mark on the ASI at not more than 90 per cent of Vdf – the maximum speed that the test pilot saw on his ASI. We call this red line Vne (Velocity Never Exceed).

So this means the red line is set at 10 per cent less than the fastest the aircraft has ever been flown. Remember we are talking about a new aircraft. You might be flying that same aircraft 40 years later – and you are still allowed to take it up to 90 per cent of its tested maximum speed. Seems a bit dodge to me – but that’s how it is.
Now we get to the interesting bit. We, as pilots, have no way of knowing whether the airspeed is limited by structural considerations or by flutter.

If the limit is there because of the physical strength of the structure and its ability to withstand aerodynamic loads, then airspeed is the thing that concerns us. After all, the structure is subjected to stresses imposed by our apparent speed through the air – the number of molecules that are pounding it. And this is exactly what the ASI reads. So indicated airspeed is the limiting factor as far as structural integrity is concerned. (For test pilots and nit pickers only: where I said indicated airspeed I should have said EAS (equivalent airspeed) which allows for position error, instrument error and compressibility error. But for us peasants it makes no real difference.)

But, if flutter is the reason for limiting the airspeed then we have to be thinking of TAS. If you were paying attention you will remember that thin air (with a high density altitude) permits flutter to happen more easily – because it has a reduced damping effect.

So to recap, IAS is the limiting factor for structural stress, but TAS warns us that we are approaching a speed where flutter is likely to start.

This is too complicated for us ordinary folks to work with on a daily basis, so the FAA made a law which basically says that aeroplanes must be able to operate safely up to the red line as long as they operate within their design parameters. These parameters include such things as gross weight and C of G. But they also include a maximum density altitude.

This means that as long as you operate within these limits you don’t need to worry about TAS and flutter, you will be on the safe side of that TAS provided you stay below the red.

Basically, they have given you a buffer so you don’t have to convert your ASI readings to TAS to make sure you are safe.

But, of course, that buffer gets smaller the higher you go. So if you install a bigger motor, or bolt on a turbocharger, or simply climb higher than the operating parameters specify – then you are entering forbidden territory and may well experience flutter, below the red line.

So that’s the story with normal light aircraft – Pipers, Cessnas, Beechcraft and so on. Now let’s see what happens with gliders and NTC (Non Type Certified) aircraft.

Gliders are slightly different. They have high structural elasticity due to their long, flexible wings. This makes them more prone to flutter. Furthermore, many are capable of operating up to extremely high altitudes – which gives them poor anti-flutter damping. And finally, they are aerodynamically slippery and are capable of high airspeeds.
This means the manufacturers can’t give them a fixed Vne that will protect them from flutter at all altitudes. So they supply a table which tells you the maximum IASs for various density altitudes. Above is an example for the very neat little 15m wingspan Pipistrel Sinus, which is a powered glider.

Non Type Certified aircraft
For NTC aircraft, the regulating authority specifies the extent to which they must meet flutter requirements. Typically you will have a red line the same as any other aircraft, but the protocol for establishing this is generally less stringent and you may not have as much protection as with a normal (type certified) aircraft. Some NTC manufacturers test their aircraft as rigorously as normal aircraft, but many don’t. So not all NTCs are equal.

How can you be sure it never happens to you?
01 Start with a proper pre-flight. Pay particular attention to the attachment of mass balance weights, the integrity of control surface and trim tab hinges, as well as their operating rods or cable attachments and tensions. If it’s got play it is potentially dangerous.

02 Plan your top of descent to be early rather than late. Descend for 50 miles at 150 KIAS rather than 30 miles at 170 KIAS.

03 If there’s a chance of turbulence during the descent keep the needle below the yellow. This is true even without flutter – gust loads at high airspeeds can break your aircraft without going the flutter route.

04 If you feel a slight hum or buzz take this as a serious warning that flutter could start at any second.

05 Whether you get the warning, or the actual flutter, immediately throttle back and raise the nose. An increase in G and a reduced dynamic pressure may save you. But it often happens so quickly that you have no time to react before the aircraft disintegrates.

06 Don’t even think of modifying anything to do with the controls – or even painting them.

07 If your aircraft has a table of airspeeds and altitudes for Vne – pay very close attention. Remember that at high altitudes your mind is not that sharp – so even more reason to remember the table.

08 If you operate outside the aircraft’s design parameters you are looking for trouble.

09 Turbo-charging or increasing engine power may not be smart.

10 When your aircraft was brand new it was tested at only 10 per cent past the red line. After 40 years of wear and tear do you still think it is a good idea to find out if that is still valid?

My thanks to engineer Mike Beresford for his technical advice with this article.

Jim Davis has 15,000 hours of immensely varied flying experience, including 10,000 hours civil and military flying instruction. He is an established author, his current projects being an instructors’ manual and a collection of Air Accident analyses, called Choose Not To Crash. Visit Jim's website by clicking here.

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What does the red line on your ASI mean? It seems to say bad things might happen if the pointer goes past that mark. Jim Davis disagrees. Here he explains why things can sometimes go seriously wrong way before the needle gets near the red.
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