Three witnesses all made substantially the same statements. They saw the wings of the aircraft flexing and oscillating up and down along the entire span and it was also longitudinally unstable. They also noticed the wheel bay doors opening and closing in time with the pitching oscillations.
- This report is to promote aviation safety and not to establish legal liability.
- The CAA’s report contains padding, repetition, poor English and incompetence. In the interest of clarity and readability I have corrected and paraphrased extensively.
The chute was deployed but failed to blossom and appeared to suffer shroud line failure. The resultant drag appeared to pitch the aircraft’s nose up; it dropped its right wing and dived vertically into the ground.
The engine, airframe and propeller had done only 15 hours since new.
The pilot was fatally injured. The post mortem found multiple rib fractures, lacerations of the heart, fracture of the spine at three places, crushed cauda equine at lumbar spine No.2. Cause of death was a blunt trauma to the chest.
Aircraft: powered glider Pipistrel D.O.O Taurus
Date of Accident: 10 August 2012
Time of Accident: 11:06Z
Pilot-in-command Licence Type: Glider / PPL
Licence Valid: Yes
Pilot-in-command Flying Experience: Total Flying 80.2 Hours on Type 80.2 Hours
Last point of departure: El Mirador (FALQ)
Next point of intended landing: El Mirador (FALQ)
Location of the accident: Game reserve in Winterton.
Meteorological Information: CAVOK wind 350/04
Number of people on board: 1 + 0
Number of people injured: 0
Number of people killed: 1
This accident is all about Vne and flutter which are both huge and complicated topics. I will keep it as short and simple as possible and stick to what pilots should know in order to recognise and avoid the conditions that can cause flutter.
What is flutter?
In its simplest form flutter is what a flag does in the wind – it doesn’t just stream out smoothly – it flutters. Sometimes in big waving movements, and sometimes in fast ripples.
Actually, there are two types of flutter. The first is a very rapid twisting of the wing, around the main spar. As the rear rises the leading edge dips – and vice-versa, as shown in the diagram. It is often so rapid that it is sometimes described as a buzz.
The second type, called divergence, is most common with long, thin, glider-type wings. It’s a much slower, spanwise flapping of the wings, and this centers about their attachment points to the fuselage. This is what happened to this aircraft
What causes it?
It’s caused by the aircraft’s structure – particularly the wings and control surfaces, being a compromise between weight and structural rigidity. To keep the aircraft light, it sacrifices stiffness. This means that aerodynamic forces can make the wings twist around the main spar, as in the diagram, or they can make the wings flap like a bird – which happened to this aircraft.
- A minor disturbance or gust lifts the wing and the aileron’s C of G gets left behind causing down aileron.
- As the wing twists about its tortional axis X, this deflects the aileron down further due to the ailerons C of G being left further behind.
- When the wing reaches its twisting (tortional) limit, the aileron’s C of G gets flung up.
- This causes the rear of the wing to move down and the whole process starts again more violently.
- The wing again twists to its elastic limit, and when it stops suddenly, the aileron’s C of G swings down, causing the wing to start twisting the other way.
Aileron flutter can start in a micro way as trim tab flutter. This happened in a catastrophic way in September 2011 when a P51 Mustang at the Reno air race had flutter on an elevator trim tab. The aircraft became uncontrollable and flew into the crowd, killing the pilot and ten spectators, and injuring another 64.
‘Flutter is relative to TAS, not IAS’
The tendency to flutter increases with:
The tracker shows the Pipistrel Taurus was doing 236 km/hr in the circuit, which was 11km/hr past the flapless Vne. However, when he extended the flaps, that put him 95 km/ hr past the new reduced Vne, and divergent flutter set in immediately.
Increased TRUE Airspeed.
Strangely flutter is relative to True air speed and not indicated air speed. This is because at high altitudes, TAS is considerably faster than the reading on your ASI, but the air is thinner and has less damping effect.
On 1 April 2010 a young charter pilot was descending from FL95 towards Swakopmund in a Cessna 210. Apparently, without warning the aircraft suddenly came apart and scattered itself over a massive area – the wings no longer being attached to the fuselage. The wreckage showed indications of flutter. I haven’t seen the accident report, but my guess is that the pilot simply allowed the airspeed to run away during the long descent.
Low air density/high altitude.
Your POH states the aircraft’s maximum certified altitude. As long as you stay below this, and your aircraft’s rigging is tight and in good condition, and you keep your IAS below the red line, in calm air, you should not get flutter.
The most common type is aileron flutter. The diagram shows how it works:
Above that height a slower red line would be needed. In fact, gliders, which can operate up to almost any altitude, do exactly that – they give you lower Vnes for each block of altitude. Here’s a chart for a Pipistrel Sinus powered glider showing massively reduced indicated Vnes at altitude.
So, in certified, powered aircraft, the FAA makes it easy by saying that in this aircraft’s block of air (say sea level up to 15 000’) the red line (Vne) will keep your TAS in safe limits.
WARNING Some NTCA manufacturers test their aircraft as rigorously as normal aircraft, but many don’t. So not all NTCAs will give you the same flutter protection as a certified aircraft. A while ago a pilot and his navigator were descending their Flamingo (NTCA) towards an air-race checkpoint near Bella-Bella, when it broke up in flight. A witness said it looked like a quick puff of smoke and then the wings ripped themselves into what looked like little scraps of paper. Both occupants died.
Balance weights are critical in limiting the tendency to flutter. I have just heard that the ex DCA (CAA) Beaver, ZS-CAJ, which was sold to a company in Canada, almost destroyed itself in a violent flutter while flying at normal airspeed in calm air. It was so badly damaged by the flutter that it was scrapped after landing safely. It turns out that during maintenance an aileron balance weight was left off.
Some time ago a Cherokee in the USA 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 preflight.
And in April 2005 two pilots died near Stellenbosch 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
Worn hinges and slack cables.
Any slop in the operating system will allow flutter to start at way below the red line. I looked at a C210 that was used by a parachute club. You could move one aileron up and down at least two inches before the stick, and the other aileron moved. It developed a flutter at just over 100 kts which ripped one aileron right off. Fortunately he managed to land safely.
‘any slop in the controls will cause flutter’
TAKE HOME POINTS TO MAKE SURE IT DOESN’T HAPPEN TO YOU.
- Start with a proper preflight. Pay special attention to the attachment of mass balance weights. Check the control surfaces for damage and all the hinges for excessive play. Trim tabs are just as important – flutter can start with a sloppy trim tab. Check the integrity of control surface operating rods, or cable attachments and tensions. Move one aileron up and down and see if the control column and other aileron move immediately. If anything has play, it’s potentially dangerous.
- 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. Air races are potentially dangerous if a checkpoint is just beyond high ground.
- 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.
- If you feel a slight hum or buzz, take this as a serious warning that flutter could start at any second. Throttle right back and ease the nose up immediately – but not violently.
- Similarly if you get 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.
- Don’t even think of modifying anything to do with the controls – or even painting them.
- 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.
- If you operate outside the aircraft’s design parameters you are looking for trouble.
- Turbo-charging or increasing engine power may not be smart.
- When your aircraft was brand new it was tested to only 10% past the red line. After 40 years of wear and tear perhaps it’s not a good idea to find out if that is still valid?