This chap posted on the Kiwi Pilots Facebook page about the last two NZ accidents as they both appear to involve strucural issues with the aircraft. Not trying to say this is the cause but it is something for us pilots to be aware of at this time of year especially. Lets be careful out there:
"After 2 incidents in 2 days involving possible inflight technical problems I'm including here an article I wrote. In summer months with turbulence from strong thermal activity, airspeed can be critical and pilot use of controls important. Hope you find the article interesting.
FACTS ABOUT “ROUGH AIR MANOEUVERING SPEED”
A few years ago I was amazed to read the accident report of an American Airlines A320 which lost its tail and crashed in New York after the co-pilot used excessive alternate rudder inputs when they encountered wake turbulence from a 747 they were following. The A320 aircraft was flying in the green arc on the ASI, still in climb and is one of the most reliable aircraft ever designed, with safeguards designed to protect the aircraft against pilot error. In this accident they were flying within the design Rough air Manoeuvering speed. That got me into thinking that if such massive structural damage can occur to a new airliner, what about our General Aviation aircraft? Are we right in what we teach student pilots? What effects will manoeuvers such as wing drop stalls, maximum rate turns, and side slipping have on the aircraft structure if performed in bumpy conditions? I then began to think especially about the effects on the airframe when flying in strong turbulence, because that’s when I have experienced the worst conditions. We were taught that when flying in the green arc of the ASI we can use full deflection of our controls without damaging the aircraft. As long as the needle isn’t in the yellow arc we feel safe no matter how hard the turbulence feels. How wrong we are. We are also taught to study the Flight Manual of an aircraft when getting a type rating and we are taught that the Va—Design Manoeuvering Speed. "Is the maximum speed at which the limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage". As it turns out, when flying in turbulent conditions, and here in New Zealand we frequently experience severe turbulence in strong westerly conditions in the lee of hills or mountains, or in summer under strong cumulus clouds, we can induce severe damage to the aircraft structure when flying at the top end of the green arc. The certification regulations do not require an aircraft design to withstand a full deflection of a control surface in one direction followed by a full deflection in another even when operating below VA which is meant to be the maximum speed at which the pilot can make abrupt and extreme control movements and not overstress the aeroplane's structures. In designing an aircraft the regulations don't require it to be designed to withstand the forces caused when two or more control surfaces are simultaneously moved to their stops. The recent studies have shown that such control movements will create G forces that will overstress the aircraft. The latest definition which changes our entire understanding from what we were taught is "The Design Manoeuvering Speed (Va) is the speed below which you can move a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane." Unfortunately many flight manuals do not publish a “Turbulence penetration speed” so we either need to calculate it or use a simple rule of thumb to estimate a speed which should give a pilot enough of a margin over stall speed to be comfortable, and if a gust does cause a sudden stall, the wing, at these speeds will recover and no damage will result. A quick rule is to use 1.7 times the published Vs (stall speed) for the weight of the aircraft during that flight. As fuel is burnt during the flight, the aircrafts weight will decrease, and as weight changes the stall speed it must be taken into account. The VA or Design Manoeuvering Speed is calculated by multiplying the stall speed by the square root of the maximum load factor. However you can calculate a new Design Manoeuvering Speed and it is a matter of simple maths using the formula: (Va-New = Va √ (WNew/WMax-Gross). A large proportion of our privately owned GA fleet in New Zealand are either 15-30 year old aircraft or relatively new LSA or Advanced Microlights. All need to appreciate the following. The older GA aircraft may suffer from corrosion or other aging processes which can make the aircraft more susceptible to structural damage in severe turbulence or excessive G force manoeuvers. Many of the newer LSA and Advanced Microlights are built to European weight limitations of 450kgs MTOW and regardless of whether it’s in the LSA or Microlight category the aircraft are practically identical. Even though New Zealand regulations specify a MAUW of 600 Kgs for LSA aircraft, they are still designed to meet the 450 Kg limitation to meet the European regulations. These aircraft are designed and built to be as light as possible and owner/pilots need to understand this and fly their aircraft accordingly. In the same vein any aerobatic manoeuvers including stall turns, chandelles and lazy eights should be totally avoided as the pull out at the bottom of the manoeuver can overstress the aircraft. Many of us have experienced severe turbulence and noted the aircraft Air Speed Indicator needle and the vario jumping all over the dials. These are the times we need to slow down and coach the aircraft through the conditions. As the aircraft ascends in turbulence or a thermal the airspeed will jump rapidly requiring an immediate reduction in power, and the contrary applies in descending air when sometimes full power is required to arrest the descent. I’ve seen a few cockpit canopies cracked and broken when struck by the pilots head so even tight harnesses don’t always hold the pilot in place. For those of us living and flying in hilly or mountainous areas of the country, and that makes up most of New Zealand the conditions I depict are normal flying conditions we experience except on those rare calm days in winter. Our Flight training manuals and student hand-outs need to now be modified to clarify the “Rough Air Manoeuvering Speed” to read "The Design Manoeuvering Speed (Va) or Rough Air Manoeuvering Speed is the speed below which you can move a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane." It is very clear that if the pilot was to deflect the ailerons and elevator at the same time, added by a kick on the rudders to avoid an in-flight upset in strong turbulence, there is a strong chance the aircraft will be bent. As for me personally I’ve always hated getting bounced around, and have always reduced speed to just above VFE (flap extension speed) in strong to severe turbulence, and it takes considerable use of the throttle to control the airspeed. It’s a much more comfortable ride and I know my aircraft isn’t suffering.
I'll post it in the Civil aviation section separately as well.
I presume the “speculation” you talk of is the RV7 Accident Report I asked Dave to post for me.
In case you wondered, it was sent to me by a friend… I didn't go looking for it.
I asked Dave to do this as I have a lot on my plate at the moment and just didn't have the time to attempt to work out how to post the Report to this site, however both of your comments need addressing so I have now no option but make the time… . This Report has nothing to do with speculation as to the cause of this tragic accident of ZK-DVS, but it does have many similarities that until I read this Report I was not aware of the possible consequences – consequencs that cannot wait for the CAA Accident Report to come out on this aircraft.(normally a couple of years). Simply put, those couple of years could cost more lives!
The first similarity I noticed was both ZK-DVS and G-GNDY had gone through their test flying period prior to the final coat of paint. Off the top of my head, I know of at least two RV's that have done the same, and I guess there are many more – and this is not only limited to RV's.
As the Report points out, once the final paint has been put on, this can cause an imbalance in the originally balanced control surfaces requiring a need to remove and rebalance them. Failure of doing this can lead to the lowering of the speed where flutter can be expected and as you know – flutter is something to avoid at all costs! This of course is only relevant to aeroplanes capable of the speeds that exceed something like 150 knots (was once told the speed but now can't remember!) This then also extends to all those aircraft that have balanced controls and have been repainted...and I guess there could be dozens of them out there.
The second similarity was that I noticed that ZK-DVS had also lost most of it's tail. You may wish to call this speculation, but does one keep his mouth shut and just wait for more to die? I'm not saying that this was the cause of the crash especially as witnesses also said one of the main wings had broken off, but of course this could well have been as a result of losing the tail. We will have to wait for the Accident Report to come out to find out the true cause, but in the meantime, I believe it is very important for the word to get out to all those builders/flyers that could have the same potential fault in their plane.
The last thing I wish to see is another inflight break-up such as this
As for my homebuilts (Maranda and SkyLux)…. they don't go fast enough!!!
Last Edit: Jan 3, 2018 20:59:22 GMT 12 by grgrimmer
Yes that is something to be wary of. I've seen aircraft test flown bare metal but those with balanced controls usually had these already painted, probably knowing what could happen post paint to the balance of the controls.
The Auster should be recognised for what it is: a gentleman's aerial touring carriage and a nice aeroplane.
Yes, I understand your concerns Peter as uninformed speculation can be cruel to the families and friends of the deceased. It can be even more cruel if as a result of people not talking there are more that become deceased, when it could have been avoided. Enough said!
Post by Dave Homewood on Jan 4, 2018 15:56:34 GMT 12
Following a phone call from a concerned aircraft owner and good friend of the pilot in the Northland accident, and further discussion with a moderator, we have decided to temporarily close this thread till the accident investigation report is released. If and when you see the report come out let me know and I'll reopen the thread.