The worst case of turbulence?
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The worst case of turbulence?
I was chatting to a friend (now flies for BA) about various flying trips I'd done and was explaining about a trip a did back from Scotland down the East coast once. The forecast was not brilliant (and with more experience now, I wouldn't have done the same trip!). Winds were forecast 50kts over the highlands, gradually decreasing further South. Cloud base was approx. 3000. (I had no IMCR at the time so was stuck under).
We'd just passed North Berwick, a few miles inland from the coast. The trip had been relatively uneventful and the further South we got, the more relaxed we were until the aircraft suddenly dropped with a huge sounding "BANG". The drop was so severe, I was lifted out of my seat, hit my head on the roof, headset came off etc. We were both suitably shaken but unhurt by it.
The friend I was explaining this to had never experienced anything like this (certainly not in a light aircraft). It made me wonder what the very worst cases of turbulence might be - and also what might be dangerous to the aircraft and me!
We'd just passed North Berwick, a few miles inland from the coast. The trip had been relatively uneventful and the further South we got, the more relaxed we were until the aircraft suddenly dropped with a huge sounding "BANG". The drop was so severe, I was lifted out of my seat, hit my head on the roof, headset came off etc. We were both suitably shaken but unhurt by it.
The friend I was explaining this to had never experienced anything like this (certainly not in a light aircraft). It made me wonder what the very worst cases of turbulence might be - and also what might be dangerous to the aircraft and me!
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From the above report
The lack of precise evidence concerning the pilot's intentions and G-BVNA's flight path and speed after it turned inland makes it impossible to define the combination and degree of gust loading and manoeuvre loading which caused the failure of the wing. However, two related aspects may be relevant.
Firstly, although the workmanship in G-BVNA appeared to be of a good standard, a number of experienced aircraft engineers commented on the design of the wing structure. One observation was that, with the main wing spar not occupying the full depth of wing, the leading edge structurewould make little contribution to the tors ional stiffness of the wing and to the stability of the main spar. Another observation concerned the wing ribs, which were of simple 'trapezoidal' construction and appeared to have low in-plane stiffness. As the AAIB have not been able to contact the original designer, it has not been possible to discuss with him the structural design and whether, the static load test in Hungary was with a wing identical to that in G-BVNA.
i.e. hard to say... there are probably a lot of planes in this category which might not be that hard to break.
The lack of precise evidence concerning the pilot's intentions and G-BVNA's flight path and speed after it turned inland makes it impossible to define the combination and degree of gust loading and manoeuvre loading which caused the failure of the wing. However, two related aspects may be relevant.
Firstly, although the workmanship in G-BVNA appeared to be of a good standard, a number of experienced aircraft engineers commented on the design of the wing structure. One observation was that, with the main wing spar not occupying the full depth of wing, the leading edge structurewould make little contribution to the tors ional stiffness of the wing and to the stability of the main spar. Another observation concerned the wing ribs, which were of simple 'trapezoidal' construction and appeared to have low in-plane stiffness. As the AAIB have not been able to contact the original designer, it has not been possible to discuss with him the structural design and whether, the static load test in Hungary was with a wing identical to that in G-BVNA.
i.e. hard to say... there are probably a lot of planes in this category which might not be that hard to break.
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Happened to me in the heatwave of '76. I was flying a glider at about 500' on final when a sudden bang pushed me down violently enough for dirt to rise up from the floor.
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It's funny that this subject is brought up, as I've been thinking about this. Yesterday lunchtime I observed a low-wing SEP pass through the Edinburgh Class D, East-West, at about 1500 AGL (Chez Smithy ~ 400' AMSL), with the aircraft visibly being vigourously rocked back and forth in the (probably severe) turbulence. Couldn't believe that some nutter was up flying, despite the fact that the surface wind at the time was 25 Kt gusting 30-odd. At 2000' probably about 40+. Unbelievable. I would be interested to know how the landing went. Even from my perspective on the ground it was obvious the aircraft was being chucked about good and proper. Not to mention no doubt its occupants 
Round my patch things can get a bit rough if the wind is >15 Kt, plenty hilly terrain in the area to upset things somewhat.
Makes you wonder how severe turbulence can get before the structural integrity of the airframe is threatened. That report made for sobering reading.
Smithy
Round my patch things can get a bit rough if the wind is >15 Kt, plenty hilly terrain in the area to upset things somewhat.
Makes you wonder how severe turbulence can get before the structural integrity of the airframe is threatened. That report made for sobering reading.
Smithy
It is quite possible that you ran into a mountain wave system. Whist it is well known that flying in the wave is incredibly smooth - even when vertical currents are very strong - it is less well known that under the wave a rotor forms that can be exceedingly rough with complete reversals of wind direction within quite short distances.
Comments on wave flight in Black Mountains at URL below.
Wave flying
The rotor turbulence may be harmful for other small aircraft such as balloons, hang gliders and para gliders. It can even be a hazard for large aircraft; the phenomenon is believed responsible for many aviation accidents and incidents including the in-flight break up of BOAC Flight 911, a Boeing 707, near Mt. Fuji, Japan in 1966, and the in-flight separation of an engine on an Evergreen International Airlines Boeing 747 cargo jet near Anchorage, Alaska in 1993.
Atmospheric rotors are intense low level vortices which form along an axis parallel to, and downwind of, a mountain ridge crest. They pose a serious aviation hazard and have been cited as contributing to numerous aircraft accidents.
Wave flying
I've certainly had my head bashed on the ceiling in PA28 sized aeroplanes, if nothing else it emphasises the importance of having tight lapstraps.
Just working from the certification rules and a bit of basic metallurgy and aerodynamics, a PA28 in download (which you had to bang your head on the ceiling) is likely to break at around -3g, but it is also designed that up to Va, which typically is a little above maximum cruising speed on a PA28, you should in theory hit a negative g stall at around -1.5g, which should more than adequately protect you from structural failure. Knocking yourself out is probably more likely.
That said, G-BVNA's accident discussion does highlight that you could make it worse with inappropriate control inputs - such as an inadvertent hard push on the yoke at the same time. This is pretty unlikely, but I'm pretty certain that if I tried hard enough I could break a PA28 that way.
A light aircraft however should be less vulnerable than a big aeroplane. Firstly we fly slower - giving poorer gust response (although they make up for that by having a higher wing loading, which does the same thing), secondly we have much less mass and so tend to accelerate all at once with the gust, third the g limits of a light aeroplane are typically about 50% better than those of a big jet.
G
Just working from the certification rules and a bit of basic metallurgy and aerodynamics, a PA28 in download (which you had to bang your head on the ceiling) is likely to break at around -3g, but it is also designed that up to Va, which typically is a little above maximum cruising speed on a PA28, you should in theory hit a negative g stall at around -1.5g, which should more than adequately protect you from structural failure. Knocking yourself out is probably more likely.
That said, G-BVNA's accident discussion does highlight that you could make it worse with inappropriate control inputs - such as an inadvertent hard push on the yoke at the same time. This is pretty unlikely, but I'm pretty certain that if I tried hard enough I could break a PA28 that way.
A light aircraft however should be less vulnerable than a big aeroplane. Firstly we fly slower - giving poorer gust response (although they make up for that by having a higher wing loading, which does the same thing), secondly we have much less mass and so tend to accelerate all at once with the gust, third the g limits of a light aeroplane are typically about 50% better than those of a big jet.
G
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It is quite possible that you ran into a mountain wave system
I recall we were about 1500-2000ft agl, winds were circa 30kts so if we'd been a couple thousand feet higher, would probably have been ok.
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Worst turbulence - just outside Omarama in a duo discus, attempting a rotor climb in the lee of a big mountain range. Regularly off the seat, but too well strapped down to hit the roof. Utterly intimidating. At times the glider was almost 'fizzing' with some very strange oscilations where both tips were in lift, the middle in sink and vice versa. Most peculiar.
Spent the first 30 mins waiting for the wings to fall off and the next wishing they would - was quite miserable. Once the wave was contacted it was surreally smooth, and well worth it! In SEP terms, midday thermals over outback aus were pretty uncomfy, but nothing major. Worst big plane ride into Denver international one windy (50kt) day. I'd guess 'moderate' turbulence in ICAO classification, lots of crying and vommiting down the back, occasionally nearly off the seat.
Spent the first 30 mins waiting for the wings to fall off and the next wishing they would - was quite miserable. Once the wave was contacted it was surreally smooth, and well worth it! In SEP terms, midday thermals over outback aus were pretty uncomfy, but nothing major. Worst big plane ride into Denver international one windy (50kt) day. I'd guess 'moderate' turbulence in ICAO classification, lots of crying and vommiting down the back, occasionally nearly off the seat.
I need to be (windspeed x 100 ft) above mountain or hill.
The official UK record height in a glider in the UK (in wave) is 11,570 metres (37,958 feet) in 1995. I'm not convinced the wind speed was of the order of 379 kts!
P.S.: World height records in gliders are 12,637 metres (41,459') for a woman and 15,460 metres (50,721')for a man. Both in wave systems.
Last edited by Jim59; 12th Jul 2010 at 11:52.
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I think a crude rule is to fly 1000ft above ridges for every 10kt of wind aloft, for any downdraughts to be smaller than 500fpm.
This has always worked for me when crossing the Alps for example, which I normally do ~ FL180 (on oxygen) which is ~ 8k above the peaks (on the relevant routing) and I have never seen any significant turbulence for wind aloft values of say 20-30kt.
This has always worked for me when crossing the Alps for example, which I normally do ~ FL180 (on oxygen) which is ~ 8k above the peaks (on the relevant routing) and I have never seen any significant turbulence for wind aloft values of say 20-30kt.
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Far from it! That may work as an approximation for avoiding low level turbulence but it won't work for wave (and associated rotor). Mountain Wave can go as low as ground level and right the way up to the heights described elsewhere. In gliding you soon learn that wherever there's air going up, there'll be air going down too. You don't want to be in the downdraught near the ground or caught in between the opposing layers!
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That's true but there are degrees of "turbulence", degrees of "mountain waves" (which exist anytime there is any wind flowing over ridges), degrees of "rotors" (comment as previous), and degrees of "wind shear" (which exists anytime one is climbing or descending with any wind present whatsoever).
Before I did my first flight over the Alps in 2004, I got all the usual dark warnings about "killer mountain waves". I think most of them were from armchair / pilot forum pilots. But knowing no better, I first flew to Wangen-Lachen in Switzerland and sat there for about 5 days, eating £20 sandwiches, waiting for wind-free conditions. Then we did the flight... being VFR, Zurich would not (completely pointlessly, as far as I can tell) let us into their FL130-base Class C so we flew most of the way across at FL129, getting nice pics like this, and sure enough it was smooth. But other flights followed, and the other week I got this pic from about 5000ft above the terrain, with I guess about 20kt wind, and it was smooth too.
So I think reasonable rules can be applied.
What you do not want to be doing is crossing mountain ridges while flying at your operating ceiling
Then, any downdraught is going to force a descent.
Before I did my first flight over the Alps in 2004, I got all the usual dark warnings about "killer mountain waves". I think most of them were from armchair / pilot forum pilots. But knowing no better, I first flew to Wangen-Lachen in Switzerland and sat there for about 5 days, eating £20 sandwiches, waiting for wind-free conditions. Then we did the flight... being VFR, Zurich would not (completely pointlessly, as far as I can tell) let us into their FL130-base Class C so we flew most of the way across at FL129, getting nice pics like this, and sure enough it was smooth. But other flights followed, and the other week I got this pic from about 5000ft above the terrain, with I guess about 20kt wind, and it was smooth too.
So I think reasonable rules can be applied.
What you do not want to be doing is crossing mountain ridges while flying at your operating ceiling
Then, any downdraught is going to force a descent.
I've just checked some of my glider's GPS flight recorder files for flight in mountain wave in Scotland, in the lee of the Cairngorms that are about 4,000' high.
I don't do much wave flying so I would guess that these figures are frequently bettered. The wind would probably have been about 40 kts at flying heights.
Climbing between 8,000 to 9,000' achieved around 1,000' per minute.
Climbing between 14,000' and 15,000' achieved around 600' per minute.
The glider used would have been sinking at at least 150' per minute at the speeds being flown so the air was rising at 1,150 and 750 ft/min respectively.
Sink at altitude was of similar magnitudes - although I did try to stay out of it!
I don't do much wave flying so I would guess that these figures are frequently bettered. The wind would probably have been about 40 kts at flying heights.
Climbing between 8,000 to 9,000' achieved around 1,000' per minute.
Climbing between 14,000' and 15,000' achieved around 600' per minute.
The glider used would have been sinking at at least 150' per minute at the speeds being flown so the air was rising at 1,150 and 750 ft/min respectively.
Sink at altitude was of similar magnitudes - although I did try to stay out of it!
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You can sometimes see the mountain waves in the high level clouds, often 50 miles or more downwind. I've seen this in the lee of the San Bernadino mountains in LA when a strong Santa Ana wind has been blowing, and the (severe) turbulence has existed as far as Catalina Island. Makes for interesting flying.
There was a case in the US of a glider / tug combo meeting a mountain wave. The tug disintegrated, the glider landed safely and it is estimated that the g forces exceeded 14g.
There was a case in the US of a glider / tug combo meeting a mountain wave. The tug disintegrated, the glider landed safely and it is estimated that the g forces exceeded 14g.