Thursday, 10 March 2016

Fifty years since the loss of a BOAC Boeing 707 over Mount Fiji, and some thoughts on mountain wave

It is 50 years on 5th March that BOAC lost Boeing 707-436 G-APFE which broke up over Mt. Fuji with the loss of all on board. The aircraft had just taken off from Tokyo and the crew routed VFR off-airways to give the passengers a view of Fuji. They flew into rotor shear downwind of the mountain and the 707 broke up. I remember this well as the day before a CP Air DC8 crashed landing at Tokyo and there was a poignant photo in the papers of FE taxying out at Tokyo past the DC8 wreckage. 18 minutes later that aircraft was also wrecked. 

Ill fated Boeing 707 G-APFE taxys out at Tokyo past the wreckage of a DC8 that crashed on landing the previous night

The mountain wave rotor induced vertical 'G' loads far in excess of design load (probably in the region of minus 8'G'). The wings outboard of the engines detached, as did all 4 engines which continued ahead powered by the fuel in the pylons and were found some distance ahead of the main wreckage. Later in the break-up sequence the forward fuselage, rear fuselage, tail, more wing parts, and control surfaces all separated from the centre section. The upward gust produced enormous induced drag which caused severe deceleration which ruptured the fuel tanks' forward faces. It was enough for the fuel in the centre section tank to slosh forward, break the front of the tank, and carry on until it reached the forward pressure bulkhead, just ahead of the flight deck. Thus the flight deck filled with fuel, drowning the crew (hopefully they were unconscious by then due the 'G' loads).

The centre section and part of the wing goes down, trailing fuel as white vapour

An absolutely horrific accident which shows the power of airflows around mountains. 

A far less serious, but nonetheless scary, personal brush with rotor turbulence

Some years ago flying to Caernarfon in the Chippy we were caught in rotor downwind of Snowdon. I was not expecting it because the surface wind at Caernarfon, a few miles away, was less than five knots (in the Fuji accident the summit winds were in excess of 70 knots on the accident day). I have never know turbulence like it - it was impossible to control the aeroplane; it was in the hands of the elements, being thrown violently up, down, and rolled in spite of any control inputs I made (not many as my backside was, despite a tightened full aerobatic harness, not in contact with the seat most of the time). I remember wisps of cloud forming and dispersing around us. Then it spat us out, and normality returned as rapidly as it had left us. The wind over Snowdon must have been far higher than the 5 knots Caernarfon were giving. I suspect an inversion was causing a 'vortex', funneling the wind through a narrow gap between the air masses over the summit. A similar meteorological effect caused the crash of a friend of mine in a Cessna 150  in lee-side sink into the flank of Kinder Scout. He and his student survived with minor injuries; the aeroplane was wrecked.

Link to crash of G-BFRP kinder Scout, 1983

Interesting discussion on lee-side wave and rotor

Lee-side wave, causing lift and sink, and also in the right conditions, vicious rotor in the shear between the air layers below the summit is a fascinating subject. Glider pilots exploit wave to soar to amazing heights, many times higher than the mountains originating the wave, and indeed I have often exploited this in the Chippy over Shropshire in the lee of the Welsh hills.

Here's a great description of wave and rotor from an experienced Glider and Tug pilot in Canada, and a video of wave and rotor over Fuji:

I have many years of gliding and towing in the lee of the Canadian Rockies. The rotor occurs downwind of the upgoing air, underneath the "crest" of the wave, where there is shear between the wave airmass and the underlying air. The rotor is often marked by very ragged, wispy, dark clouds, that are visibly rotating. 

The rotor can be extremely rough and quite violent. A very experienced tow-pilot friend of mine used to say "The rotor's not rough until you get rolled inverted!" 

Typically, the rotor only occurs at or below the ridge level of the mountain range that is generating it. The largest gust "jolts" I have experienced while flying commercially in the western US and Canada, have been while climbing or descending through the altitude of adjacent mountain ridge lines, on windy days. Based on my "Chipmunk calibrated posterior", I estimate some of them to have been ~2G. Quite scary for non-pilot passengers.

A typical wave-tow consists of flying up-wind through the rotor until you reach the upwind side and then turning to track parallel to the mountain front and climb in the disorganized lift until the air becomes magically smooth. At this point, the glider releases and by the time the towplane has done a 180, the glider is a 1000' or more higher.

The laminar flow in a wave is uncanny. Often the only indication in a glider that you are moving, is the altimeter winding upwards.

PS I've never towed into a wave associated with isolated peaks, but I've seen the so-called "UFO" lenticulars that form downwind of the Cascade volcanoes, like Mt. Rainier. I would image the rotor associated with those waves might be even more violent, because of air coming around the mountain, as well as over it.

Here is a nice time-lapse video of cap clouds and lenticulars at Fuji, although the labelling at the beginning is wrong. What is labelled as "Rotor Cloud" is the stack of lenticulars, marking the laminar wave. The actual rotor can be seen as fast moving wispy clouds, just above the foreground ridgeline. There is a good example at 0:25:

Time lapse of wave and rotor over Mount Fuji

It is more than likely that the day of the BOAC accident would have been a "blue wave day", where there was not enough moisture in the airmass to form clouds, so there would have been no visual warning for the crew of what they were flying into.


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