- Flying a Chipmunk similar to this one, the author explored mountain waves which were produced by the relatively low mountains in the North of England and in Wales. The aerobatic Chipmunk was a good mount for this task, and these encounters left the author with a healthy respect for mountain waves.
It was an intriguing thought.
My previous gliding experience had been primarily by using thermals or hill lift. I was well used to circling in a thermal, inching upwards with eyes glued to the variometer, trying to keep centered in the rising air. Even in the sleek Blanik sailplane, success was not guaranteed, as British thermals were generally not as strong as those in warmer countries. Imitating the seabirds by using upcurrents in front of a hill could also keep you airborne, but you were generally limited to the maximum altitude that could be reached, not much higher than the hill itself.
Wave systems were strange. The combination of the right atmospheric conditions, wind speed and direction, and mountain topography could set the atmosphere oscillating vertically many miles downwind of the hills, with the effects stretching to much higher altitudes than the hills themselves. Wave-soaring provided the glider pilot with a stepladder to reach heights that could otherwise only be attained within the turbulent and dangerous interior of a cumulo-nimbus cloud. The world absolute altitude record for gliders had stood for many years at forty-six thousand feet, held by Paul Bikle flying in the Sierra wave in California. Even the modest Scottish mountains had produced height gains in the region of thirty thousand feet.
From our house in Preston, the Pennines to the east rose to barely 2,000 feet, while across the Irish Sea to the south-west the Welsh hills were topped by Snowdon at 4,000 feet. To the North the Lakeland hills also rose to four thousand feet. With this topography, I reasoned, wind from any direction might trigger some sort of a wave system, given the correct atmospheric conditions. Waves certainly were not impossible, as during my initial gliding from Hawarden and Sealand, near the Welsh border, occasionally our Sedburghs had managed to connect with lift which could only have been part of a wave system.
My curiosity was aroused. Could I make a systematic investigation to see if usable wave lift existed in our area? The random nature of the wave system meant that it was not really feasible to play a waiting game at the glider fields. However, there was an alternative. I could use a suitable powered aircraft to investigate a possible wave system. Using the Chipmunk I could get airborne from Blackpool within the hour. So I just had to be patient until the weather might indicate that a wave system was forming.
Throughout the early part of 1974 I played a waiting game, studying the weather maps and looking for any sign of the wave. On 25 April an unusual weather system with high pressure to the north resulted in a weak warm front advancing from the north east, giving light rain over Lancashire in the afternoon.
At five o’clock in the afternoon the sky began to clear and as the layer of stratus broke, a single bar of lenticular cloud appeared, stretching from north to south between Lancaster and Preston, maintaining its position despite the easterly wind.
This looked like a good opportunity to check out my theories. I was airborne from Blackpool within the hour and as the Chipmunk climbed eastwards I could see two well-defined lines of multi-layered lenticular clouds which had now formed upwind of the original bar. As I climbed above the tattered layer of stratus that hung over the coast a further flat lenticular was revealed over the port wingtip, stretching southwards down the coast from Fleetwood. This thin veil floated at 6,000 feet, but there was no lift discernible in the immediate area.
Over the nose I could see the most spectacular cloud sitting over Preston. Its base was at 7000 feet and the cloud rose in a stack of six pale gray saucers, linked laterally by bars of cloud to a second stack. It was an awesome sight. Further north, towards Lancaster, I could see another stack of lenticulars partially visible, with the upper reaches lost in an ill-defined curtain of stratus. I could not climb over Preston because of the proximity of Amber One Airway, so I decided to look more closely at the northern cloud.
Flying eastwards between these two towers of cloud, trimmed straight and level at 8,000 feet, I turned gently to port on to a northerly heading along the windward side of the lowest lenticular. Having heard stories of pilots encountering the dreaded rotor cloud associated with wave systems, I checked my parachute harness and snuggled myself tightly into my seat. But initially nothing happened. I was puzzled. This looked like a wave system, but would the facts bear this out?
The air was still glass-smooth, but I suddenly noticed that my vertical speed indicator was indicating a 200 ft/min rate of descent. This was promising. Then the VSI needle flickered into showing a 200 ft/min climb. After a few moments of this intermittent behavior, the VSI needle swung to show a solid 500 ft/min rate of climb.
This was more like it.
As the northern edge of the cloud drifted past the port wingtip, the VSI needle swung further up until it was showing over 1,000 ft/min rate of climb, better by far than the normal rate of climb at full power. Yet here I was, still at cruise power, trimmed for cruise. This was indeed a respectable wave system. The lift continued as I turned downwind round the edge of the cloud, with my rate of climb tapering off until the needle hit zero, then as I emerged clear of the cloud the VSI started to show a growing sink rate. At nine thousand feet I could see that the Cock Hill gliding site, deserted on this weekday evening, lay almost underneath the southernmost stack of lenticulars.
I was pleased that I had confirmed the existence of the wave system. It might be months before conditions were right again for the wave to form, but I had confirmed it was there.
Late into the evening the lenticular clouds glowed eerily high in the sky, but the next day dawned to reveal a cloudless sky.
The wave had gone…
On another occasion, later that summer, the sun was glinting on a smooth sea as I climbed away from Blackpool airport for an evening session of aerobatics. A light south-westerly wind was enough to fill the sails of the yachts scurrying about the Ribble estuary. The sky was peaceful, marred only by a broken layer of stratus at about 5,000 feet. My climb-out northwards along the coast was uneventful under the cloud until abeam Blackpool Tower when passing through 2,500 feet the Chipmunk hit a patch of turbulence. The motion was sharp-edged and random, fitting perfectly the characteristic “cobblestone” description of clear-air turbulence. It was so unlike the normal convective turbulence that I began to suspect at once that this was some sort of rotor or shear flow. But after a few seconds of being jolted around, the air smoothed out completely.
As the Chipmunk passed out into the sunshine once more, I automatically scanned the instruments: airspeed seventy knots, engine temperature and oil pressure normal, vertical speed indicator zero…Zero? I performed a rapid double-take, but zero it stayed. The altimeter confirmed the situation. The Chipmunk was suspended in space, still in a climbing attitude with the Gipsy Major still putting out full power.
Suspicion was now hardening into certainly. I must be in the descending air of a mountain wave system, even though the nearest hills upwind of me were forty miles away in North Wales and the winds seemed too light to support the formation of a wave system. I turned downwind, tracking across the gap between two clouds and barely maintaining altitude. Approaching the upwind edge of the next cloud, I surmised that the air must be starting to rise again and I banked the Chipmunk round until I was flying parallel to the cloud.
I was watching the needle of the VSI like a hawk. After a few seconds the needle reluctantly left the zero mark, inched up to show a climb rate of 200 ft/min and then, as smoothly as the sweep hand of a clock, swung up to a sustained 800 ft/min rate of climb. The Chipmunk shot upwards like a champagne cork past the thin layer of cloud. As the altimeter passed 7,500 feet the rate of climb peaked at 1000 ft/min, almost double the still-air rate of climb at this altitude. By 8,500 feet I leveled out and came back to cruise power. The aircraft was still rising at 400 ft/min.
At this height visibility was excellent and as I turned westwards an awe-inspiring panorama was spread out beyond the cowling. The whole of the Lancashire coast was now visible, with the wave system’s characteristic lenticular clouds forming at different heights. An irregular pattern of glowing bars of lenticular clouds stretched westwards at high altitude across the Irish Sea. Away to the north a stack of saucer-shaped clouds hung above more extensive cloud cover over the Lake District. Yet more saucers hung over the Pennines to the east.
In one sense it was a sobering thought. Had I been a few miles further east over the rising ground and in solid cloud, the torrent of descending air in the down-going side of the wave could easily have pushed the Chipmunk down into the hills. But still the thought of the potential of using these waves niggled at me. On the soonest possible occasion I would try to use the wave when flying a glider.
However, this was not to be. Within a few weeks I was to be plunged into the Tornado flight test program, which left little opportunity for the time-consuming sport of gliding. I wrote up my observations on these occurrences and they were published as articles in Flight International and Sailplane and Gliding.
That was all.
By the time the pressure of the Tornado program had eased somewhat, I was involved even more deeply with powered aircraft, and building an aerobatic biplane. It was to be some years later when I started flying gliders again in California. I only met up with the Lancashire wave one more time, flying a powered aircraft in instrument conditions, and I remember being exasperated by an aircraft that insisted on climbing and descending with a will of its own, until I realized what was causing the problem.
From this flying I was left with a healthy respect for mountain wave systems, and the knowledge that one day I might be able to make use of my experiences.