The 800 Knot Bonk

This article is published with the kind permission of and is copyright,

Jerry Lee
Test Pilot - BAE Warton 1973 - 1982
Chief Test Pilot - BAE Warton 1983 - 1985

Article first Published in JETS Winter 2000

To dispel any misconceptions which might arise, the term `bonking' or ‘Firing bonkers' has been around in the test flying business since the stressmen and aircraft structural engineers decided that an aircraft's structural integrity could best be demonstrated by firing small explosive charges located around the aircraft's structure in flight to measure the response to the shock and the structural damping that resulted. 

Bonking had always been a popular way  to excite structural response by the engineers at Warton well before any other connotation had been attributed to the word. One summer's day it fell to me and my back seater crew, Les, to make the final runs of the Tornado ADV's structural test programme by carrying out the 800-knot bonk.

The Chief Test Pilot at that time was a bearded ex-Navy pilot in the Captain Birdseye mould. He cheerfully proposed that l should carry out this flight accompanied by Les as if it was the greatest privilege on offer within his gift. It was this which convinced me that I had drawn a short straw, and I had. So had Les for that matter.

The Tornado aircraft representing the test vehicle was the first of the F2 prototypes (ZA254). It had had a short but extremely tough life and it showed it, like successful rock stars look when they reach their 50th birthdays. It looked as it had been through the hardest and most exacting of times. It had been pulled and pushed to the limits, flown to the extremes of the envelope where no production Tornado will ever need to go, kinetically heated at the highest of speeds in the bonking programme. Its paint had flaked off in the most exposed of places. Bits and pieces had been bolted onto it and later removed. Outside and inside it had been subjected to touch-up repainting which gave it a lived in look.

It was inevitably fitted with bonkers and miles of instrumentation wiring. It represented the peak of sophistication as a test vehicle. The downside was that because it was such a valuable working test asset, it was what is known in the trade as in a very low `mod state'. Together with its early prototype siblings, it had been responsible for the development of a number of small but essential modifications to the aircraft structure and systems which would be incorporated in to the production aircraft to make all the difference between a smooth five-star product and a rough no-star poor buy. All these modifications would pass it by. Its RB199 engines were similarly lacking in endowment.

All this meant that the aircraft could be quirky in terms of irksome limitations and required some extra action by the crew to work around the low mod state. The telemetry team, whose leader worked the radio link with the aircraft and was known as `Boffin', were well versed in the quirks and foibles, and they seemingly monitored every conceivable aspect of the aircraft and its behaviour during bonking flights. Boffin briefed Les and me on the flight we would make to complete the structural test programme to the fastest speed the Tornado was designed to go. Basically Boffin and the structural test engineers wanted us to fly to high supersonic speed at altitude then descend to progressively thicker air to reach increasingly higher indicated airspeeds, firing the bonkers at specific airspeed test points, culminating at 800 knots at Mach 1.4 at around 2000 feet over the sea. When the bonkers fired the structural damping of the aircraft would be measured straight away `in real time' on pen recorders in the telemetry room, to be matched with the predictions of the flute music men, and a mass of data stored for them to do detailed work on later.

It was a massive example of co-ordinated teamwork involving the two aircrew, upwards of a dozen test engineers, an RAF tanker to provide in flight refuelling of the Tornado on what was a fuel-thirsty exercise, and air traffic controllers linked to the telemetry station to co-ordinate the refuelling, clear the airspace and control the Tornado down the supersonic flight corridors over the Irish Sea.

Good weather was essential for the last of the bonking runs as the Tornado crew needed to have visual contact with the sea throughout the dive for the final test, simply because the altimeters in the cockpit were subject to a large error when supersonic (another example of the low mod state).

it being a good summer of sunny day after sunny day, the weather was perfect for the very day of the final briefing, so Les and I took to the air on cue to climb up to meet up with the tanker to top up with fuel then climb up for the first run.

On the climb things were quiet. Boffin tested all his monitoring kit and watched the air temperature as we climbed into cooler air. He always did this on a bonking trip to find out where the cool layers were located, if there were any, and at what precise altitude the tropopause, which divided the troposphere from the stratosphere, was sitting. All this information was useful for working out how best to achieve a rapid acceleration to the highest speeds required as this was best done in the coolest air available.

Apart from routine air traffic instructions to guide us to the tanker the radio conversation between Boffin and us revolved around outside air temperatures which both Boffin and Les recorded to assess our best acceleration profile. To anyone from the outside world listening in on our radio frequency they would have sounded like a couple of demented weathermen.

We soon reached the tanker's altitude, deployed our refuelling probe and hooked up for a refuelling top-up. We stayed hooked up gently taking on fuel until we arrived close to the supersonic test run corridor then disengaged, retracted the probe and climbed to the tropopause for the first acceleration. We accelerated and flew down an ever steepening dive to reach the first test point. At which point Les would call on the radio, "Stand by to bonk! Three-two-one bonking now!', and fire a bonker.

Boffin would reply, "Bonking copied, you are clear to continue." The team in telemetry would have examined the pen traces and deemed the damping satisfactory. The flute music men would be nodding and grinning.

Les and I climbed in a steep zoom and decelerated to subsonic speed. Air traffic gave instructions to pick up the tanker again for some more fuel. We refuelled once more and stayed hooked up for the flight back to the top of the supersonic corridor.

The next test was to be the big one, where we would fire the bonkers at the two final high speed test points. We disengaged the tanker once more, positioned at the beginning of the supersonic corridor and started the supersonic run.

As we accelerated in the ever-steepening dive towards the sea just as Les was preparing for the penultimate bonk, the emergency warning klaxon sounded and the red attention-getter lights flashed. The warning panel showed a light which indicated that the canopy was loose on its mountings. There was a loud roar as its seal deflated, the air-conditioned cabin air supply shut off and hot air started to come into the cabin from outside under the canopy rail. The noise was deafening even when wearing a sound deadening flying helmet. Les, who would really be poorly placed if the canopy were to come off, ignored the disruption and yelled over the radio, "Stand by to bonk! Three-two-one bonking now!"

Shortly afterwards Boffin yelled back. "The bonk was good, you are clear to continue!"

We carried on down our steep dive in noisy and increasingly hot conditions. The only reason we were suffering was because we were in a low mod state aircraft. On later aircraft the canopy didn't lift and cause bedlam.

Kinetic heating made the conditions like a sauna in the cockpit. Just as Les was preparing for the final 800-knot bonk and I was peering out through the smoke assessing our altitude over the calm sea, Boffin yelled, "Both your engines have oil fires!"

l yelled to Les, "Go for it Les!"

Les yelled back, "Stand by to bonk! Three-two-one bonking now!"

I cut the engines slowly to idle power and zoomed steeply to 30,000 feet. The Tornado climbed like a homesick angel, as the saying goes. I reset the canopy warning and retrieved the normal flow of cool air-conditioned air.  We were both sodden, with insufficient fuel to find the tanker or to do much else. We hoped that the engine gearbox oil fires would decide to go out soon before all the oil got burnt. Again we faced a problem which would be eliminated on production engines, which would not over pressurise the gearbox and thus cause the oil to overheat and catch fire. We were soon over the top of RAF Valley on Anglesey when Boffin announced, "I've got some good news for you. The fires have gone out!"

I checked the engines, closed the throttles to idle once more, and we made a gentle glide for Warton to cross the hedge and touch down at a minimal fuel state.

We taxied in and shutdown. On climbing down the ladder the air smelt pure and sweet of the airfield grass. We were suddenly made very conscious of just how hot and dirty we were as we took off our flying helmets.

We were surrounded by ground crew and members of Boffin's team, amongst whom was one of the most vocal and happy-go-lucky Rolls Royce field support engineers, called Chris, who in his piratical West Country burr said, "Were the engines all right Jerry?"

I confess that I threw my flying helmet at him. Chris has always remembered the incident vividly, and recalls it at dinner parties and other social occasions to this day.

(C) Jerry Lee