Wednesday, July 15, 2009

Chapter 7:Flight Testing

The Jet Provost T.5 Flight Testing and the JP 5 production line was at Warton. Much flight testing was completed to sort out the stall and spinning behavior of the jet trainer. Production Flight Testing was one of my assignments.

Photo: John Luke

The Flight Test Department at British Aircraft Corporation’s Military Aircraft Division at Warton was a busy place. In my first few years there we had Canberras and Lightnings, Jet Provosts and the first Jaguars to be flight tested.

Flight Testing was enjoyable, if hectic. I had acquired a taste for this aspect of the design process during my Cranfield course. At that time Roland Beamont was the Director of Flight Operations at Warton; Jimmy Dell was Chief Test Pilot and D.R.H.Dickinson was my boss in Flight Test. My initial tasks were concerned with stability and control trials on the later versions of the supersonic Lightning, but I was also involved with experimental and production test flying on the Jet Provost.

We were without our Chief Test Pilot for some months, as Jimmy Dell was recovering from injuries sustained when he was involved in a landing accident with a two-seat Lightning, 55-710, destined for Saudi Arabia, when it was blown off the Warton runway by a strong crosswind and broke apart at the cockpit. You could not have seen a happier man when Dell returned and flew a Canberra on his first flight back.

Production test flying was another addition to my experience. I have never really considered the final product in practical terms. Once the aircraft was designed, it went through flight testing, then into production. Surely, I reasoned, at that stage the aircraft just rolled from the production line one after another like peas in a pod. Not quite. That was a mistaken assumption on my part. Each aircraft had its own idiosyncrasies, depending on the tolerances of the thousands of parts used in its manufacture. The production shop had the job of fine-tuning the aircraft as they came off the line so that they all performed in the prescribed manner. Production flight testing was the means by which this was achieved.

My initial flying as a flight test engineer was done on Mk 3 Jet Provosts, undergoing acceptance checks after refit. Originally manufactured by Hunting Percival, the single-engined Jet Provost was a nice-flying jet trainer which had its ancestry in the piston-engined Provost which appeared just after the Second World War. It had survived the conversion to jet power quite well and RAF pilots from the fifties onwards had started their careers on the various early versions of the Jet Provost. High altitude navigational sorties in RAF service started exposing the crews to medical problems. More specifically, the students and instructors were being exposed to the bends. This is a painful condition of the joints caused by repeated exposure to low atmospheric pressure, a problem exacerbated by the unpressurized cockpits of these early Jet Provosts.

The logical solution to this problem was to modify the Jet Provost to incorporate a pressurized cockpit. This resulted in the Jet Provost Mk 5. Warton had inherited the Jet Provost project when Hunting Percival had been taken over by British Aircraft Corporation. Most of the design work had been completed at Luton, by Hunting Percival, and two prototypes, XS 230 and XS 231 had been built. By 1968 both prototypes had been moved to Warton. The pressurized cockpit was a fairly major change to the aircraft. This new cockpit gave the aircraft a more streamlined but a more bulbous look around the front end, and the changed airflow altered the handling characteristics to such an extent that a flight test program was required to re-examine the stalling and spinning characteristics of the aircraft.

Stalling problems had been isolated to flow separations at the wing root. It was determined that it would be cheaper to fly the prototypes rather than embark upon an expensive wind tunnel testing program. So a prototype was extensively tufted and flown with a Cessna 172 chase plane, with a photographer taking photos of the tuft movement during low-speed flight so that the airflow patterns near stall speed could be established. The Flight Test Engineer aboard the Jet Provost took a mirror on a stick with him so that he could check on the flow patterns on the fuselage at the wing root and behind the cockpit. It took quite a few iterations in perfecting the new wing root fairings, which came from Warton’s own fiberglass shop. But eventually a suitable fairing shape was found and the flow was tamed. The wing leading edge further outboard was roughened to improve stall warning.

Spinning was another problem. Whereas the spinning behavior of the earlier Jet Provosts had been conventional, on the Mk 5 Jet Provost the spins tended to become oscillatory. These oscillations in pitch gave a very uncomfortable and disorienting ride. Our prototypes were pressed into service to test a number of modifications to calm the spin. During this phase of testing, pilot Reg Stock and Flight Test Engineer John Scutt set a record by carrying out no less than thirty-two separate spins during one sortie. Their tenacity was rewarded. With a number of aerodynamic modifications added, the spinning behavior was tamed, the aircraft was cleared for RAF use and Jet Provost 5s started rolling off the production line.
Come with me during a production check ride of a Jet Provost

The walk across the ramp from the Flight Test building at Warton seemed endless as I walked out to Jet Provost Mk 5 serial XW 364. In addition to my flying kit I was laden down with helmet, life preserver, kneepad and stopwatch, together with a spring balance device so that the aileron forces could be checked in flight. This aircraft was still in its factory finish with serial markings stenciled on the pale green primer paint. It had already flown once and I had noted that a few snags were recorded on its first flight. An adjustment had been made to the aileron trim, to even out the aileron forces, and we were to recheck the ailerons on Flight #2.

Pilot Reg Stock joined me at the aircraft and he conducted a careful pre-flight inspection while I strapped into the right hand Martin-Baker ejection seat which would blast me clear of the aircraft if things went drastically wrong. Reg bantered with the ground crew while I strapped in. The Martin-Baker seat in the Jet Provost was one that I could never be comfortable in. With my six foot plus height I inevitably ended up with my seat fully down so that my helmet did not hit the canopy. Reg quickly strapped into the left hand seat and we went through the ritual of taking out the ejection seat safety pins. The seats were now armed. I stowed the pins in a rack on the cockpit wall. We were ready for business.

Reg completed his pre-start checks and hit the starter button. Way behind us the wail of the Viper turbojet rose to a scream, while I concentrated on timing the engine acceleration as it wailed up to idle rpm. It was a gray and overcast day, and a chill wind blew across the cockpit. Only when Reg closed the canopy did things become a little more comfortable. I was busy scanning the panel and checking off the items on my schedule for this production acceptance flight. These minutiae were the reason for Flight 2 and we had a busy time ahead.

We taxied out and backtracked to the head of Runway 09, turning until the nose pointed down the gray strip of the runway. I made of final check of the oxygen, with the rubber mask pinching my cheeks and the oxygen doll’s eye blinking on the panel in sympathy with my breathing. We were both ready to go. Reg advanced the throttle to full power. The rpm stabilized and I wrote down the rpm and jet pipe temperature readings. When I had finished, I nodded across the cockpit to Reg and as he released the brakes I started my stopwatch.

Acceleration to take-off speed in this jet was smooth, with none of the commotion of a piston-engined aircraft. We lifted off and were almost immediately in cloud, climbing under radar direction and already turning left towards our test area over the Irish Sea. I was busy writing, so absorbed in recording engine parameters and elapsed times each five thousand feet that only the sudden appearance of sunlight on the instrument panel told me that we had broken through the overcast. We were climbing smoothly north-westwards with the Viper whistling like a vacuum cleaner somewhere behind us. Despite the lack of drama, we were already climbing through twenty thousand feet in the time it would have taken my Chipmunk to climb to five thousand feet. But then a discordant note appeared. As we climbed through twenty-five thousand feet a gentle but persistent fore-and-aft thrust pulsing showed that one of the engine controls which compensated for the changing altitude would need a minor tweak before the next flight. I noted this down, and as we continued to climb the thrust pulsing died away.

At thirty thousand feet we leveled out and I snatched a glance out of the canopy to see the coastline rimming the Irish Sea, with England over my shoulder, Ireland in front to the west, Wales to the south and Scotland to the north. Under the nose lay the perfect outline of the Isle of Man, some six miles below. We slowly turned south, and off to our left our own fading condensation trail marked our climb path through the sky. During our cruise southbound we checked the radio and navigation gear, contacting in turn the various radio stations out to maximum range. Everything checked out with a clean bill of health.

We progressed to the next item on our checklist, a dive to the limiting Mach number to check handling. Reg eased the nose down and we accelerated, the Mach meter creeping up as we dived until the magic M=0.75 was reached. Occasionally a JP 5 would start to buffet and pitch at increasing Mach as the local flow went supersonic over part of the airframe, giving the crew a graphic demonstration of transonic aerodynamics, but this particular aircraft gave no problems at all and we pulled back up to altitude.

We continued to the next part of the acceptance schedule: spinning behavior. As the throttle came back to idle, a touch of airbrake slowed us further, with some buffeting as the airbrakes came out. The buffeting faded away as the spoilers retracted. As we approached the stall Reg applied full rudder, the jet yawed, rolled over onto its back and started spinning. I rather enjoyed spinning and religiously counted off the turns as we did this first spin to the right, then recovered. No problems in either the spin or the recovery, so we climbed back to altitude and repeated the spin to the left this time. There were no abnormalities in either the spin or the recovery. Another item was duly checked off my list.

It was time to check those aileron forces to see if the adjustment by the ground crew after Flight #1 had been successful. I passed the spring balance to Reg. As he pressed the balance against the stick to give full roll deflection, the Jet Provost obediently started a smart roll through 360 degrees. We were absorbed in checking the figures, quietly conversing between the two of us, oblivious to the fact that outside the cockpit the world was rotating rapidly. The balance gave a figure that was within limits so we repeated the roll in the other direction, getting a similar reading from the balance. No asymmetry in the control system now.

During a routine check of the fuel quantity, I flicked the rotary switch between the tanks only to get a zero fuel indication. This was enough to catch anyone’s attention. In a single-engined jet out over the water, as we were at that moment, this could be serious. However the engine kept running and we decided that it was probably an electrical snag. A reselection of the switch miraculously brought back our missing fuel.

By this time we were coming back in over the coast. During our flight the clouds had burned off over the land and we found a clear patch of air to carry out our required high speed dive to 450 knots. This meant diving steeply at fairly low altitude. Admittedly this was not my favorite phase of the flight, strapped passively in the right hand seat as we hurtled down into the bumpy air at this impossibly steep angle with the fields expanding all round us. Then I was straining to resist the g-force as Reg pulled out of the dive and coasted up to ten thousand feet again. No problems there.
We now checked the operating times for the landing gear and flap systems. I was kept busy with my stopwatch. Occasionally the gear might be sluggish, or a door might not close properly, but on this aircraft everything proved to be within limits. During this time Reg had been keeping a close watch on our position. Our home field at Warton now lay off the left wingtip and we were ready for our next check, an engine shutdown and restart. A prudent pilot would always be near enough to reach base in the unlikely event of the relight of our single engine not being successful. Reg pulled the throttle (and HP cock) back and stopped the engine. We glided silently for a while with the airflow whispering eerily over the canopy before he restarted the single Viper and it wailed back into life.

By this stage of flight my kneepad was always full of scribbled notes and reminders. It was just this attention to detail which was required to produce a successful aircraft. Each item might seem of small significance, but there is no room for error in the world of flying, especially when this aircraft would be flown by fledgling aviators for most of its service life. Perfection is demanded and this aircraft would be delivered in a perfect state.

With our production schedule concluded, we returned overhead the airfileld on initial, entered the break onto the downwind leg, and curved round on a fighter-type approach down to the runway. The mainwheels touched but when we were slowing and the brakes were applied there was some brake judder, shaking the aircraft as we slowed on the runway. Reg shook his head. The brakes would have to be fixed. I noted this snag for rectification.

On the whole, XW364 was not a bad ship. It would take probably one more flight to iron out those problems which I had noted down on this mission. Then the aircraft would be wheeled into the paint shop to emerge in its smart red and white RAF training colors. In a couple of weeks it would be ferried to its new home, joining the training fleet.

In the ten-year period from 1969 Warton produced 110 Jet Provost 5s, with many of these coming back to be updated with new avionics during their career. The armed version, the BAC 167 Strikemaster was also built at Warton and had a successful life, serving with ten different nations. Once again many of them returned to Warton to be rebuilt and delivered to export customers.
Production testing was just one of the responsibilities that Flight Operations and Flight Test had in the overall organization. When I joined the department, it was a time of transition, when experimental flight testing was moving away from the use of the paper trace recorders and notepads that had been the means of noting down the aircraft behavior during trials. My responsibility was to assess the directional stability of the Lightning during the supersonic flight trials that were under way on the Mk 6 for the RAF and the Mk 53 and Mk 55 export Lightnings for Saudi Arabia. This was a serious concern. The Lightning trainer , in which the crew sat side-by-side, had suffered a couple of in-flight structural failures of the vertical fin during supersonic flight. The first had resulted in test pilot John Squier baling out and drifting about the Solway Firth in his dinghy for a couple of days before he was washed ashore.

This had resulted in a redesign of the vertical fin and subsequent Mk 5 trainers had this modified fin. All went well until Jimmy Dell and Graham Elkington had a somewhat similar fin failure during supersonic maneuvering. Jimmy Dell, who had a fine sense of humour, said in later years that he always worried when flying with a Flight Test Engineer that the FTE would be so wrapped up in writing down results that if a catastrophic failure occurred, the FTE would not realize that anything was wrong and would be left behind when the pilot ejected.

In this case, Dell said, his fears were unfounded. He heard three bangs in quick succession. The first was the fin failing, the second was the canopy being jettisoned and the third was Elkington ejecting, leaving Dell alone in the cockpit. Pausing only to give a quick “Mayday” call, Dell pulled his ejection handle and ejected. They both drifted down under their canopies and were picked up from the Irish Sea.

I filled the spot vacated by Elkington, so there was a big incentive for me to do the job right.

With the advent of the Jaguar, and a requirement of carrying out an intensive flight test program to clear the aircraft for both French Air Force and RAF service, a comprehensive system of recording data on magnetic tape was introduced. The most immediate result was a vast reduction in the turn-around time to analyze data between flights, and a huge increase in the numbers of parameters which could be patched in at short notice to investigate specific engineering or aerodynamic problems.
Production testing, whether on the Jet Provost, Strikemaster, Lightning, Canberra or Jaguar was in some ways an anachronism, as the production aircraft did not have extensive instrumentation. It was more akin to flight testing in the old days, when it was largely the pilot’s experience which determined if a potential problem existed. From my point of view it was a welcome break from experimental flight testing, and I found it very useful to keep in touch with the product that was going out into service.

No comments: