Tuesday, August 4, 2009

Chapter 13: Test Pilot for a Day

An NF-104 greets the visitor to the USAF Test Pilot School at Edwards Air Force Base. Flown by students in the 60s to give experience in high-altitude flight, it still sports the rocket engine at the base of the vertical tail which enabled it to reach over 100,000 feet altitude.

A scorching wind from the high desert of Southern California moaned around the needle nose of an F-104 Starfighter, permanently earthbound but pointing skyward on a pylon outside the low building housing the USAF Test Pilot School at Edwards Air Force Base. A rocket engine mounted at the base of the vertical tail showed that this was one of the handful of NF-104s which explored the edge of the atmosphere in the sixties.

Of the three original NF-104s with their 6,000 lb thrust LR-121 rocket engines and reaction controls, one of the three survived a mid-air explosion of the rocket motor and made it back to Edwards and the sanctuary of Rogers Dry Lake, landing with tail feathers in shreds. Another was lost when Chuck Yeager, then Commandant of the Test Pilot School, got into a flat spin during a ballistic trajectory flight at over 100,000 feet. He rode the bird down to fourteen thousand feet before baling out, only to be severely burned by the rocket of the ejection seat.
In those days, and for the previous twenty years, Edwards was the fabled home of the test pilots, later to be immortalized in Tom Wolfe’s book The Right Stuff. A breed who would fly anything, anywhere.

Now, in the eighties, had anything changed?
Test Flying, even during the time I had been involved with it in England, was becoming more of a science than an art. As aircraft became more sophisticated, test flying was increasingly concentrated on systems testing, not just the handling qualities of the aircraft. However, at the USAF Test Pilot School the classical techniques of performance and flying qualities were still taught in addition to systems testing. The performance phase included aerodynamics and test techniques, while students were taught the skills required to evaluate all aspects of aircraft performance. One unique part of this course was the low lift/drag flying applicable to lifting body or shuttle operations. (Most of the astronauts were alumni of this exclusive school)

During the flying qualities part of the intensive 46-week course the students learned to evaluate the handling of a range of aircraft in the school’s fleet. Resident aircraft ranged from the F-4 to A-37, from Beaver to KC-135, together with a variable stability Learjet and a T-33 to demonstrate both the problems and the solutions encountered when assessing handling qualities. The students and staff assured me that it was the best job in the Air Force.
What was it like, learning to be a test pilot? I was given the opportunity to find out when I was invited to fly with the USAF Test Pilot School.

Our mount for this mission was the supersonic Northrop T-38, nicknamed the White Rocket during its long career with the USAF and NASA. I flew with Colonel Mel Hayashi, commandant of the Test Pilot School, for a mission which would cover a sample of the teaching techniques used during the handling and performance portions of the course.

In addition to the handling points on our flight, we briefed for a simulated shuttle approach which would culminate in a lakebed landing. This was treated routinely as just another segment of the course dealing with low L/D flying. As Colonel Hayashi explained, we would simply aim for a triangular marker painted on the lakebed, then switch to another marker as we flared. From my point of view a 280 knots power-off descent with flaps, airbrake and gear down promised to be interesting. I had watched the NASA Gulfstream apparently dive-bombing the lakebed doing precisely this maneuver when the astronauts were training for their shuttle landings. It was terrifying to watch. I started to wonder why I had volunteered for this mission.

Before we walked out to the aircraft Colonel Hayashi gave me a comprehensive briefing on emergency procedures in the T-38, bale-out procedures and the actions required of me if we should have a bird strike. We were scheduled to fly at two o’clock in the afternoon and I had spent most of the morning being fitted with flight suit, boots, helmet and oxygen mask. Last of all came the g-suit. A tight fit was essential for it to do its job correctly, and I was carefully hooked, laced and zipped into the g-suit.

It was eighty degrees Fahrenheit and a hot dry wind was sweeping across the acres of concrete of the Edwards ramp as we walked out to the white stiletto of our T-38. It was a long walk out across the ramp,helmet in hand, with the buckles on my back-type parachute clinking and the bulky g-suit around my lower body and legs making it awkward to walk. The tower had given the visibility as eighty-five miles and the snow-capped San Gabriel mountains to the south shimmered incongruously in the distance as we approached the T-38. It was a small aircraft, but looked wickedly fast just sitting on the ramp.

The supersonic Northrop T-38 is used extensively by the USAF Test Pilot School, and provided the opportunity for the writer to be introduced to the world of a student test pilot, if only for a day.

I climbed the rear ladder and parked my helmet on the far cockpit sill, tucking my gloves and kneepad up onto the coaming of the rear blast screen. I swung the parachute into the rear seat, hooked up the survival pack connectors and then climbed in. Our crew chief helped me connect up the parachute harness, g-suit and the seat straps while Colonel Hayashi pre-flighted the aircraft and then climbed into the front cockpit.

Isolated in the rear cockpit, I donned helmet, oxygen mask and gloves. Plugging in the various connections I was reassured by the doll’s eye oxygen indicator blinking down by my right knee as I breathed in and out. As the intercom came alive we established communications.

As Colonel Hayashi hit the starter button the right hand J-85 started to motor over. At fourteen percent rpm the engine lit with a rumble, sending the turbine temperature needle scurrying round the dial. The engine wound up to idle power, whining away behind me while the left hand engine was started. With both engines running, the flight controls were checked for full movement. Forewarned, I made sure my knees were out of the way of the stick. By now our crew chief had stationed himself in front of the T-38 and pantomimed the control surface movements as the stick was moved. Here in the rear cockpit, way ahead of the wings, it was difficult to see the control surfaces, even in my rear view mirrors on the canopy arch.

The controls checked out OK. All that remained was to pull out the safety pins for the canopy jettison and the ejection seart, hold them up for the crew chief to see, then stow them away. Our callsign was COBRA 01, the number reserved for the School Commandant, and the tower cleared us to taxi for the active runway. A burst of power persuaded COBRA 01 into motion and we taxied down the ramp, our open canopies nodding as we rumbled over the joints in the concrete.

The Edwards ramp was crowded with the resident F-4s and F-111s. We rolled on down the taxiway, past the tower and out across the field until we stopped at the “Last chance” area near the end of the runway. Here two crewmen made a final visual inspection of each aircraft, checking for leaks, loose panels or other discrepancies. For safety’s sake our gloved hands were kept in full view, away from any switches. I rested my hands on my canopy arch, well clear of the controls while the crew scurried about under the aircraft. Satisfied, they ran clear and waved us back to the taxiway.

It was time to button up and I reached up and manually lowered my canopy, pushing the locking lever forward on the right hand wall until it clunked home and the canopy warning light extinguished. My head was almost touching the canopy and I blipped the electric seat adjustment down until I could get some clearance between my helmet and the canopy.
We rolled gently out onto Runway 04, checking that there was no-one on finals, and lined up precisely on the centerline that stretched out to the shimmering lakebed in the distance. Mindful of the warning about bird strikes I lowered my dark visor as COBRA 01 was cleared for takeoff.
As the power levers advanced to the Military Power detent and the brakes were released, we started to move. The T-38 accelerated rapidly, keeping straight initially with differential braking, then the power levers went fully forward to maximum power and the nozzles opened as the afterburners lit. The right hand nozzle was fractionally slow, but once both were lit the acceleration continued to increase inexorably. At 155 knots the T-38 flew off the ground and started to accelerate rapidly once the gear and doors were tucked away. As the ground dropped quickly away we came out of afterburner and the acceleration decreased perceptibly.

I set up the climb as briefed at 350 knots, still at Military Power. My task was made easier by the special instrumentation fitted in the School’s T-38s. In these aircraft the standard ASI and altimeter had been replaced by single-pointer instruments. As the ASI was graduated in knots, with one complete revolution reading one hundred knots, airspeed could be read to an accuracy of one to two knots. Initially I found that I could not match that accuracy and we pursued a slightly roller-coaster path as I got used to the powered controls of the T-38. Ailerons and pitch control were considerably lighter than the Cessna 172 RG that I had been flying the week before. We settled into the climb at 4000 ft/min and some three minutes later I leveled out at 15,000 feet.
Visibility was spectacular from the T-38. The rear seat was set some ten inches higher than the front seat and I could see straight ahead over the forward ejection seat. Apart from the canopy frames and the glass blast screen between the cockpits I had an unobstructed view out of the canopy ahead and to either side. By craning around I could just see the wings following us. It was easier to check to the rear using the rear-view mirrors on my canopy arch.

Colonel Hayashi pointed out the landmarks in the local area while we positioned for our first test point. We were heading towards Owens Dry Lake, some 60 miles to the north, with the white-capped saw-tooth bulk of Sierra Nevada range of mountains stretching off our left side and extending north for over a hundred miles. The highest, Mount Whitney, towering to 14,494 feet was clearly visible out front to our left. Below us stretched the dusty sagebrush wastes of the Mojave desert. Volcanic plugs dotted the plain below and although it was a completely alien landscape I realized with a sense of déjà vu that of course I did know this scene. While I had been growing up in England this had been the backdrop for all that spectacular footage of test flying from the X-1, Skyrocket and X-15 rocket plane projects. These were hallowed skies for anyone involved in any way with test flying. It was a strange sensation.

Colonel Hayashi started by showing me how a student was taught to set up a typical stable test point. No screaming dives to the edge of the envelope on this flight. The essence of test flying was accuracy. The limits for me to fly to were plus or minus two knots and we were aiming to establish a data point at exactly 15,000 feet and 300 knots.

In a strange aircraft, the test pilot first had to estimate the power to set up the first point, so we chose a deliberately low power setting to start. At exactly 15,000 feet the power was inched up to eighty-seven percent on the J85s; stick forces were trimmed out on the coolie hat trim switch on the top of the hefty stick grip and we waited for the aircraft to stabilize. Airspeed settled at 295 knots, so rpm was inched up to eighty-nine per cent.
This gained us two knots and so the power was increased in increments and the process repeated until at ninety-one per cent we were indicating exactly 300 knots. At this point our hands were taken off the controls to see if we were trimmed out. The T-38 hummed along, rock-steady. It seemed easy enough.

Now it was my turn. My task was to set up a test point at 240 knots. With throttles back to idle we slowed to about 220 knots and I started inching the power up. In my unpracticed hands the exercise proved to be a slightly more lengthy procedure than I had anticipated. By the time I had the situation more or less under control and had trimmed out the stick forces, we were within five miles of the eastern boundary of our Restricted Military Airspace and the controller wanted us to turn back west. I began to appreciate why the minimum requirements for entry to the Test Pilot School included a thousand hours of jet time. Flying to this accuracy had to be second nature before embarking on a venture like this.

Next on our flight card was an acceleration from 250 knots to Mach 0.9 while maintaining exactly 15,000 feet. At M0.9 we would bring back the power to idle, start a turn and decelerate under a constant 3g until we hit the aerodynamic limit which would be marked by heavy airframe buffeting.

As we accelerated I was aware that the tiniest corrections in pitch were constantly required to maintain us at constant altitude as the speed increased. As the angle of attack decreased I could see the nose inching lower against the horizon. Above M0.85 a small-amplitude directional snaking appeared. This was a transient effect of the yaw damper system.

At M0.9 as the power levers were brought back to idle we flipped into a steep left hand bank. As the needle of the accelerometer locked onto 3g my g-suit inflated, pressing against my legs and abdomen to restrict the flow of blood away from my head. The turn tightened and the speed drastically reduced as the induced drag increased.

It was now noticeably difficult to breathe as I strained against the g-force and tried to write notes on my kneepad. Below three hundred knots the ride started to get louder and rougher as the airflow started breaking away behind the canopy and over the wings. At 250 knots the T-38 was protesting by giving us a very uncomfortable ride. The stick went forward to unload the g-force and the T-38 came back into level flight.

Now it was my turn to try this. Bringing the power levers back up to Miltary Power, I watched the gauges start to wind up. Flying this accurately took me back to the days when I was sweating through my instrument rating. It was just as challenging. I found that by concentrating hard on the altimeter I could maintain altitude precisely at 15,000 feet, although as the Machmeter crept past M0.8 it was becoming increasingly difficult to maintain altitude exactly.

At M0.9 I brought the power levers back to the idle stop and attempted to set up the required 3g deceleration in the turn. In an all-but-vertical bank my attention was divided between keeping the altimeter at 15,000 feet and maintaining the correct bank angle and the g. This task was easier said than done. There were few cues for speed and attitude outside the cockpit and while I was trying to keep the horizon in the right place I found that the g-force backed off and the g-suit deflated. My eye finally caught the movement of the g-meter needle and I pulled back on the stick until we hit 3g.

We hit the buffet at around 280 knots and it got progressively worse until I could feel the aircraft start to wallow around in the heavy airframe buffet. That was enough of a limit for me. Pushing the stick forward to unload the gs I powered us back into level flight to set up for the next point on the test card, a constant airspeed descent at 300 knots. After a suggestion from the front cockpit that I might try 80% rpm for this, I was on my own. I pulled the throttles back, checked that the speedbrake was out and we started sliding down the sky. It was difficult initially for me to keep the speed constant; I was still tending to overcontrol. In jets, small changes in attitude generate large variations in the rate of descent.

We leveled at 10,000 feet to demonstrate the effects of flying on the back of the drag curve. The T-38, designed by Northrop in the fifties to introduce pilots to the flight characteristics of the Century Series supersonic fighters, had all the characteristics of the early supersonic jets, with a low aspect ratio wing giving high induced drag at high angles of attack. This meant that the slower you flew, the more power was needed to combat the drag.

Flight behavior in this part of the flight envelope had its own characteristics. Our demonstration test point was 190 knots and 10,000 feet altitude, with gear down and flaps at 60 per cent. I tried to remember the classroom briefing. In this regime, altitude was maintained with power while speed was now controlled by pitch, reversing the normal technique at higher speeds.
For our budding student test pilot, accuracy was the goal, as it was at the higher speed. The technique demonstrated was to bring the power up in increments until 94%rpm on each engine kept the altimeter locked on to 10,000 feet; meanwhile the speed was maintained at exactly 190 knots by tiny fore-and-aft corrections on the stick. At this speed we were getting into an area of small-amplitude wing rock.

At this point I took control of the aircraft again to set up a test point at 180 knots and 10,000 feet. I met with indifferent success initially, finding that the power required just to stop us descending had the throttles almost up at Military Power. It was fast becoming a juggling act as my changes in pitch to keep the speed constant were in turn magnified by instrument lags and the fact that I was not letting things settle out. Gradually the situation was sorted out and we settled at the required 180 knots, nose way up in the air and engines almost all the way up to 100% as the engine thrust fought the induced drag from the tiny wing. This was the purpose of the exercise and I was learning fast.

At the eastern limit of our airspace again we turned west, pausing while an F-16 flashed past some 2,000 feet above on a reciprocal course. Edwards airspace was very congested and civil aircraft were warned to keep well away. With a dozen or so military jets airborne on various test missions at any one time, maintaining separation was a full-time task for the controllers.
My next task was to find the g-force value at 300 knots at which the T-38 could just maintain sustained flight at Military Power. This would give a check point on the specific excess energy curve of the aircraft. Combat aircraft live or die on specific excess energy in combat. Aircraft designers needed to know how their aircraft measured up to their calculations by checking on the specific excess energy curves relative to other aircraft. My life as a designer had centered on this for years.

I set up a level turn at Military Power and gradually increased the bank angle, pulling back on the stick to keep the g increasing while maintaining the speed at exactly 300 knots. We could just maintain 2.4g at this flight condition, with thrust equal to drag at this point. Easing the stick back gave fractionally more g and sure enough the speed started to drop. This checked out with the book figure.

Colonel Hayashi gave me a break from this hard work and we took five minutes out to fly the T-38 through some aerobatics. Barrel rolls were especially fun, using the distant snow-capped peak of Mt. Whitney as a reference point. I found the T-38 a delightful aircraft to maneuver but typical of any fast jet it certainly used a lot of sky as we effortlessly soared and rolled between 8,000 feet and 15,000 feet. Aerobatics were exhilarating and roll rate was spectacularly fast, as good as the Pitts Special.

Watching the fuel gauges, we could see that it was time to head back to Edwards. However, we found that Murphy strikes in flight test scheduling just as much as in other types of aviation. By now the wind had strengthened from the east and the traffic pattern in use now cut right across the airspace over the lakebed. We needed that airspace to complete our scheduled Shuttle-type steep approach and lakebed landing. Peering over the side I could see the aiming markers sharply contrasted against the white lakebed, but regretfully we had to pass that item by.
As a precaution, before flight we had briefed a couple of alternate test points for just this eventuality, so now we started to descend towards the lakebed for a practice tower fly-by. This was a pure flight test exercise in which the student had to set up a precise flight condition. Another of those tests requiring the utmost accuracy. The exact parameters of speed and altitude had to be maintained while flying past an observer who was in a special fly-by tower out on the lakebed. By sighting on the aircraft as it flew past, the observer could establish its exact altitude. Later comparison of the aircraft’s known position with the instrument readings in the tower and in the aircraft enabled a check to be made of the pitot and static pressure errors in the aircraft system.

The cockpit was very quiet as we descended. The tower gave us updated pressure readings for the altimeter which I set in on my panel. COBRA 01 was now cleared for a tower fly-by. As the throttles went forward we accelerated to our briefed speed of 400 knots, heading east, as the airfield slid past on our right hand side, with the gleaming white surface of Rogers Dry Lake beyond the base.

Down now to 200 feet over the desert the airspeed stabilized at 400 knots and the power levers were left alone from this point on. We banked hard right over the eastern portion of the lakebed, aiming for a stubby pylon which marked the beginning of the tower fly-by run. From the base of the pylon a black line arrowed off across the glaring white lakebed into the distance.
I strained against the g-force as the pylon flashed past less than a hundred feet below and we flicked back into wings-level flight, seemingly balanced over the black line while the lakebed streamed past to either side.

”This time of day would not be good for taking data,” Colonel Hayashi remarked conversationally as the turbulence over the lakebed was causing the airspeed to fluctuate two or three knots either side of our 400 knot datum, enough to invalidate our data. For serious training, these tests were invariably carried out in the early morning, before the sun had heated the desert floor.
We streaked past the tower and on across the airfield.

Still at low altitude, Colonel Hayashi showed me what the performance of the T-38 was like at low level. The throttles went forward and accelerating in Military Power the airspeed was now up to 500 knots. It was an exhilarating ride at this low level. Out of the corner of my eye I could see our shadow flicking across the sagebrush-covered desert. We were now heading north and as the terrain below got rougher the jolting ride became worse in the turbulence. Even at this speed the cockpit was still quiet. I was writing more notes when we ran into a patch of rough air and I was slammed against my straps, with the airspeed fluctuating violently.

Enough of that. The quickest way out of the turbulence was upwards and we pulled up at 5g. This was definitely enough to stop me taking notes. We arrowed up to 10,000 feet, coasting inverted over the top of a giant loop and rolling upright in an Immelmann maneuver to head back to base for some pattern work.

Descending to traffic pattern altitude, we ran in over the field. Fuel was down to 1,100lb as we broke to the right at 300 knots some 1,500 feet above the field. Power levers were pulled back and as the T-38 decelerated through 240 knots, gear and flaps came down. It all happened very quickly. Speed was down to 175 knots on a tightly curving base leg, tapering off to 155 knots on finals. Remembering the behavior of the T-38 at low speed earlier in the flight, I was not surprised to find that the power was way up just to keep us flying with the high drag from gear and flaps added to the induced drag from that vestigial wing. The angle of attack indicator on top of the instrument panel was in the green. Over the runway overrun we went to military power, cleaned up and overshot. We were now at 1,000lbs of fuel. The J85 jets were thirsty at low altitude and the fuel gauges were dropping rapidly.

Another T-38 and an F-16 joined us in the pattern and we decided to make the next landing a full stop. With fuel down to 900lb, this time we banked round on our curving approach with three greens, flaps full down and airspeed at 155 knots. We touched down on the numbers, then whistled along the runway, nose still high for aerodynamic braking until the speed dropped to one hundred knots. The nose was allowed to drop, then gentle braking brought the speed down to a reasonable pace and we turned off the runway.

This Northrop T-38 belonging to the USAF Test Pilot School taxies back from its test mission at Edwards Air Force Base. The Supersonic T-38 has been the mainstay of the TPS test fleet for many years. It is maneuverable, fast and a challenging aircraft to fly well. The T-38 is also great fun to fly

We cracked the canopies open to taxi back. The desert air was dry and pleasantly cool. At the end of the taxiway we waited while an all-white A-7 taxied past, part of the never-ending pattern of flying at Edwards. Colonel Hayashi wheeled the T-38 back into our parking slot. I inserted my seat pins as the J85s whistled down into silence.

This flight had given me an insight into the challenging world of teaching pilots to be test pilots. I had found out the hard way that there is a world of difference in some respects between theory and practice. Classroom work is not enough. Flying the T-38 and the other aircraft in the fleet at the TPS showed the students the practical differences in handling between various designs.
In this age of computers and sophisticated simulators, there is not a lot of mystique left in flying, but maybe it’s still there in test flying. The outsider rarely gets to appreciate the hard work that must be completed before a test pilot emerges from this Test Pilot School and the other military test pilot organizations around the world.

It had been a rare privilege to fly with the USAF Test Pilot School, to experience what it was like to be a student at the Test Pilot School, if only for a day…

No comments: