Citius, Altius

Some recollections of the author’s aerial frolics in the quest to go faster, higher.

  

Mach Busting

The sight and sound of the roaring Starfighter at a Pakistan Day flypast was largely responsible for stirring up my pre-teen notions of high speed flight, in the early sixties.  An upward swish of the hand, complete with an electrifying ‘fffiyaaaon’ sound-effect, is how I would explain supersonic flight to my tongue-tied school friends.  Growing a little older, I read about another fighter, the Mirage that was said to be the most elegant solution to penetrating the sound barrier.  Leaving any pursuer hopelessly baffled because the Mirage could be seen but never caught – as the name suggested – was an absolute knock-out idea for my adolescent mind.  Little did I know that one day, I’d be routinely doing high-speed air tests or Functional Check Flights (usually required after a maintenance inspection) with great relish.

An FCF was the only occasion when one could do the ‘Mach[1] Run’ to the aircraft’s limit, which was, theoretically, Mach 2.2 for the Mirage III/5 that I was flying in No 8 Squadron at Masroor.  Practically, Mach 1.6 or thereabout was the fastest one could go during an escape or a chase in air combat, and you had to be high in the troposphere[2], giving yourself away with those tell-tale contrails[3].  Mach 2 was a little-needed capability, though as a full blooded fighter pilot, the experience of having gone through it was de rigueur if you were worth your salt.

My turn for FCFs came after I had mustered the regulation 200 hours on-type.  It took a few sorties to learn how to rapidly check scores of items off a busy checklist, while the fuel counter of the clean-configured airplane ticked off anxiously.  High speed runs were usually limited to Mach 1.6, which is easily achievable in level flight on a cool day.  That speed was more than enough to check the absence of engine duct buzz, while verifying the function of shock cones to ensure that they bend the shock waves in a way so as to keep disruptive supersonic airflow from entering the intakes.  Anything beyond that speed entails: 1) keeping a keen eye on the precarious fuel state; 2) very careful airspace management, as Mach 2 is usually achieved in a straight run of nearly 100 nm; 3) ensuring that no one hears you, for many a fighter pilot has been hauled up for causing ‘sound and fury’ over the city.

My first experience at going bisonic was a non-starter, though a personal best of Mach 1.6 was achieved easily.  I just could not figure out why the aerodynamically perfect Mirage was not racing any faster, till I got in touch with the Dassault’s resident technical representative, Mr Cerveau.  Cerveau, who knew his job well, had a little laugh at my brute force approach, and with a nationalistic slant to his explanation, told me how the French do it.  “You climb at the optimum speed to the tropopause[4], the altitude of which varies with ambient surface temperature; you will have to calculate it accordingly.  Then, accelerate to Mach 1.6 while maintaining altitude.  After reaching this speed, put the aircraft in a very shallow dive till you reach Mach 1.8.  Next, gently raise the nose a few degrees above the horizon and maintain it there.  You will see the Mach-meter registering a steady rise while still in a climb.  On a cold day, you will hit Mach 2 or even beyond, before you reach the aircraft ceiling of 50,000 ft above mean sea level (AMSL).  This technique is not given in the Pilot’s Flight Manual,” he whispered, “but our test pilots do it routinely.”  “As the engine is not very powerful, we have to exploit the unique properties of the atmosphere.”  Heaving his shoulders and pouting his lips, Cerveau let out a noisy sputter which I understood to mean, ‘voila’!  “Merci,” I gratefully responded with French etiquette.

For the next FCF, I had prepared my mission quite meticulously, in line with Cerveau’s instructions.  Luckily, the test flight was limited to checking the result of a minor rectification, so I had all the time and fuel to fly as suggested.  I had also studied the aircraft flight envelope and engine limitations, thoroughly.  As I sped up in afterburner and followed the ‘dipsy doodle’ profile, everything seemed in order, except for the ground radar controller who thought something was amiss.  Quite obviously the blip on his scope was moving faster than he had ever seen.  He first confirmed my call-sign, to be sure it matched with the flight plan.  Some time after the usual banter about ‘parrots and angels[5],’ he broke in to ask about my speed, rather curiously.  I countered by asking how fast was he clocking me, to which he replied that probably AP (anomalous propagation) was causing some error with his speed readout.  “Sir, it is giving me nearly 1,200 knots.  That is not possible,” he hastened to clarify.  “Yes, I have the about the same speed on my gauge too,” I assured him, as he sat enthralled by the runaway blip.  By this time, I had also reached the aircraft ceiling.  After video recording the Head-Up Display speed readout of Mach 2.1 and an altitude of 50,000 ft AMSL for the eyes of the non-believers, I threw in a turn for a recovery back to the Base.  After rolling out, I popped open the speed brakes and disengaged the afterburner, making sure there was no snappy movement of the throttle.  “The Mirage engine never flames out if you handle it properly.  If it does, it never relights,” our previous OC’s wise words rang in my ears, as I gently eased the throttle for saner airspeed settings.

Descending to a safer altitude where the engine was no more starved of oxygen, standard recovery procedures were followed.  Fuel, area limits and sonic bangs had been controlled well in the mission.  Having finally acquired the rare bisonic credentials, I thought it prudent not to advertise the deed as I could be put on the leash by a jittery Flight Commander.  A few months before, on my first FCF mission, I had inadvertently banged out and had to face an unsavoury brush with the authorities, following this embarrassing bit in the newspaper:

Coming in the wake of SAM-7 missiles and reports of increased security all around, last Thursday’s mysterious thunder ruffled the nerves of a large number of people.  The deafening blast, as they later reported, was located in the trans Lyari area where things can always be expected to happen.  Was it a bomb? In Liaquatabad?

Rumours which circulate like phantoms in our haunted minds were afloat until late in the afternoon. Anxious calls were made to newspaper offices, police stations and friends believed to be knowledgeable. For a while there was a sense of community. People were tied together in a knot of fear and apprehension.

Then, anti-climatically, it turned out that all that sound and fury had signified nothing. For it must have been a sonic boom. Some military aircraft, it seemed, had crossed the speed of sound to send the shock-wave to the ground.

(DAWN 10 February 1984 – Karachi Diary, ‘Sounds and Fury’ by Ghazi Salahuddin.)

A few years later, I went on to try the bisonic stuff on the superlative F-16 to see how its simple intake would behave, without a shock cone or ramp to keep the disruptive supersonic airflow out of the engine inlet.  Flying a flight profile quite similar to that of the Mirage III/5, I did manage to get to Mach 1.85.  I suspect that the huge pressure and temperature rise resulting from the turbulent airflow impacting the compressor causes considerable energy loss.  Any more speed would, therefore, have to come at the expense of bulldozing over some more distance which area limitations did not permit.  (I had already caused some frayed nerves as I approached the border near Sialkot.)  It has always been a disappointment for my F-16 friends to learn that the under-powered Mirage actually went faster, though it should be some consolation that the F-16, at least, has the potential of doing so.

Several attempts to go bisonic on the F-7 (Chinese derivative of the early model MiG-21) also never got me past Mach 1.8, primarily because of extreme shortage of fuel in a clean-configured aircraft. Perhaps not yet familiar with the concept of supersonic drag management known as ‘area ruling',[6] the aircraft designers left it to the fairly powerful engine to push the aircraft through the brick wall of supersonic wave drag.  The nimble little aircraft could, however, only do so much on the limited fuel it had, as I discovered. The later versions of the MiG-21 were better endowed fuel-wise and, I am sure many intrepid pilots world-wide, would have experienced the thrill of watching the Mach needle swing past 2.0.

In the last week of my tenure as Base Commander, which also brought a poignant end to my flying career spread over thirty years, I managed to fly four FCFs: three on the F-7 and one on the Mirage-5, all in a single day.  I had come a long way for one last swish at twice the speed of sound.  Can’t think of a better adios.

X-treme Racing
 

While the F-16 was not designed for speeds in excess of Mach 2.0 at high altitudes, it has the capability of reaching the terrific speed of 800 knots at sea level, despite the air being the densest, generating maximum airframe drag.  Only a few of the modern fighters are capable of reaching this extreme speed at low level, as only an airframe fabricated from the strongest of alloys and composites can withstand the immense dynamic loads.  Fighters of the seventies and earlier were limited to 750 knots at sea level (corresponding to Mach 1.13), which was still fast enough, as I had experienced in a couple of sorties involving trials for a drag chute container modification on the Mirage 5.  

Flights involving supersonic flight at low level are rare due to the definite possibility of sonic bangs over populated areas.  With a maritime role of my squadron, I was well trained to fly over the sea.  Sonic bangs were thus of little concern, except for some surface vessels which would be on the receiving end of the nasty shock waves.  I went through the trials which showed that the drag chute container was sub-standard and had failed to retain the chute every time; the vendor was then made to come up with a better modification.  I think it was useful to have taken the aircraft to the limit, both for the sake of man and machine.

A similar opportunity for conducting trials on a locally manufactured training missile came up on the F-16.  Testing the missile for extreme dynamic loads was once again only possible, I thought, by subjecting it to the highest speeds in the densest atmosphere ie, sea level or thereabout.  Thal Desert, west of Sargodha offers a vast sandy plain, with an odd gypsy settlement here and there.  Going supersonic at low level would, thus, be possible without a whimper from any quarter.

With just two sorties available for the trials, I decided to utilise the first one for some high-G manoeuvres to see if the missile stayed on-board without being bent.  The method was rather unsophisticated, but the PAF had been quite satisfied with this makeshift approach for many decades, before formally qualified test pilots came in for the JF-17 development.  A dual-seater F-16B was taken up for the trials, as the rear seat occupant could monitor what was going on. Everything went well, so the next mission on the following day was planned for the fast stuff. Intriguingly, there were no volunteers to occupy the rear seat, so I decided to fly alone in the dual-seater.

The sky was perfectly clear on the 7^th of April 1988.  Even though there was no ground turbulence, a height of 500 ft AGL was considered low enough.  Afterburner was selected and its five stages cut in, quite out of synch with my fast pounding heartbeats.  The acceleration was very swift and in no time the aircraft sped past Mach 1.0, without any sensation inside the cockpit, thanks to an excellent flight control system that dampens any extraneous tug or tweak, except what is pilot-induced.  The aircraft was tearing up the sky and everything on the ground seemed like a blur.  Before I could feel anything like a vibration or a buzz, the reading on the Head-Up Display rolled up to show 800 knots.  There was little room for error at that stage as the aircraft was close to its structural limits, not withstanding the built-in safety margins.  No flight limitations had been crossed, though.

Slowing down was not much of an issue; a mere 1/8” movement of the joystick was enough to rocket me straight upward to 25,000 ft before the aircraft fell on its back, and I collected my gasping self for a recovery to the Base.  That is when I remembered to look at the missile, which was still there.  Barring any cracks that a post-flight inspection might reveal, it was ready for use as far as I was concerned.  

It was a while before I realised that I could count myself amongst the fastest men over the sand dunes. Hallelujah!

Astronautics

Converting students on to the F-16 could become a drag for instructors in No 11 Squadron, so a call from the flight lines for an FCF was always a welcome break.  An engine had been changed and a full-envelope test had to be done, which meant it had to be taken to the speed and altitude limits, besides others.  Since such extreme testing was prone to some risk, the emphasis was more on checking the various temperatures and pressures at the heights and speeds normally flown during training.  For me, exploring the ceiling limit of 55,000 ft AMSL had, however, been almost an obsession, so this was the day for the climb – a cool morning of 10^th November, 1987.

Since it was a dual-seat F-16B, I got hold of a student by the name of Flt Lt Ashfaque Arain, to give me company.  The usual FCF take-off – as steep as you could get away with – was followed by successive stops at various altitudes to check engine parameters.  Climbing to 40,000 ft was usually considered good enough, but I was going higher.  The engine temperatures and pressures were in perfect green range, as were the equally vital oxygen and environmental control systems.  Ashfaque, who hadn’t been up to such heights before, was breathing steadily, so I gathered that he was enjoying the scene.  The powerful engine was pushing us steadily upwards as we went past 40-, then 45- and then 50,000 ft.  I had been to such extreme altitudes a few times in the Mirage, but with quite a bit of effort.  This time it was a breeze, and the vertical velocity indicator (VVI) was registering a healthy rate of climb for the altitude we were at.  The clear sky was getting to be a darker shade of grey and one could even discern the earth’s curvature.  It was almost like an optical trick being played on the terrestrially attuned eyes.

Reaching the aircraft ceiling of 55,000 ft, I rechecked the cockpit pressure as well as the oxygen supply, both being within limits.  The engine too was working normal, without a cough or a grunt.  The VVI was still showing a positive rate of climb, measly though it had become.  At the spur of the moment, I thought why not let the aircraft go higher?  Ashfaque seemed quite confident about my abilities for he did not show any apprehension.  So we went on … and on, till we reached 62,000 ft.

All of a sudden a thought crossed my mind: what if we had to eject for any reason, or the cockpit pressurisation system simply blew out?  If I recalled my aero medical course lectures correctly, at this pressure altitude exposed body fluids such as saliva, tears, urine, blood and the liquids wetting the alveoli within the lungs will boil away without a pressure suit, and no amount of breathable oxygen delivered by any means will sustain life for more than a few minutes. We had reached what is known as the Armstrong Limit[7] (no relation to the moon man).  It was time to get down post-haste.

During descent, the first priority was to quickly get below the authorised aircraft ceiling, for any malfunction in the prohibited regime would be squarely blamed on the pilot.  A dip of the nose sufficed to get the aircraft in the proper flight envelope.  Thereafter, use of the speed brake did the trick for quite a while; the engine throttle was reduced to descent settings only when all seemed safe, around 40,000 ft.  After an uneventful descent and landing, the FCF was duly signed off as ‘cleared.’

It was much later that I learnt about No 24 Squadron’s RB-57F (‘Droopy’) aircrew who used to routinely cruise at 65,000 ft AMSL in the lower reaches of the stratosphere[8], while flying ELINT missions along the Wakhan strip in the early sixties.  That was the optimum altitude for ferreting out Soviet telemetry signals from the Baikonur-Tyuratam Missile Test Center in Kazakhstan, which was associated with the launch of newly developed missiles of the Cold War era.  Pressure suits were, of course, mandatory for such extreme altitudes.  If I had been aware of Droopy’s cruising altitude, the temptation of busting it would have been too great, for it was a matter of just a few thousand feet.  Nonetheless, it is a jolly thought to muse over, that the mission saw me gate-crashing into the 60K Club – though on the wrong side of the operating limitations the mission was, I have to abashedly admit!

Full Stop

While performing a desk job at the Air Headquarters in 1998, I had been detailed to conduct a second phase of flight evaluation of the recently developed, double-delta winged F-7MG in China.  Having been off flying for several months, some familiarisation sorties on the largely similar F-7P were, therefore, in order.  I preferred the home ground of Rafiqui (Shorkot), where I had commanded a Flying Wing in my previous appointment.  Besides, a course-mate of mine was then in command of the Wing and could facilitate a prompt check-out.

On a pleasant windy morning of 7^th April, a dual check ride in a FT-7 was followed by a ‘general flying’ mission involving some aerobatics in a single seater.  It was a routine affair till the landing, which was planned to be a short one.  I had especially wanted to compare the F-7P with the F-7MG which had a slightly lower landing speed, along with a hydraulic brake system that allowed unlimited braking (the open loop pneumatic system of the F-7P would lose air pressure after every brake application).

Approaching for Runway 15, a head wind of 10 knots was reported, which was quite welcome.    A dragging approach, coupled with extended speed brakes that warranted a high power setting, was flown; this was to ensure faster engine spool-up, should it be required in an unfavourable gust of wind.  On short finals, a speed of 280 kph (150 kts) was diligently cross-checked, which was 20 kph (11 kts) slower than stipulated. At the runway threshold, the drag chute was deployed while still about 15-20 ft up in the air, something I had done many a time on the Mirage, though each time the height judgement had been a nerve-racking exercise.  As if trapped by an invisible genie, the aircraft was jolted back, but managed to land just ahead of the arrester barrier which lay prone.  Immediately after touchdown, the nose wheel was lowered and the bicycle type brake lever on the control stick was squeezed, hoping for the anti-skid system of the wheels to work properly.  The brakes were released and then reapplied to prevent wheel jamming, just in case.  The aircraft snaked a bit as the drag chute swayed in the wind and, came to a full stop in an incredible 1,100 ft (Eleven hundred feet)!  Those familiar with the runway at Rafiqui would know that exactly at this distance from the threshold is the first taxi-way link, normally used to turn-off after aborting a take-off from Runway 15.  Clearing the runway at this point, I was in utter disbelief at the stopping distance, for I had never experienced anything like it before.

The Senior Air Traffic Controller, Sqn Ldr Gohar Saeed, who was expecting me to be rolling down somewhere half way down the runway, in front of the ATC tower, called out anxiously about my position.  He was as relieved as he was shocked, when I told him that I had turned off at the Charlie Link and was proceeding back to the flight lines.  It took him a while to be sure that I had not skidded off the main runway and, deposited myself on the parallel taxi-track.

Gp Capt Shaheen Hameed, the OC Flying Wing, had overheard the unusual R/T exchange on his walkie talkie and found it ‘convenient’ to receive me at the flight lines.  Seeing that the wheels of the aircraft were not smoking too badly and, there was no tail scrape, we exchanged agreeable smiles and drove off.  I wryly assured him that I now had a solid data base and would be able to thoroughly check the F-7MG’s landing performance against its venerable ancestor. 

During the evaluation trials of the F-7MG at Chengdu, a very short landing was made as well, but due to an inexplicable absence of runway distance marker boards at the flight test facility, nothing conclusive could come out.  Shall we say then, that the F-7P stands next to the Harrier as the shortest landing jet fighter?

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[1] Named after Austrian physicist Ernst Mach. Mach No denotes the ratio of the speed of an aircraft (or any object) to the local speed of sound. As speed of sound is inversely proportional to the surrounding air temperature, a decreasing temperature from the surface up to the tropopause results in an increasing Mach No. In other words, an aircraft flying at 661 kts true airspeed at sea level, which equals Mach 1.0, would need to fly at 573 kts true airspeed (13% slower) at 36,090 ft AMSL to be at Mach 1.0. This means that shock waves will be experienced in exactly the same manner and, air flow parameters and relevant aircraft design features can be implemented simply with respect to the Mach No, rather than a varying slope of airspeed versus altitude.

[2] The lowest layer of the atmosphere characterised by a decreasing air temperature. Most of the weather phenomena are also found in the troposphere.
[3] Condensation trails of the water vapour in jet exhaust that freezes at high altitudes.

[4] The tropopause marks the border between the first atmospheric layer, the troposphere and the second layer known as the stratosphere. At the tropopause (36,090 ft AMSL in standard atmospheric conditions), the air temperature stops decreasing with an increase in altitude. Most commercial airliners prefer to fly at the tropopause, as it offers the optimum fuel burn for turbojet and turbofan engines due to low temperatures.

[5] ‘Parrots’ traces its origins to a WW-II code for Identification Friend or Foe (IFF) equipment; ‘Angels’ is a two-numeral brevity code for altitude in thousands of feet (eg, Angels 20 = 20,000 ft), used mostly by military radars.
[6] Area Ruling aims at preventing an abrupt drag rise in supersonic flight. This is achieved by narrowing the fuselage at the wing root, whence the wing drag starts to add up to the overall frontal drag. An Area Ruled fuselage, thus, takes a contoured shape, much like that of the classic Coca Cola bottle.
[7] Named after Maj Gen Harry G Armstrong, a USAF flight surgeon and aero-medicine researcher who first discovered the phenomenon.

 [8] Starting above the tropopause, the stratosphere is marked by an ‘inversion’ of temperature, which remains steady at -56.5°C till 65,000 ft AMSL and then starts to rise, with a consequent decrease in efficiency of turbojet and turbofan engines. Long endurance flights like strategic reconnaissance are best flown around 65,000 ft, due to a suitable combination of low temperature (for better engine performance) and reduced air density (for lower airframe drag). Considerations of longer radio or radar line of sight may entail flights at even higher altitudes, by aircraft with specially designed engines.