Hugh Pryor

When you go to a party where you don’t know anybody, you should always go for the ‘Ugly’ girl, because she will be happy that you chose to spend time with her.

More than likely, you will enjoy a splendid evening full of laughter and genuine friendship, and you will meet her at later venues, where you will  enjoy each other’s company. There is none of the hassle which you get if you are escorting a racy blonde bombshell.

Similarly, the Pilatus Porter PC-6 is ugly, but fun and a great friend in difficult circumstances.

In comparison, Lear Jets and in particular the Lear 25, looks racy and sexy, but like many beauties, she can be cantankerous and occasionally extremely dangerous.

Based on a failed Swiss jet fighter, the Lear Jet was born in the days before supersonic flight became the norm for fighter aircraft.

Aeronautical engineers then did not really understand the problems of compressibility in trans-sonic flight. Notably, pilots did not appreciate that, as the wings approach the speed of sound, a shock wave builds up, rather like the bow-wave of a ship, above and below the wing, more above than below. As the speed increases, the shock wave moves back across the chord of the wing.

Wikipedia  describes how Mack Tuck affected even the piston powered fighters of WW2, noting that “The fastest World War II fighters were the first aircraft to experience Mach Tuck. Their wings were not designed to counter Mach Tuck because research on supersonic aerofoils was just beginning; areas of supersonic flow, together with shock waves and flow separation, were present on the wing. This condition was known at the time as compressibility burble and was known to exist on propeller tips at high aircraft speeds.

The P-38 Lightning was one of the first 400 mph fighters, and it suffered more than the usual teething troubles. It had a thick, high-lift wing, distinctive twin booms and a single, central nacelle containing the cockpit and armament. It quickly accelerated to terminal velocity in a dive. The short stubby fuselage had a detrimental effect in reducing the critical Mach number of the 15% thick wing centre section with high velocities over the canopy adding to those on the upper surface of the wing.

Mach tuck occurred at speeds above Mach 0.65; the air flow over the wing centre section became transonic, causing a loss of lift. The resultant change in downwash at the tail caused a nose-down pitching moment and the dive to steepen (Mach Tuck). The aircraft was very stable in this condition, making recovery from the dive very difficult.

The lift in the sub-sonic air behind the shock-wave is much greater than the lift in front of it and eventually, for the early Lears, at precisely .87 of the speed of sound, (Mach 1,) the horizontal stabiliser ran out of authority to keep the nose up.

When the wing stalls, it does it so viciously that the subsequent nose-over imposes negative Gs on the airframe, powerful enough to eject the passengers and crew through the roof with their seats and to cause the wings to fail downwards.

To prevent this, they installed  a ‘Mach Limiter’ to pull the nose up at .84 Mach in order to prevent the plane from nosing over into ‘Mach Tuck’.

During the year when I was a co-pilot on the aircraft, seven Lear 25s fell out of the sky in similar circumstances. The violence of the destruction spread the remains over such a wide area that it was difficult to identify the primary reason for the disintegration and it was not until investigators recovered two of the primary circuit breaker boards that they discovered the probable cause of the disasters…on each of the circuit breaker boards, the Mach limiter circuit breakers had been pulled.

On further investigation, after talking to other Lear 25 crews, it turned out that there was an ongoing competition among Lear 25 crews to see who could get from San Francisco to Las Vegas quickest, and if they pulled the circuit breaker, they could operate at .86 with literally a couple of knots between ‘winning’ and losing everything.

There were several other details which I found vaguely annoying about the Lear 25. The type-rating involved a course in gymnastics in order for the flight crew to gain access to the cockpit, without breaking any switches or bones.

If you saw a Lear 25 crew walking down the street, how would you know which one was the Captain and which was the First Officer? The Captain will have his head resting on his right shoulder and the co-pilot’s head will be on his left shoulder, because that is how they have to sit on the small cramped round flight deck.

The fuel tanks were installed like torpedoes on the tips of the wings and if you were refuelling an empty aircraft, it was necessary to half fill one tank and then fill the other one, before topping off the first one, because if you filled one tank with the other one empty, the plane would fall over.

Then there were the leading-edge de-icing strips on the wings. In order to work efficiently, they had to be spotlessly clean. There was a joke among the engineers; What has more finger prints than the FBI? The answer was the Learjet de-icing strips.

And did I mention the windscreen? It was made out of Perspex, more than an inch thick and it was difficult to polish without introducing distortions and scratches, so it was tempting just leave it until it was virtually opaque and then cleaning and polishing it would take an age.

Okay, the Lear may turn heads with its sleek lines and sexy figure, but give me the good old Porter any day! We go back a long while together and she gave me more than six thousand hours of trouble-free friendship.

Maybe I am just an old Bush Bumkin, but I prefer to fly aeroplanes which can take me to places where there are no officials trying to justify their existence by giving me a lot of ‘up-hill’, through airspace unmolested by unintelligible controllers and air-traffic-jams of enormous flying hotels, flown by pilots who spend their lives being told what to do by computers.