Flying Machines 5: Light Twins

Piper Pa31 Navajo

The first light twin I flew, admittedly under a lot of supervision, was the Piper PA31 Navajo. There have been quite a few versions of the Navajo, but the basic model had Lycoming TIO-540 engines of 310 hp. It cruised at about 180 knots and with the turbocharged engines could get to a good altitude. The

Piper Navajo 310

Piper Navajo 310

highest I ever flew one was 25,000ft on a photo survey in Tanzania. The controls were the usual Piper mess with the pitch stability involving a large spring in the elevator circuit; this and the rather stiff main landing gear made most landings a bit of an arrival. In most aircraft the engines drove three bladed constant speed propellers which made a delightful humming noise (cruise RPM was 2200 RPM). The engines seemed pretty bomb proof and rarely gave any problems.

I first flew in one when Sam St. Pierre who worked as a survey pilot, but was also a keen glider pilot took me on an air test and later flew Mike Carlton’s aircraft to Le Touquet. My first professional flying job was with Air Foyle and after a few weeks flying the Aztec I was packed off to Africa to fly a survey aircraft. The aeroplane was in Douala in West Africa and the survey in Tanzania in East Africa, so the first job was to fly across Africa. The story is a good one, but too long to put here. I also flew a Navajo westwards across the Atlantic.

I later flew Navajo 310s, 325s (bigger engines) and the Navajo Chieftain (Longer and even bigger engines) on air taxi and night freight flights and flew one across the Atlantic; another story, too long for here. In the early 1980s a pilot could almost always get a job the next day if he had the Navajo on his licence.

Piper Seneca 1

Piper Seneca 1

Piper PA34 Seneca 1

My night and instrument training at Oxford was in a Piper Seneca 1. This sad apology for an aeroplane had started life as an extension to the single engined Cherokee 6. With bigger wings and two engines (the prototype was said to have three!), Piper’s attempted to sell it as a replacement for the ageing Twin Comanche. It was a dog, the controls were awful and the engines nowhere near powerful enough. I was once in one with two other people and with one engine feathered and full power on the other it would not climb at all. As with many earlier twin engined aircraft the second engine merely took you to the scene of the accident. The later versions were fitted with turbo-charged engines and performed a bit better. The last time I flew one was the day I passed my instrument rating I am pleased to say.

Piper Aztec C

Piper Aztec C

Piper Pa23 Aztec

Soon after I had gained my CPL IR I was introduced to the Piper Aztec. This was a much more honest aeroplane than the Seneca above. It was a much older design with the same wing section and planform as the Super Cub and two healthy IO-540 engines. As an Air Taxi it didn’t really cut it, as the passengers still had to climb up onto the right wing to get in and the rear two seats were fairly cosy.

The things that went against it were the lack of any supercharging of the engines which made it run out of puff much above 7000 ft and that the early models only had a hydraulic pump on one engine, if you lost that engine you could not retract the undercarriage and with the gear down on one engine it was a strictly downhill problem. Even worse was an engine failure with the gear half up!

Cessna 404 Titan

Cessna 404 Titan

Cessna 404 Titan

A few years later I was introduced to the Cessna 404 Titan. The Titan’s airframe was good, it lifted a good load, the controls were light and effective and the avionics were good for a light twin. The problem was the engines, they were continental GTSIO-520s of 375 hp which were developed from the 250 hp IO-520. The truth was that in trying to pull 375 hp out of these engines they had gone a bit too far. The propeller shaft was gear driven from the crankshaft with a reduction gearing which meant the engine turned at over 3000 RPM at full power. They had had to fit dynamic counterweights which worked fine with the throttles well open and the engine having plenty of air to chew on, but less well if the throttle was closed which would lead to cracks in the crankcase. The only way to stop this happening was to maintain cruise power until almost in the flare using the gear and flaps to reduce speed.

375 hp was not really enough for this aeroplane, the “book” rate of climb on one engine was 175 fpm at 109 knots, in a tired aeroplane with tired engines it would descend. I understand that fitting PT6 turboprops to this (or at least a very similar) airframe in the Caravan 2 resulted in a very good aeroplane.

De Havilland Canada DHC-6 Twin Otter

Twin Otter in the Old Brymon Colours

Twin Otter in the Old Brymon Colours

The DHC-6 Twin Otter is in a totally different class, in fact it isn’t really a light twin being certificated in other countries to 14500 lbs, in the UK it was limited to 12,500 lb to allow single pilot operation. It is very sturdy, has controls that were designed by the same person who designed the Chipmunk and best of all has Pratt & Witney Canada PT6-27 engines. The PT6-27 is really a 850 hp engine derated to 620 hp which it can maintain up to 12,000 ft.

The “Twotter” was designed out of the DHC-3 Otter which was a smaller single engined tail dragger with a Pratt & Witney R1340 Wasp radial piston engine of 600 hp. Both aircraft were intended for bush operations, but the Twin Otter also became attractive to small airlines in more normal places. The “bush” design made it very reliable as did the PT6 engines. The only flaw was the crosswind landing problems; being a nosewheel aircraft the twin could not do a wheeler landing as its single brother could. The nosewheel

Twin Otter in the New Brymon Colours

Twin Otter in the New Brymon Colours

steering was operated through a lever behind the left control yoke in much the place an indicator switch would be on a car. There was no manual reversion and no rudder fine connection and a pilot’s attempt to grab the lever as he lost aerodynamic control would result in the into-wind wing rising, the aircraft weathercocking into wind with the downwind wheel digging in sideways and the aircraft ending up on its back. There was a way out using asymmetric reverse, but it took a lot of practice.

Apart from the crosswind problem it was a delight to fly and I even forgave it the rather low cruising speed (150 knots). As I am unlikely to fly any new types, the Twin Otter will always be my favourite.

Partenavia P68

Partenavia P68

Partenavia P68

The Partenavia was an aeroplane that I managed to avoid flying commercially. It is not really suited to air taxi or night freight being a bit small and, with only O-360 engines, rather inclined to carburettor icing. However as a club aircraft it is very good. As an aside it proves the lie to my long held definition of a big aircraft as one with the engines behind the pilot and a wing high enough to walk under.

My experience of the Partenavia is being lent one, free of charge, for a day. The reasons for this offer are too complex to explain here, but they involve the UN and a long ferry flight to come. I and the aircraft were in Plymouth and I decided to go to Booker to pick up Helen and Amy (‘Becca was yet to come) and return to Plymouth. A couple of Brymon staff came along for the ride. A delightful trip in good weather with splendid views of the ground with no wing in the way. We duly picked up Helen and A and set off for Devon. The weather stayed good as far as Exeter, but beyond it was a bit “Iffy”. However, unlike the other light aircraft going that way, we did not have to turn back, but having two engines declared IFR, climbed to 4500 ft and did an instrument approach into Plymouth.

The only briefing I was given by Don Sainsbury who lent it to me was “It’s just like a miniature Twin Otter, have fun”.

Beechcraft Duchess

Beech Duchess

Beech Duchess

Another aeroplane I only flew in once was the Beech Duchess, in fact it wasn’t me that flew it, it was Helen. Helen had just completed her instrument rating and as a bit of a celebration we put Amy and ‘Becca in the back and flew to Guernsey (by way of Bournemouth to clear customs) for lunch and back by the same route. I thought it a splendid aeroplane and in fact it worked out cheaper than using a single engine C 172 because it cruised much faster. Even more impressive was bringing the castings for the steamboat engine back with us.