Flying Machines 7 DHC – 7

Brymon Dash 7 at Plymouth before the LCY colour scheme and with a stormy south-west sky

Brymon Dash 7 at Plymouth before the LCY colour scheme and with a stormy south-west sky

The DHC -7 is a bit of a peculiar aeroplane to be an airliner. It was intended to take over when a Twin Otter was not big enough for a route involving short runways. Going up to 46 passengers from 20, of course, meant a much bigger aeroplane and that in turn meant it became subject to a lot of “big aeroplane” regulations. The power went up from two 620hp engines in the Twin Otter to four 1200 hp engines, the biggest PT6’s that were ever built. The short runway performance came very much from a low wing loading which in turn meant a low cruising speed. If the route didn’t involve a short runway, you didn’t want a Dash 7.

The aeroplane was built to the very high standards that DHC are famous for. Only 30% of the roll control came ailerons, the rest of it from roll spoilers. the controls were split in that twisting a lever in the cockpit disconnected the two control wheels leaving the Captain with the ailerons and the First Officer with the roll spoilers. The idea was that if there was a jam in any of the control runs you could regain control of the aeroplane. There was a similar system with the elevators, left control column operating the left elevator and right the right. In all normal operation, of course, the control columns would remain connected together and either would operate all the controls.

The rudder was also very powerful being a double hinged affair. With these controls and use of the “wing down” technique it coped very well with cross winds. The book limit was 45 knots across (this with an airspeed over the threshold of just over 80 knots) and that could result in you doing the last bit of the approach looking at the runway through the other windscreen. I was told that the book limit was 45 knots because it had not been tested in any stronger crosswind; I certainly never found a 45 knot crosswind any problem except that the rudder would stall at about 40 knots, so it was as well to get the nosewheel on the ground fairly quickly.

The view forward of the approach to a short runway. When you first see this you can't believe that anyone could think an aeroplane that big could land on that tiny runway

The view forward of the approach to a short runway. When you first see this you can’t believe that anyone could think an aeroplane that big could land on that tiny runway

The other thing that the Dash 7 was really designed for was steep approaches. A normal ILS approach is three degrees, the -7 was cleared up to seven and a half degrees. Even at 82knots that gave a rate of descent of over 1300 fpm and an attitude of up to 17.5 degrees nose down (in the early days of London City Airport all approaches were flown raw data as the flight system couldn’t cope with the attitude).

The steep approach was achieved by the use of very large flaps that deployed to nearly 90 degrees (it was placarded as “Flap 40”, but that was the angle between the leading endge of the wing and the trailing edge of the flap) and some very special propellers. Because of the massive angle and the worry about the failure case and asymmetric operation, the movement from 25 to full flap was split into three sections on each wing that were individually powered by hydraulic rams. The movement was very quick (about a second) and as the drag was enormous they would retract just as quickly if the throttles were moved more than about a third forward. To get the weight firmly on the wheels for braking they would also retract on touchdown.

The clever propellers were designed to go into beta in the air on a steep approach to give more drag. Effectively the propellers became rotating air brakes, someone worked out that on a steep approach 10% of the weight of the aircraft was being supported by the props. If you have not flown a turbo-prop and don’t understand the term beta you need to Google it.

In normal operation take off and landing was made with flap 15 or 25 with the full flap 40 only used on short runways. Only the outer engines were started on stand with the inners started just before take off. On some occasions when given an unexpected line-up clearance at Heathrow I have known these engines go to full power almost as soon as the start cycle had ended, which is hard treatment for a turbine engine, but they were well built and we had few problems. After landing the flaps were retracted only to the next take-off setting to avoid wear in the flap tracks. Because of Brymon’s intense use of the aircraft and very short turnaround times (they did 14 sectors a day), the after landing check list more or less set the aircraft up for the next take-off. Turnround times at Plymouth and Newquay were scheduled at 15 minutes and 10 minutes respectively, but we often shaved 5 minutes of of both of them. My best was 4 minutes at Newquay with complete shutdown, passengers and baggage on and off.

In general the aeroplane was very pleasant to fly, if a little slow as it cruised between 200 and 220 knots. It was pressurised, although we normally only flew it at 12 to 13,000 ft, it was only worth going up to its ceiling of 25,000 ft if you were flying a long sector. The flight director and autopilot worked well, although it did have one trap for the unwary. The aeroplane was capable of quite high rates of climb and descent and if you thought it was approaching the level a bit too fast you could reduce the rate by adjusting the pitch attitude thumbwheel. Normally all would be well, but if you moved the thumbwheel just at the instant that the system was changing to alt acquire, all vertical modes dropped out without any warning and the aircraft would make no attempt to level off. I would like to say you only got caught once, but sadly…….

The engine controls were typical P&WC PT6. Four throttles that controlled the gas generator and therefore torque when airborne , four propeller controls/engine condition levers that controlled prop RPM/feathering and the HP cocks in one lever. Reverse thrust was controlled by moving the throttles back from the flight idle gate which opened as the weight-on switches on the undercarriage. The flight idle gate would close with a loud “Clack” as the aircraft left the ground and open with an equally loud one when you landed. A really good landing was announced by a series of clacking noises as the aircraft tried to make up its mind if it had landed or not. below the centre instruments there are the two yellow controls to disconnect the control wheels from each other.

The cockpit

The cockpit

The centre of the main instrument panel was taken up with the engine instruments. Top to bottom Torque, Prop RPM, Gas Temperature, Gas Generator (HP) RPM and Fuel Flow. The oil temperature and pressure gauges were outboard of the bottom row of main instruments. The square panel to the left of centre with the coloured lights on controlled the engine inertial separator ice protection system.

The top centre panel has the autopilot/flight director selectors, autopilot engage buttons and the landing light switches. The outer top panels are dominated by the two rather complex clock/stopwatches which had far more functions than any pilot could need or understand. On the captain’s side there are lights to show the operation of the spoilers and the Flight/Ground switch; this controlled the ground spoilers on the ground, this had to be put to “Flight” when you lined up for take off and to “Ground” after landing to close the ground spoilers. If you forgot to put it to “Flight” it would go there automatically when you opened the throttles.

The other side of the clock had the very important “Propeller in Ground Range” lights and another set that would sense an engine failure. The propellers had an auto feather system that would, as their name suggests, feather the propeller if the engine failed; the system was switched on for take off. It was quite clever in that if an engine failed on the take off run it wouldn’t auto feather (which would produce thrust as the prop ran down) but go into a braking mode as the throttles were closed.

The panel just to the right of the captain’s control wheel showed the position of the powered control surfaces.

The electrical system was a little complicated due mainly to the PT6’s 28volt starter/generators and DHC’s need for AC power for some bits of the aircraft. The main system was 28v DC with inverters to supply AC to the instruments. However, there were frequency wild AC generators driven from the propeller gearbox; these supplied power to various heating loads (notably the propeller de-icing) and also to the pumps in the fuel transfer system.

The pneumatic system power source was bleed air from the engines which for some reason didn’t have air for the control system from the No. 4 engine. This was silly because No. 4 was always started first and it could lead to some problems in the air-conditioning system that I really can’t remember. All the Brymon aircraft had been modified to make No. 4 fully part of the team, but it was a problem when we flew leased aircraft. Air conditioning was by two packs, one in each wing root. Hydraulics were a fairly simple two system arrangement with the left system powered by the left engines and the right by the right. I remember the mnemonic for remembering what the powered was FANGIA for the left and LOSER for the right, but I now have no idea what they stand for except F was flaps and L was landing gear.

The cabin looking forward from the rear galley

The cabin looking forward from the rear galley

The cabin was comfortable with four abreast seating for 46 passengers. Both the cabin crew seats were at the rear of the cabin as was the main door/airstair on the left. The cockpit door also formed the back to the cockpit jump seat and a stewardess opening the door to supply tea and forgetting about the third person in the cockpit could easily have him falling out backwards.

There were two emergency exits between rows 1 and 2 (I think) and another, door sized, one on the right side of the galley.

The cargo hold was on the deck level behind the galley normally accessed through a loading door outside the aircraft on the right side. There was a door in the rear of the galley that gave access to the hold in flight. The Brymon aircraft always carried a quantity of ballast in the form of bags of stones in the hold as the aircraft was nose heavy with a full load of passengers unless there was a lot of baggage.