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SOME ENGINEERING CHARACTERISTICS OF THE BOEING 707-138
by Bill Fishwick
| These are some of the
engineering characteristics of the B707-138 which were not on the
B707-138B aircraft as they were removed earlier or during modification
to 138B status and the engine from JT3C-6 to JT3D-1. These recollections
are written in 2011 about the aircraft in 1959 to 1962, that is some
50 years ago, so there might be some errors due to memory and the
passage of time. Flares: At the very end of the aft fuselage on the underneath there is a figure eight shaped access door. This was where two parachute flares were fitted. In the case of a ditching at night these flares could be deployed individually. The flare fell from the aircraft a certain distance but still connected to the aircraft by a cable. At a determined distance the cable broke and flare ignition started. If the flare switch was activated on the ground then the flare would only fall from the aircraft and not ignite, as the distance to activate it was greater. After a year or two the flares were removed permanently, but I do not know the reason. The outside panel was made of some very light weight material. If it had to be replaced we would use the same material that was used for the sliding window shades in the cabin. Combustor Starter: There was no auxiliary power unit fitted so the engines were started using a ground cart that delivered air at 40 psi pressure. The ground cart was a miniature jet engine which connected to the aircraft via a hose to a point on the lower fuselage near the wing leading edge on the right side. To be able to start the engines at a station where there was no ground start cart, the aircraft had a combustor starter system fitted to engines #2 and #3. Air and fuel were fed into the combustor, ignited and the resulting gas was used to drive the starter. An air bottle to supply the air was fitted in the right wing aft fairing, just forward of the rear cargo door. At the forward end of the right wheel well on the bottom of the keel beam was installed a compressor, driven off the hydraulic system to recharge the air bottle. When the system was de-activated the four mount points for the compressor could be seen. I expect it would still be there on VH-XBA. The air bottle could also be recharged from an external source in the right wheel well. The crew and passenger oxygen bottles were stowed in the rear cargo hold just aft of the door. When the air bottle was removed from the aircraft one of the electrical/instrument foremen suggested placing the oxygen bottles in the space where the air bottle was, thus freeing up space in the cargo hold. This earned him a top award in the Staff Suggestion Scheme. Water Injection System: A void between the main gear wheel wells was the full height of the well, and ran almost the full length with two walls enclosing it, approximately 12 inches apart. In this area was a tank for the water injection system. It contained 375 Imperial Gallons Also there were two rubber bladder tanks each of 117 Imperial Gallons located at the rear of the wheel wells and connected to the main tank. It was filled overwing from a point in the port wing to body fairing. De-mineralised water was used because impurities in normal water would affect the compressor and turbine blades in the engine. A couple of times in Sydney de-min water was not available so local tap water was used as this was acceptable for emergency usage. At the bottom of the tank at the rear were two injection pumps. These were easy to work on if both main gear doors were up, but a bit restricted if one was down and well nigh impossible if both were down, as access was between the doors. The pumps were located in a fore and aft line; one fed the engines on the right wing and the other the left wing. This was a problem for the Flight Crew on a water injection assisted take off as they had to calculate when the water would be used up, and switch off the pumps before this occurred, otherwise the front pump would run out of water first causing the aircraft to yaw. Then the Flight Engineer had to open a drain valve which was located aft of the water injection pumps to drain any residual water remaining in the tank before it froze. The valve could only be closed on the ground and a number of times the valve was still frozen and the operating system sheared. One of the ground engineers suggested making a tapping off the cabin air to flow over the valve to keep it warm and this solved the problem. There were injection pumps mounted on each engine to boost the pressure before the water was injected. The water was injected at two points, at the intake of the first compressor and at the beginning of the combustion chamber. The injection point at the compressor could be selected off at low ambient temperatures to prevent the water freezing. Anti-icing: The 138 had anti-icing on the wing leading edges, engine intakes, nose cowl and inlet guide vanes via hot air bled off the engines. There was also protection for pitot tubes and windscreen via electric heating. The stabiliser leading edges at the tail were protected by a rubber boot that had electrical wires embedded in it, covered by an outside layer of stainless steel. This was fine until there was a lightning strike that struck the leading edge and burned a hole in the stainless cover into the rubber boot and severed heating wires. Consequently, Boeing did further tests with mock ice on the leading edges and found that anti-icing of the tail was not needed. As a result, the system was removed. Pre-set Re-fuelling System: The B707-138 came with a pre-set re-fuelling system. On the Flight Engineer's lower panel was a guard covering switches and rheostat controls. When re-fuelling, the guard was lowered, and the switch for the applicable tank moved from a normal to a re-fuel position. With the rheostat , the fuel quantity gauge was wound up to indicate the amount of fuel required in that tank (or it might have been the amount to be added). Re-fuel started and when the gauge wound down to zero a light was illuminated on the underwing fuelling station. The person there would then close the re-fuel valve. [At least I think that was how the switching and the rheostat worked with the fuel gauge]. After some time this pre-set re-fuelling system proved to be unreliable so its use was discontinued. To re-fuel then, dripsticks in each tank were pulled down at the lower wing until the desired amount in inches was showing. When the tank got to the desired level fuel would enter the dripstick top and run down inside to pour out sideways onto the ground. You had to be careful where you stood. A call was made to the person on the underwing re-fuel station to shut off the fuel flow. After allowing the fuel level to settle, the dripstick reading was taken and it was then pushed up into its recess and secured. The 'Greenies' these days would not like the thought of fuel pouring out onto the ground from dripsticks! While at Longreach in 2007, I noticed that the pre-fuel system was still in place at the Flight Engineers station of VH-XBA. |