4-Engine Overhaul Era

by William Maxfield
Superintendent of Maintenance, TWA

With the inauguration of 4-engine. supercharged airplanes. new problems arrive for the maintenance man to solve.

Efficiency and economy in airline maintenance work depends to large extent upon the successful correlation of all duties to create a steady routine and turnover in aircraft and engine work.

Picture, then, the situation a year ago in the maintenance department of Transcontinental and Western Air, Inc, when it was announced that the airline was purchasing five 307-B Boeing Stratoliners, which were equipped with new devices never before used on transport planes, including a cabin supercharging system, autosyn instruments, retractable tail wheel, electricity-operated flaps and two-speed superchargers on the engines.

For five years, TWA had been operating with Douglas equipment exclusively, and the maintenance work on the DC-2, DC-3 and SDT models had been organized to the point where inspection and overhaul had reached an ultimate in smoothness.

But within a few months, the entire maintenance department was to be equipped and personnel trained to take charge of the big four-engine planes when they arrived from Boeing's plant at Seattle.

Starting from scratch, without benefit of precedent because no domestic airline had yet flown four-engine planes on regular schedules, the first step was to organize a program that would dovetail Douglas and Boeing maintenance as much as possible.

Mechanical and maintenance personnel spent months in Seattle, while the Stratoliners were in the final stage of construction, to familiarize themselves with every detail of the planes. As a result of their studies a maintenance manual was prepared and placed in the hands of all personnel concerned before the delivery of the first ship.

A third member of the flying crew — the flight engineer — was to be added for flights of the four-engine planes, and it was decided that the position would be filled by non-flying mechanical experts to be selected from mechanic personnel within the company who held both airplane and engine certificates of competency. Two years before delivery of the Stratoliners a correspondence course was prepared and given to all interested and qualified personnel. Those qualifying with highest grade were called to Kansas City for interviews and 18 were selected for further schooling and flight duty.

After completion of the ground course, the flight engineers and their supervisors visited all stations to give lectures and answer questions concerning the Stratoliners. In addition, all maintenance supervisory personnel from outside stations were sent to Kansas City for instruction. By the time the first Boeing plane was delivered to Kansas City the maintenance department was completely schooled.

During the pilot and field checkout program, before actual schedule service began, the maintenance department became thoroughly familiar with the ships and during the same period all flight engineers were checked out with a minimum of 20 hours actual flying time.

To synchronize Boeing and Douglas equipment, special platforms, jacks, ladders and towing apparatus were designed not only for maintenance work but also for loading and servicing on scheduled flights.

The Douglas carries mail, express and baggage in front and rear compartments and the Boeing Stratoliner carries its cargo in belly compartments. One of two belly hatchways in the Boeing is used for a crew entrance to the control room while Douglas crews enter either through the passengers' cabin door or through a small baggage door in the nose. These belly entrances required a special low platform of an entirely new design. The loading steps used on the Douglas planes could be utilized for the Boeings by adding two steps and lengthening the handrails. The two extra steps can be retracted when the platform is used for Douglas passengers.

For maintenance work in the hangars, workstands that were suitable for the twin-engine Douglas models had to be extended to accommodate the Boeing's outboard engines. New facilities for hoisting the Boeing also had to be provided since the plane weighs 30,000 pounds empty compared to the Douglas DC-3 empty weight of 16,228 pounds and regular overhead hoists could not be used. Special heavy duty jacks were therefore necessary.

Larger battery carts, designed to accommodate 24V batteries, also were necessary. Fuel trucks were altered to provide two hose lines in order that both left and right wing tanks could be filled simultaneously. Because the Boeing plane carries a gas load of 1,700 gallons, two gas trucks otherwise would have been necessary to prevent schedule delays. New wing mats and wing ladders were designed because Stratoliner wings are higher and wider than those on a Douglas plane.

It was learned that the large size of the Boeing passenger cabins required servicing by two air conditioner trucks in excessively hot or cold weather. The heated, or cooled, air enters through two vents in the right rear of the plane directly into overhead ventilator ducts.

The Boeing wheels weigh approximately 400 pounds each and cannot be handled by hand so a special dolly was designed that carries the wheel upright and can be adjusted up or down so the wheel can be placed on its axle directly from the dolly.

For work on the vertical stabilizer, which is 20' 9" high, a special trestle has to be assembled each time a Stratoliner goes into the hangar. Due to the size of the trestle, it was not designed for permanent use and is disassembled when not in use.

Differences in actual operating parts of the Boeing and the Douglases also came in for a thorough study for maintenance purposes. Hydraulic power activates most of the movable controls on a Douglas while electricity is the Boeings' principal power agent. This includes landing flaps and landing gear.

Because of the extensive use of electricity the Boeings utilize two heavy-duty 24V batteries compared to two 12V batteries for the Douglas planes.

Entirely new in design and principle was the cabin supercharging system on the Stratoliners, which makes possible the high-altitude flights from 16,000 to 20,000 feet. The supercharging, or "altitude conditioning" system increases the density of the air in order to hold the cabin altitude between 8,000 and 12,000 feet although the plane actually may rise to 18,000 or 20,000 feet. It was learned that the air, when subjected to the compression of the supercharger, would increase about 37° in temperature and a method of cooling the air was necessary. A satisfactory intercooler system was then devised. The intake for the supercharging system is in each wing and another intake was drilled in the wings several inches from the supercharging intake. This second hole opened to another duct that extends past the part of the supercharger that compresses the air. (Over 100 5/16" tubes were inserted in the supercharger duct where the new duct intersected. The tubes were sealed together at each end so the compressed air could pass only through the tubes. The air, entering through the new intake, circulates through these tubes and cools the compressed air before exhausting through electrically-operated shutoff valves located in the exhaust duct in the underside of the wings. The flight engineer, who is in charge of the supercharging system, can control this valve and can close it in cold weather, thereby relieving the load on the steam heating system by allowing the compressed air to retain its heat.

The instrument department maintains the condition of the supercharging system, and overhaul experts are assigned to inspection work to insure cabin door and hatch seals are in perfect condition at all times, otherwise it would be difficult to maintain a steady cabin pressure.

It is interesting to note that a few passengers making their first trips on the Stratoliner have informed the air hostess that the "altitude conditioning system bothered them, when the system had not yet been turned on, and, on the other hand, the same passengers did not notice when the system actually began to work. The supercharging system is one of the prides of TWA, first to introduce high-altitude flying on domestic airlines.

The Stratoliner's engines are equipped with two-speed supercharger blowers, which also makes necessary a different system of engine service overhaul compared to the Douglas planes.

Some of the Stratoliner's control surfaces are fitted with hydraulic booster units to aid the pilots' manual operation and these are so balanced that overcontrol is impossible.

In general, various maintenance operations on the Boeings are practically the same compared to the Douglases, but the Stratoliners have many more items to be checked, and, as a result, more maintenance man-hours are necessary.

Today, TWA's maintenance program again is operating in routine fashion for both Douglas planes and Boeing Stratoliners. With the preliminary program completed and overhaul work running smoothly, it is a question only of added manpower to handle the new duties required for the country's largest land transport planes.

This article was originally published in the December, 1940, issue of Aviation magazine, vol 39, no 12, pp 44-45, 130.
The original article includes 3 photos.
Photos are not credited, but are probably from TWA.

Photos: