Pan Am Maintenance

Aero Digest May, 1930 Page 61

Maintenance Methods of the Pan-American Airways System

W. Overton Snyder

Divisional Engineer, West Indies Division Pan-American Airways, Inc.

"In air transportation all things are possible." In spite of its optimistic implications, this statement is most often used as a vivid reminder that any air transport enterprise may encounter disaster if there is the slightest lack of farseeing precaution or any diminution of daily, vigilance. In this conviction, then, the maintenance department of the Pan American organization has labored to set a standard which precludes failure either of man or machine.

The first necessity for starting any enterprise is a plan. This was provided for the Pan American maintenance department by Mr. Andre A. Priester, who brought to his task a wealth of practical experience, vision, and engineering training. His basic principle, roughly, was that safety was paramount in air transportation, and that, moreover, safety was a true friend to profitable operation. He made it plain that if ample funds of money and of man-power were early invested in the interest of safety, the company would so much the sooner establish itself impregnably in its chosen field, and dividends would be so much the sooner forthcoming.

Pan American operates over such great distances and from such widely separated bases, that each "division" must be practically self-sufficient in daily operation. It was decided, therefore, to put the maintenance work of each division under a divisional engineer working under the general guidance of the chief engineer. The essential qualifications for these positions included extensive experience in aircraft manufacture or operation, and graduation from a recognized engineering university. This set a basic principle in the personnel plan.

Another fundamental decision was to centralize maintenance equipment and supplies so far as possible at one point on the division rather than at a number of smaller bases. As an example of the execution of this policy, Fokker F-10 planes based at Miami now regularly complete a trip of 2,890 miles to San Juan, Porto Rico, and return. They receive a complete, routine fifteen-hour servicing at San Juan. but have all special repair work done at Miami. In like manner, Sikorksy S-38 amphibions based at Miami regularly complete the 1,868-mile trip to Tela, Honduras, and return. Following the same principle, a very complete stock of replacement parts and materials is kept at Miami for issuance to the intermediate stations as required. Of course, the latter stations are-all stocked with the parts and materials which experience has shown to be necessary to maintain the airplanes safely on schedule.

Another factor to be considered was whether Pan American planes should be operated at a very low maintenance cost, expecting early obsolescense, or whether they should be kept in a very excellent state of repair, with a view to almost indefinite life. It was felt that, in spite of higher hourly maintenance cost, this latter policy would be most profitable in the end, since ships could be operated in a serviceable condition long after they had been entirely "depreciated" on the books. It was further felt that this policy would tend to keep the mechanics up to a very high standard of workmanship, and that safety would accordingly be served. It is for this purpose of preserving morale that visible copper fuel lines on Pan American airplanes must be highly polished and tires kept painted black, details more of appearance than of mechanical safety.

With the more fundamental policies established, a system of routine maintenance had to be set up. Naturally, any such system varies with the type of airplane, climate, and operating conditions, but general standards can readily be ascertained and established. At every overnight stop every ship, with its engines, is given a "Short Service," which consists briefly in the following steps:

Fill gasoline
Check oil level
Remove and inspect front spark plugs
Remove rocker box covers; inspect and lubricate valve rocker mechanism
Remove and clean gas strainers
Check rear spark plugs for tightness
Check motor mounts, fuselage structure, control system, and safety wire and pins
Check landing gear. including brakes
Lubricate control hinges

After fifteen hours of flying time, each airplane, with its engines, is given a "Long Service," which consists briefly of the following steps:

Fill gasoline
Change oil
Change spark plug
Lubricate valve rocker mechanism and check valve clearance
Check magneto breaker assembly, especially the points for wear and clearance
Remove carburetor jet plugs and flush carburetor
Remove and clean gas strainers
Remove and clean oil strainers
Flush hull and pontoons on seaplanes
Inspect and grease landing gear. including brakes
Lubricate control hinges and pulleys and clean control cables
Lubricate all zerk fittings
Clean engines and ship

After approximately 150 hours of flying time, carburetors are changed, since experience has shown that sufficient corrosion and sediment have collected by this time to warrant a carburetor overhaul.

After 250 hours of flying time the engines, together with all their accessories, and the propellers, are removed for a complete overhaul. At this time the airplane is given a very minute inspection, and all possible repair items completed.

After 1,000 hours of flying time the whole airplane is disassembled and completely overhauled. (This period may be somewhat shorter in the case of amphibions, and slightly longer in the case of landplanes.)

When the fundamentals of the Pan American maintenance plan were made clear, there followed the problem of putting them into effect. The way in which this has been done can best be made clear by describing the handling of a ship at the Miami terminal when it comes in from a twenty-eight-hour San Juan trip. As soon as the cargo is unloaded and the engines are stopped, the "line crew," with its tractor and dolly, transfers the ship to the fueling pits and immediately fills the gasoline tanks up to the standard level required of all outgoing planes. The tanks are filled thus immediately partly to prevent undue condensation of water inside empty tanks, and partly to ensure the ship's being ready for flight at any time on the shortest possible notice. While the filling operation is going on, an inspector is going over the ship checking every detail of the entire structure, making note of all defects. Simultaneously, the pilot who brought the ship in has reported to the operations office and then has gone personally to the divisional engineer to explain in detail any peculiarity or fault which he has noted while flying the airplane. Detailed notes of his remarks are made on his pilot's flight report, a copy of which the divisional engineer always receives. In addition to the inspector's report and the pilot's report, information is secured from a "Mechanic's Log Book," a small notebook which is always carried in the ship. In this book the mechanic at each station along the route describes what work he has done while the ship was at his station, and his observations. All of these various sources of information come within an hour to the superintendent of the Miami shops, who uses them all in preparing his "Work Order" for the airplane in question.

When the fueling has been completed, the ship is brought into the hangar and placed with its wheels and tail skid within boxes painted on the floor. The position of each ship in the hangar is scheduled in advance by the superintendent in order that the outgoing ships the next day may be recovered without moving any other ships.

When the ship is in the hangar, the superintendent's Work Order is placed on that plane's individual clip-board and hung on the ship. This Work Order, whether it be for a Short Service or a Long Service, has printed on it the routine items to be executed. Under these are written any special items. Opposite each natural group of items is a space for the name of the man assigned to that job, followed by a space in which he must sign when he has completed the work, followed in turn by a space in which the man's foreman must sign when the work is acceptable to him. Similar to the regular Work Order is a special sheet for the employee who specializes in checking emergency equipment, such as rubber life-rafts, life preservers, safety belts, and Very pistols. This sheet provides space for him to indicate what items he has inspected and what quantity of each item was on the ship when it was finally approved for regular service.

When all of the work has been completed on the airplane, the inspector once more goes over it in every detail, with particular reference to those items which were on his preliminary inspection report and on the plane's Work Order.

Each inspector reports directly to the divisional engineer, and is fully empowered to forbid a ship's being put into service until it has passed inspection to his satisfaction.

When the airplane is approved for service, the line crew brings it to the warming-up platform and brings the engine temperatures to forty degrees Centigrade in the usual manner. Each engine is then run up to full throttle, tested separately on each magneto, and instrument readings recorded on a "Final Check" card. In addition to giving instrument readings, this card states the amount of gasoline and oil in the ship at the time of departure, whether or not the emergency equipment was in place, and similar essential details. When this check has been completed, the engines are stopped and the ship towed to the loading platform at the passenger terminal, where the engines are started again a few minutes before departure time. It is worth noting that Pan American airplanes are always towed on the ground from the landing gear axle, in order to avoid the possibility of excessive fuselage strains involved in towing from the tail skid or tail wheel.

The example used above was typical of a ship performing its regular tour of duty. A somewhat different procedure is followed in the case of a plane which has completed 250 flight hours. The routine work on the airplane proper is conducted as above. The engines and propellers, however, are removed and transferred to their respective overhaul shops. The motors are set up on conventional engine work stands and completely disassembled, the parts being put on a parts rack, of which there is one for each individual engine. This rack is simply a large wooden tier of shelves mounted on castors. Major parts of the engine, such as cylinders, pistons and connecting rods, are tagged in order that they may be reassembled in the locations from which they were removed. All of the engine's accessories, such as magnetos, carburetor, starter and fuel pump, are removed to the accessory overhaul shop, where they are entirely reconditioned and tested. Magnetos, after overhaul and re-magnetization of the rotors, are run by means of an electric motor, their firing qualities being visually inspected on a conventional series of spark gaps. Carburetors and fuel pumps are attached to an overhauled engine and run on an engine test stand before being pronounced fit for flight duty. In the same shop an entirely new shielded ignition harness is constructed, for installation on the engine when completed.

In the meantime, the cylinders are carefully inspected, and the valve guides replaced when necessary. Then the cylinders are scraped clean of paint, repainted and baked, since it has been found that painting over the old enamel reduces the heat-dissipation efficiency of the cylinders and thus materially increases engine temperatures. Finally, the valve seats are reamed down and the valves lapped in by mechanics who specialize on that work. (The valves in the meantime have been refaced on a conventional Kwik-Way machine.)

Simultaneously, the internal engine parts have been cleaned of carbon and oil by a combination of chemical baths and brushes, and have been carefully inspected for defects. Whenever shown necessary by checks on dimensions, main bearings are replaced, bushings renewed, or entire new parts procured. When new piston rings are to be installed, they are carefully lapped down to provide proper side and end clearance. Ball and roller bearings are minutely checked both with feeler-gauges and with manual tests for smooth running. When all parts are ready for reassembly. one group of mechanics starts setting up the blower-section, checking each part as it goes in to see that it is within the tolerances published by the engine manufacturer. A number of special tools have been developed by Pan American employees to assist them in this work. When the blower section is completed, it is transferred to another group of mechanics who assemble the main-section and the nose-section. In like manner, all parts which must come within certain tolerances are carefully checked with gauges. Throughout the whole assembly process, all parts are inspected by experienced mechanics.

When an engine is completed, it is put on a conventional test stand, driving a four-blade club propeller, and is run for five hours at 1,400 r.p.m., then for an hour at 1,700 r.p.m. and finally for 5-15 minutes at full throttle. During the whole test period, complete instrument readings are taken every fifteen minutes, including cylinder head and flange temperatures taken on selected cylinders by means of a thermocouple. If the engine's behavior has been normal on the test stand, it is installed on an airplane, checked by an inspector, run for an hour on the ground, and finally test-flown for an hour at various speeds in the vicinity of the airport. When all minor adjustments have been made at the conclusion of the flight, the airplane with its newly overhauled engines is pronounced fit for regular flight duty.

Simultaneously with the engine overhaul proceeds the overhaul of the propellers which are to be fitted to the newly conditioned engines. The propellers are first disassembled, the blades etched in caustic soda, and the hub clamps and bolts polished clean. Then all parts are inspected with a magnifying glass for any sign of fatigue-cracks or intercrystalline corrosion. If no such faults are apparent, the parts are reassembled and the blades filed down to remove dents and leading-edge abrasion. Next, the blades are set to the proper angle and the assembly adjusted for track and balance. Finally, the blades are buffed to restore their original polish.

Certain other sections of the maintenance department schedule their work not according to 250-hour overhaul periods, but according to current repair requirements. Notable among these is the instrument repair shop, which is equipped to handle all types of fine instrument maintenance. In its equipment is included a jeweler's lathe and watchmaker's tools, an air speed calibration column, an altimeter calibration bell-jar, a calibrating device for pressure gauges, and a tachometer test stand. Under the direction of a skilled watchmaker, especially trained in aircraft instruments, this shop is able to handle any normal repair and replacement work. The instrument shop is charged with the maintenance of all compasses, for which work it requires a "compass rose" or turn table device for swinging ships. This is simply a circular concrete platform situated a considerable distance from any hangars. At its center is a small turntable on which one wheel of the airplane is placed. The tail of the ship is raised by a hydraulic jack onto a high "dolly" which rolls on castors around the circumference of the concrete platform. The standard method of "swinging" compasses in aircraft is too well known to require further comment, it is interesting to note in passing that, with the type of airplanes now used by Pan American, it is not necessary to run the engines while swinging the compasses. Numerous tests have proved that running the engines has no effect on the compasses. (This might not be true, however, in other types of aircraft.)

Of the other small but important sections of the maintenance department should be mentioned that concerned with spark plug overhaul. It is standard practice in the Pan American System to install completely reconditioned plugs at every fifteen-hour engine service. In being overhauled a spark plug is first completely disassembled and all parts cleaned on motor-driven wire brushes. Then the plug is reassembled and the gap adjusted with a special tool. Next the threads are inspected and a die run over them to remove any possible burrs. Finally, the plug is visually inspected for its firing qualities when under 100 pounds of air pressure. If satisfactory for service, it is given a mark to show that it has been overhauled and is then returned to the hangar for installation in a motor.

A system of airplane repair and overhaul, as distinct from engine and accessory overhaul, cannot possibly follow a rigid routine, and hence cannot be so readily described. One large hangar at the Miami terminal of the Pan American System is devoted exclusively to airplane repair work. It is equipped with an adequate sheet-metal and welding shop, a machine shop, a wood-working shop, a fabric and doping room, and a plentifully-supplied stockroom. It has been found that seaplanes and amphibions require fairly frequent attention, chiefly because of the susceptibility of duralumin to corrosion. This attention ordinarily consists in removing the paint from the hull, cleaning, and repainting, and in uncovering the lower wing structure in order that incipient corrosion may be cleaned away and new protective coatings applied. Obviously, such attention provides an opportunity not only for checking corrosion, but also for an essential inspection of important structural members, such as wing spars and strut fittings. When applied to duralumin operating under salt water conditions, the old saying, "A stitch in time . ." becomes trebly true. After 750 to 1,000 hours of flight time, each Pan American amphibion is completely disassembled and rebuilt, with new parts when necessary.

The maintenance of landplanes involves somewhat different problems, since ordinary maintenance work is concerned less with the actual structure of the ships than with their accessories, such as control and piping systems. Occasionally, through the agency of a hard landing, for instance, it becomes necessary to make minor repairs on a fuselage, for which work little more is needed than a good welder or sheet-metal worker and skillful engineering supervision. Certain landplanes operated by Pan American use all-wood plywood covered wings which require for their maintenance skilled cabinet-makers. As an example of the work which can be readily accomplished along this line can be cited the case of a seventy-nine-foot wooden wing which was entirely recovered with plywood while still in place on the airplane. When the work was completed and the ship test-flown, it was found to be in excellent aerodynamic balance.

The critical problem in repair work of this kind is not that of getting the work done, but of getting it done safely and efficiently. The first requisites are good material, adequate equipment, and skilled workmen. Equally important is an experienced and able shop superintendent to supervise work as it progresses. Again equally important is a highly skilled aircraft inspector, who can pass on the quality of material and workmanship going into the repair. Finally. and also essential, is the supervision of trained aeronautical engineers who can lay out the general scheme of repair with due regard for stress analyses and the strengths of materials. There must be the fullest cooperation between these various groups if the repair work is to measure up to a high standard of safety.

No mention of a maintenance system would be complete without a brief description of the stockroom and the cost accounting department. Without adequate parts and materials, no shop can operate efficiently; yet with an excessive inventory, no enterprise can show a profit. Therefore, a trained storekeeper and a carefully organized system of controlling and issuing materials is an absolute necessity. A "perpetual" inventory is kept for each stockroom in the Pan American System. Each item at each station is represented by a stock card, on file in the "Central Stores Department" at a main base such as Miami. All requisitions on stores and receiving reports pass through the hands of the clerks in charge of these stock cards, so that the proper additions and subtractions can be made, and orders can be placed for new materials when the stock reaches a specified "low limit."

Hand in hand with the stockroom must go the cost accounting department, which is concerned not only with material requisitions against specific planes, engines and jobs, but also with labor charges against those units. The Pan American cost department furnishes regularly to the divisional engineer exact figures on the maintenance and overhaul cost of each airplane and engine in service, as well as on special jobs done in the shop for outside customers, inventory and permanent equipment. The divisional engineer regards these data as of prime importance to him in the economical control of his maintenance system.

The first few paragraphs of this article outlined the general plan upon which the Pan American maintenance system is based. The middle paragraphs showed briefly how this plan is executed from day to day. No plan of this kind. however, can be successfully carried out over a long period of time without continual, skilled and vigilant control. Therefore, these concluding paragraphs will sketch a few of the ways in which the system is kept under control.

The first essential is the supervision of trained, conscientious engineers, and of their important aides, the inspectors. To them must go for solution problems of structural repair, abnormal engine performance, unusual aerodynamic characteristics, and the many similar problems which require prompt and skilled judgment if danger is to be forestalled. Data on these problems must come from the pilots, from the inspectors, and from carefully kept records of engine performance, fuel consumption, and the like. The importance of keeping on file detailed "life histories" of each airplane and engine cannot be overestimated. Such data must include pilots' flight reports, inspection reports, fuel consumption data, engine test logs and airplane flight test logs.

To assure control of individual workmanship, the system of requiring personal signatures for work completed proves successful, in that it definitely places responsibility on specific employees. If an error is detected, it is almost invariably possible to go directly to the employee at fault in order either to educate or to discipline him as the case may require (the former is usually much the more desirable).

Since the final criterion of any airplane or engine work is an adequate flight test, it is a rigid rule that all changes or repairs on airplanes or engines require a flight test before the ship is put into regular service.

The fuel used in aircraft engines has a direct and important effect on both safety and efficiency, as is well known. In order to control the quality of the fuel supplied to Pan American planes over many thousands of miles of airways in foreign countries, a complete laboratory is established in Miami. It is equipped to make not only the routine tests prescribed by the American Petroleum Institute, such as gravity, corrosion, gum and distillation tests, but is also equipped with the latest model of knock-rating engine to determine the detonation characteristic of each sample of fuel. Specimens come to the laboratory regularly from each station in the system: Tests are promptly run. If any deviation from standard specifications is observed, the use of that particular batch of fuel is immediately stopped. It is to the credit of the refiners supplying aviation gasoline that they have cooperated in developing laboratory equipment of this kind, and then have so improved their product as to make any deviation from specifications comparatively rare.