TM 1-410. Technical Manual, Airplane Structures 1941: Section 16 - Hydraulic Jacks

SECTION XVI: HYDRAULIC JACKS

61. General.-It is not feasible to outline herein detailed jacking instructions for all types of airplanes and for all conditions under which airplanes must be raised. This information can be secured from the basic handbook of instructions for each particular airplane. However, some general information on this subject is included with illustrations showing approved methods and. equipment.

62. Assemblies.-a. A standard jack assembly used by the Air Corps is shown in figure 67. This unit is of the tripod base type, designed to use a hydraulic jack as a lifting medium and is available with rated capacities of 10,000 and 20,000 pounds. These jacks should be located under the airplane as shown in figure 68. When using these jacks, instructions given on the instruction plate should be observed. During jacking process, the locking pin should always be kept in the lowest possible hole in the sliding shaft and the lock on the height adjustment locking ring must be fully engaged, except when supporting structure for the hydraulic jacking unit is being raised or lowered.

b. The jack assembly shown in figure 69 is intended for use as a tail jack for large airplanes and as a jack for the main landing gear of small pursuit type airplanes. This jack has a double telescoping ram which gives it a very high lift in spite of its low unextended height.

63. Use.-a. In procedure of raising airplanes by use of jacks it must be ascertained that the jacks to be used are of sufficient capacity to support the airplane. Care must be taken that the jacks rest on a level area so that their motion will be vertical. When raising small airplanes the jacks may be placed directly on the floor or ground, and in the case of large airplanes the jacks should be placed on suitable timbers or bases so that a minimum of extension is required and as great a length of the ram remains in the cylinder as possible. All jacks must be operated simultaneously so as to maintain equal loads on them. No attempt should be made to lock the hydraulic ram in position by using clamps. If a jack should leak, it is better to allow the ram to come down slowly than to risk the possibility of its dropping suddenly when releasing a clamp. In the event it becomes necessary to leave an airplane unattended for any length of time while supported on hydraulic jacks, care should be taken to see that equipment, ladders, etc., are not located in such a position as to permit the airplane to come in contact with them in the event of leakage of the jack cylinders.

b. When airplanes in hangars are being supported by jacks, use of auxiliary cribbing or supports may be dispensed with as standard

FIGURE 67.-Tripod jack assembly.

Air Corps jacks are designed to withstand all vertical loads with large margins of safety. When the tail wheel is to remain in contact with the floor, yaw may be prevented by locking the tail wheel assembly, by securing the tail wheel with a steering fork, or by arranging

FIGURE 68.-Positions of tripod jack assembly.

FIGURE 69.-Tail and wing jack assembly.

a rope sling around the fuselage and from this sling extending ropes from each side of the fuselage to convenient anchor points on or near the ground. However, the tail wheel must be free to roll fore and aft during raising and lowering the airplane.

c. When raising airplanes in the open they are first headed down wind and in no case raised when exposed to winds in excess of 15 miles per hour. If airplane jacking points are close together, yawing moments due to wind may cause jacks to flex and cramp alarmingly and due to the large area of vertical tail surfaces of bombardment type

FIGURE 70.-Wing jack and auxiliary support.

airplanes, a small side wind may cause a rather severe side load on tail jacks. It is important therefore to shelter the airplane if any perceptible wind prevails. Personnel working on or under airplanes supported by jacks in the open are to be protected front injury in case of failure of the jacking equipment by one of the following means:

(1) Heavy timber cribbing, top members of which are felt-padded and snugly fitted under the wing spars extending along 3 or 4 feet of the spar.

(2) Large felt-padded timbers that fit the chordwise contour of the wing, extending several inches to the front and rear of the spars, and supported with wooden horses or stands of suitable strength.

(3) Suitable strong benches upon which sandbags have been placed to bear against the wing spars.

(4) Adjustable height steel stands bearing against such units of the primary airplane structure that are capable of supporting the airplane weight, as shown in figure 70. The center of gravity of the airplane must be aft of the point of contact of supporting stands.

(5) In no case are the timber cribbing, benches, stands, etc., specified above placed outboard of the fuselage a distance greater than one-third the semispan or beyond the minimum distance necessary to provide clearance for retracting the landing gear. Overhead hoisting equipment may be used as an auxiliary safety support where authorized by local engineering officers. Capacity of the equipment and its suspension must be checked carefully and specific authority given for its use on each particular type of airplane.