20 50 1O 20 80 80 FIGURE 11-24. Map showing average number of thunderstorm days per year.
of the local nature of thunderstorms and the sparsity of observations from some areas. July and August are the months with the greatest number of thunderstorms over most sections of the United States, while December and January have the least number.
Thunderstorms are not the only concern of aircraft owners, fixed-base operators, airport service crews, etc., from a tiedown sense. There are also hurricanes and tornadoes. Figure 11-25 is a map showing the tornado frequency in the various states during a recent 10-year period.
A map showing the principal storm belts in the continental U.S.A. is presented in figure 11-26.
Precautions Against Windstorm Damage The best protection against windstorm damage is, of course, to fly the aircraft out of the impending storm area when there is sufficient time. The next best protective measure is to secure the aircraft in a stormproof hangar or other suitable shelter. The remaining alternative is to assure that the aircraft is tied down securely. When securing aircraft, fasten all doors and windows properly to minimize damage inside the aircraft. Engine openings (intake and exhaust) for both reciprocating and gasturbine type should be covered to prevent entry of foreign matter. Pitot-static tubes should also be covered to prevent damage.
Be prepared for the worst storm conditions; for example, pouring rain, and gusty winds with intermittent sheets of water flowing across the runways, ramps, and parking areas, with perhaps no hangar facilities available. With such conditions in mind, responsible service crews should plan in advance by becoming familiar with their aircraft manufacturer's instructions for the following: (1) Tiedown ropes; (2) installation of tiedown rings for attachment of tiedown ropes; (3) securing nosewheel type aircraft vs. tailwheel type aircraft;
and (4) aircraft weights and relative wind velocities that would make varied tiedown procedures necessary for pending weather emergencies.
The following suggestions will materially reduce aircraft damage from windstorms:
1. Partially disassembled aircraft which are outdoors (particularly light aircraft with 513
£OI 16 12 14 17 32 28 116 80 20 139 20 96 10 186 64 62 377 90 50 00~ 8+1 191 2560 1215 989 266 e+z 138 .C 80 187 58 99 149 90 167 87 1102 175 915 412 Fairbanks
•0 Honolulu S ALASKA HAWAI!
UPPER NUMBER - Number of tornadoes LOWER NUMBER- Number of tornado days FiGurE 11-25. Tornado frequency during a recent 10-year period.
engines removed) should be hangared as soon as storm warnings are received.
Loose wings should never be tied against a fuselage; they should be stored inside a hangar.
2. Whenever possible, fly aircraft out of anticipated storm danger zones. If impossible, hangar the aircraft in a stormproof hangar.
3. Observe the minimum recommended strength for tiedown ropes.
4. A single row of properly secured sandbags or spoiler boards (2" X 2') on the top of a wing's leading edge will serve as an effective spoiler and reduce the lifting tendency of the wings. Do not overload the wings with sandbags. If the anticipated winds will exceed the lift-off speed of the aircraft, the makeshift spoilers should run the entire length of the wings.
Another means for tying down light aircraft (of various types and sizes) utilizes continuous lengths of parallel wire ropes passed through U-bolt anchors secured to ground tiedown points (figures 11-27 and 11-28). Tiedown chains are attached to the wire rope with round-pin galvanized anchor shackles. This allows tiedown chains to "float" along wire rope and gives a variable distance between anchor points so that a variety of aircraft can use a vertical tiedown without loss of space.
The vertical anchor significantly reduces impact loads that may occur during gusty wind conditions.
The distance between ropes will depend upon the types of aircraft which use the tiedown area.
The diagram in figure 11-28 shows a proper vertical anchor using wire rope tiedown line, straight link coil chain for connection between the wire rope and aircraft wing. One link on the free end is then passed through a link of the taut portion, and a safety snap is used to keep the link from passing back through. Any load on the chain is borne by the chain itself instead of the snap.
Primary hurricane danger zone.
Fringe area.
Tornado belt.
FIGURE 11-26. Storm belts in the continental U.S.
FIGURE 11-27. Typical aircraft tiedown using a wire rope system.
FIGURE 11-28. Wire rope tiedown line using vertical anchor chain.
Securing Multiengine Aircraft Multiengine aircraft will obviously require stronger tiedown facilities because of the additional weight of these aircraft. The anchor should be capable of a holding power of 4,000 pounds each for the lighter executive twin-engine aircraft.
Much higher load capacity is required for the heavier transport-type aircraft.
Do not depend on the weight of the multiengine aircraft to protect it from damage by windstorms.
It is possible for a sudden, severe windstorm to move, damage, or even overturn such aircraft.
Multiengine aircraft should, therefore, always be tied down and chocked when left unattended for any length of time. Gust locks should be used to protect control surfaces. If the landing gear uses downlock safety pins, these pins should be inserted at the time the aircraft is being secured.
Securing Helicopters Structural damage can occur from high-velocity surface winds. Therefore, if at all possible, helicopters should be evacuated to a safe area if tornadoes or hurricanes are anticipated.
When possible, helicopters should be secured in hangars. If not, they should be tied down securely.
Helicopters that are tied down can usually sustain winds up to approximately 65 m.p.h.
For added protection, helicopters should be moved to a clear area so that they will not be damaged by flying objects or falling limbs from surrounding trees.
If high winds are anticipated with the helicopter parked in the open, the main rotor blades should be tied down. Detailed instructions for securing and mooring each type of helicopter can be found in the applicable maintenance manual. Methods of securing helicopters will vary with weather conditions, the length of time the aircraft is expected to remain on the ground, and location and characteristics of the aircraft. Wheel chocks, control locks, rope tiedowns, mooring covers, tip socks, tiedown assemblies, parking brakes, and rotor brakes are used to secure helicopters.
Typical mooring procedures are as follows:
1. Head the helicopter in the direction from which the highest forecasted wind or gusts are anticipated.
2. Spot the helicopter slightly more than rotor-span distance from other aircraft.
3. Place wheel chocks ahead of and behind all wheels (where applicable). On helicopters equipped with skids, retract the handling wheels, lower the helicopter to rest on the skids, and install wheel position lockpins.
4. Install a tiedown assembly on the end of the blade (figure 11-29 and align the blade over the tail boom. Secure the tiedown straps under the structural tubes of the tail boom. Tie the straps snugly without strain.
During wet weather, provide some slack to avoid the possibility of the straps tightening 516
FIGURE 11-29. Securing helicopter blades and fuselage.
5. Fasten the tiedown ropes or cables to the forward and aft landing gear cross tubes and secure to ground stakes or tiedown rings.
Securing Seaplanes and Aircraft on Skis Aircraft mounted on floats or skis should be secured to tiedown anchors or "deadmen" sunk under the water or ice. When warning of an impending storm is received and it is not possible to fiy the aircraft out of the storm area, some compartments of the seaplane can be flooded, partially sinking the aircraft. In addition, the aircraft should be tied down securely to anchors:
Seaplanes tied down on land have been saved from high-wind damage by filling the floats with water in addition to tying the aircraft down in the usual manner. Operators of ski-equipped aircraft sometimes pack soft snow around the skis, pour water on the snow, and permit the skis to freeze to the ice. This, in addition to the usual tiedown procedures, aids in preventing damage from windstorms.
MOVEMENT OF AIRCRAFT General Movement of large aircraft on an airport and about the flight line and hangar is usually accomplished by towing with a tow tractor (sometimes called a "mule or tug"). In the case of small aircraft, most moving is accomplished by hand, by pushing on certain areas of the aircraft surface.
Aircraft may also be taxied about the fight line, but usually only by certain qualified persons.
Towing of Aircraft Towing aircraft can be a hazardous operation, causing damage to the aircraft and injury to personnel, if done recklessly or carelessly. The following paragraphs outline the general procedure for towing aircraft; however, specific instructions for each model of aircraft are detailed in the manufacturer's maintenance instructions and should be followed in all instances.
Before the aircraft to be towed is moved, a qualified man must be in the cockpit to operate the brakes in case the tow bar should fail or become unhooked. The aircraft can then be stopped, preventing possible damage.
Some types of tow bars available for general use (figure 11-30) can be used for many types of towing operations. These bars are designed with sufficient tensile strength to pull most aircraft, but are not intended to be subjected to torsional or twisting loads. Although many have small wheels that permit them to be drawn behind the towing vehicle going to or from an aircraft, they will suffer 517
Tow hitch plate.
Locking pin.
Spreader plate.
End hooks. Tubing.
Locking pin hole.
Tailwheel Main wheel FIGURE 11-30. A front or rear tow bar.
less damage and wear if they are loaded aboard the vehicle and hauled to the aircraft. When the bar is attached to the aircraft, all the engaging devices should be inspected for damage or malfunction before moving the aircraft.
Some tow bars are designed for towing various types of aircraft; however, other special types can be used on a particular aircraft only. Such bars are usually designed and built by the aircraft manufacturer.
When towing the aircraft, the towing vehicle speed must be reasonable, and all persons involved in the operation must be alert.
When the aircraft is stopped, the brakes of the towing vehicle alone should not be relied upon to stop the aircraft. The man in the cockpit should coordinate the use of the aircraft brakes with those of the towing vehicle. A typical tow tractor (or tug) is shown in figure 11-31.
The attachment of the tow bar will vary on different types of aircraft. Aircraft equipped with tailwheels are generally towed forward by attaching the tow bar to the tow rings on the main landing gear. In most cases it is permissible to tow the aircraft in reverse by attaching the tow bar to the tailwheel axle. Anytime an aircraft equipped with a tailwheel is towed, the tailwheel must be unlocked or the tailwheel locking mechanism will be damaged or broken.
Aircraft equipped with tricycle landing gear are generally towed forward by attaching a tow bar to the axle of the nosewheel. They may also be towed forward or backward by attaching a towing bridle or specially designed towing bar to FIGURE 11-31. Tow tractor.
the towing lugs on the main landing gear. When an aircraft is towed in this manner, a steering bar is attached to the nosewheel to steer the aircraft.
The following towing and parking procedures are typical of one type of operation. They are examples, and not necessarily suited to every type of operation. Aircraft ground-handling personnel should be thoroughly familiar with all procedures pertaining to the types of aircraft being towed and local operating standards governing ground handling of aircraft. Only competent persons properly checked out should direct an aircraft towing team.
1. The towing vehicle driver is responsible for operating his vehicle in a safe manner and obeying emergency stop instructions given by any team member.
2. The person in charge should assign team personnel as wing walkers. A wing walker should be stationed at each wingtip in such a position that he can ensure adequate clearance of any obstruction in the path of the aircraft. A tail walker should be assigned when sharp turns are to be made, or when the aircraft is to be backed into position.
3. A qualified person should occupy the pilot's seat of the towed aircraft to observe and operate the brakes as required. When necessary, another qualified person is stationed to watch and maintain aircraft hydraulic system pressure.
4. The person in charge of the towing operation should verify that, on aircraft with a steerable nosewheel, the locking scissors are set to full swivel for towing.
The locking device must be reset after the tow bar has been removed from the aircraft. Persons stationed in the aircraft should not attempt to steer or turn the nosewheel when the tow bar is attached to the aircraft.
5. Under no circumstances should anyone be permitted to walk or ride between the nosewheel of an aircraft and the towing vehicle, nor ride on the outside of a moving aircraft or on the towing vehicle.
In the interest of safety, no attempt to board or leave a moving aircraft or towing vehicle should be permitted.
6. The towing speed of the aircraft should not exceed that of the walking team members. The aircraft's engines usually are not operated when the aircraft is being towed into position.
7. The aircraft brake system should be charged before each towing operation.
Aircraft with faulty brakes should be towed into position only for repair of brake systems, and then only with personnel standing by ready with chocks for emergency use. Chocks must be immediately available in case of an emergency throughout any towing operation.
8. To avoid possible personal injury and aircraft damage during towing operations, entrance doors should be closed, ladders retracted, and gear downlocks installed.
9. Prior to towing any aircraft, check all tires and landing gear struts for proper inflation. (Inflation of landing gear struts of aircraft in overhaul and storage is excluded.)
10. When moving aircraft, do not start and stop suddenly, For added safety, aircraft brakes must never be applied during towing except in emergencies, and then only upon command by one of the tow team members.
11. Aircraft should be parked in specified areas only. Generally, the distance between rows of parked aircraft should be great enough to allow immediate access of emergency vehicles in case of fire, as well as free movement of equipment and materials.
12. Wheel chocks should be placed fore and aft of the main landing gear of the parked aircraft.
13. Internal or external control locks (gust locks or blocks) should be used while the aircraft is parked.
14. Prior to any movement of aircraft across runways or taxiways, contact the airport control tower on the appropriate frequency for clearance to proceed.
15. An aircraft should not be parked in a hangar without immediately being statically grounded.
Taxiing Aircraft As a general rule, only rated pilots and qualified airframe and powerplant technicians are author- 519
Lights Meaning Flashing green - Cleared to taxi.
Steady red. _ _Stop.
Flashing red Taxi clear of runway in use.
Flashing white. Return to starting point.
Alternating red and green... Exercise extreme caution.
FIGURE 11-32. Standard taxi light signals.
ized to start, run up, and taxi aircraft. All taxiing operations should be performed in accordance with applicable local regulations. Figure 11-32 contains the standard taxi light signals used by control towers to control and expedite the taxiing of aircraft. Refer to the following section, "Taxi Signals," for detailed instructions on taxi signals and related taxi instructions.
FIGURE 11-33. The taxi signalman.
Taxi signalman FIGURE 11-34. Position of the taxi signalman.
Taxi Signals Many ground accidents have occurred as a result of improper technique in taxiing aircraft.
Although the pilot is ultimately responsible for the aircraft until the engine is stopped, a taxi signalman can assist him around the flight line. In some aircraft configurations, the pilot's vision is obstructed while he is on the ground. He cannot see obstructions close to the wheels nor under the wings, and has little idea of what is behind him.
Consequently, he depends upon the taxi signalman for directions. Figure 11-33 shows a taxi signalman indicating his readiness to assume guidance of the aircraft by extending both arms at full length above his head, palms facing each other.
The standard position for a signalman is slightly ahead of and in line with the aircraft's left wingtip.
As the signalman faces the aircraft, the nose of the aircraft is on his left (figure 11-34). He must stay far enough ahead of the wingtip for the pilot to see him easily. He should follow a foolproof test to be sure the pilot can see his signals. If he can see the pilot's eyes, the pilot can see his signals.
Figure 11-35 shows the standard aircraft taxiing signals published in the Airmen's Information Manual by the Federal Aviation Administration.
It should be emphasized that there are other standard signals, such as those published by the Armed Forces. In addition, operating conditions in many areas may call for a modified set of taxi 520
FLAGMAN DIRECTS PILOT TO SIGNALMAN'S POSITION SIGNALMAN DIRECTS
SIGNALMAN IF TRAFFIC TOWING
CONDITIONS REQUIRE
STOP COME AHEAD EMERGENCY STOP CUT ENGINES
START ENGINES PULL CHOCKS INSERT CHOCKS SLOW DOWN ALL CLEAR (O.K.) LEFT TURN RIGHT TURN NIGHT
OPERATION FIGURE 11-35. Standard FAA hand taxi signals.
START ENGINE ENGAGE ROTOR STOP ROTOR STOP
MOVE BACK MOVE FORWARD MOVE RIGHT MOVE LEFT
TAKE OFF ANDIN( GO UP GO DOWN
DIRECTIOI
SWING TAIL SWING TAIL
TO RIGHT TO LEFT FIGURE 11-36. Helicopter operating signals.
manner as day signals with the exception of the stop signal. The stop signal used at night is the emergency stop" signal. This signal is made by crossing the wands to form a lighted "X" above and in front of the head.
FIGURE 11-37. Night operations with wands.
signals. The signals shown in figure 11-35 represent a minimum number of the most commonly used signals. Whether this set of signals or a modified set is used is not the most important consideration, as long as each flight operational center uses a suitable, agreed-upon set of signals.
Figure 11-36 illustrates some of the most commonly used helicopter operating signals.
The taxi signals to be used should be studied until the taxi signalman can execute them clearly and precisely. The signals must be given in such a way that the pilot cannot confuse their meaning.
It should be remembered that the pilot receiving the signals is always some distance away, and must often look out and down from a difficult angle.
Thus, the signalman's hands should be kept well separated, and signals should be overexaggerated rather than risk making indistinct signals. If there is any doubt about a signal, or if the pilot does not appear to be following the signals, the "stop" signal should be used and the series of signals begun again.
The signalman should always try to give the pilot an indication of the approximate area in which the aircraft is to be parked. The signalman should glance behind himself often when walking backward to prevent backing into a propeller or tripping over a chock, fire bottle, tiedown line, or other obstruction.
Taxi signals are usually given at night with the aid of illuminated wands attached to flashlights (figure 11-37). Night signals are made in the same
JACKING AIRCRAFT The aviation technician must be familiar with the jacking of aircraft in order to perform maintenance and inspection. Since jacking procedures and safety precautions vary for different types of aircraft, only general jacking procedures and precautions are discussed. Consult the applicable aircraft manufacturer's maintenance instructions for specific jacking procedures.
Extensive aircraft damage and serious personal injury have resulted from careless or improper jacking procedures. As an added safety measure, jacks should be inspected before use to determine the specific lifting capacity, proper functioning of safety locks, condition of pins, and general serviceability. Before raising an aircraft on jacks, all workstands and other equipment should be removed from under and near the aircraft. No one should remain in the aircraft while it is being raised or lowered, unless maintenance manual procedures require such practice for observing leveling instruments in the aircraft.
The aircraft to be jacked must be located in a FIGURE 11-38. Typical tripod jack.
Jacking Complete Aircraft Prior to jacking the aircraft, an overall survey of the complete situation should be made to determine if any hazards to the aircraft or personnel exist. Tripod jacks of the appropriate size for the aircraft being jacked should be placed under the aircraft jacking points and perfectly centered to prevent them from cocking when the aircraft is raised. The legs of the jacks should be checked to see that they will not interfere with the operations to be performed after the aircraft is jacked, such as retracting the landing gear.
At least three places or points are provided on aircraft for jacking purposes; a fourth place on some aircraft is used to stabilize the aircraft while it is being jacked at the other three points. The two main places are on the wings, with a smaller one on the fuselage near either the tail or the nose, depending on the landing gear design.
Most aircraft have jack pads located at the jack points. Others have removable jack pads that are inserted into receptacles bolted in place prior to jacking. The correct jack pad should be used in all cases. The function of the jack pad is to ensure that the aircraft load is properly distributed at the jack point and to provide a convex bearing surface to mate with the concave jack stem. Figure 11-40 FIGuRE 11-39. Typical single-base jack. illustrates two types of jack pads.
level position, well protected from the wind.
A hangar should be used if possible. The manufacturer's maintenance instructions for the aircraft being jacked should be consulted for the location of the jacking points. These jacking points are usually located in relation to the aircraft center of gravity so the aircraft will be well balanced on the jacks. However, there are some exceptions to this. Wing jack pad assembly On some aircraft it may be necessary to add weight to the nose or tail of the aircraft to achieve a safe balance. Sandbags are usually used for this purpose.
Tripod jacks similar to the one shown in figure 11-38 are used when the complete aircraft is to be jacked.
A small single-base jack similar to the one shown in figure 11-39 is used when only one wheel is to be raised. The jacks used for jacking aircraft must be maintained in good condition; a leaking or damaged jack must never be used. Also, each jack Forward jack fitting has a maximum capacity, which must never be exceeded. FIGURE 11-40. Typical jack pads.