Chapter 8. Glider And Banner Tow-Hitch Installations 

Section 1. Towplane Considerations. [Deleted] Change 1 Section 2. TOW-HITCH INSTALLATIONS 126. STRUCTURAL REQUIREMENTS. The structural integrity of a tow-hitch installation on an aircraft is dependent upon its intended usage. Hitches which meet the glider tow criteria of this chapter are acceptable for banner tow usage.  However, because the direction and magnitude of maximum dynamic banner towline loads occur within a more limited rearward cone of displacement than do glider towline loads, hitches which meet the banner tow criteria of this chapter may not be satisfactory for glider towing. Due to the basic aerodynamic difference between the two objects being towed, glider and banner tow-hitch installations are treated separately with regard to loading angles.

a. Glider tow hitches.

Protection for the towplane is provided by requiring use of a towline assembly which will break prior to structural damage occurring to the towplane. The normal tow load of a glider rarely exceeds 80 percent of the weight of the glider. Therefore, the towline assembly design load for a 1,000 pound glider could be estimated at 800 pounds. By multiplying the estimated design load by 1.5 (to provide a safety margin), we arrive at a limit load value of 1,200 pounds. The 1,200 pound limit load value is used in static testing or analysis procedures per paragraph 127 of this handbook to prove the strength of the tow hook installation. When the hook and structure have been proven to withstand

the limit load, then the MAXIMUM breaking strength of the towline assembly is established at the design load of 800 pounds. Thus, the towline will break well before structural damage will occur to the towplane. {See Figure 8.1.}

Another approach can be applied if the limit load carrying capabilities of a tow hook and fuselage are known. In this case, the known load value can be divided by 1.5 to arrive at the design load capabilities if the tow hook and fuselage limit loads are known to be 1,800 pounds. By dividing by 1.5 (1800 divided by 1.5 = 1,200) we arrive at a design load value of 1,200 pounds. Thus, the maximum breaking strength of the towline assembly is established at 1,200 pounds and provides protection for the towplane.

Thus, in considering tow hook installations, one may establish maximum towline breaking strength by:

(1) Dividing the known limit load capabilities of the fuselage and two hook installation by 1.5; or

(2) Knowing the design load needs of the towline assembly and multiplying by 1.5 to arrive at a limit load. Then by analysis or static testing, determine that the hook and fuselage are capable of withstanding that limit load.

b. Banner tow hitches.

Install the hitch to support a limit load equal to at least two times the operating weight of the banner.

127. STRUCTURAL TESTING.

Adequacy of the aircraft structure to withstand the required loads can be determined by either static test or structural analysis.

a. Static testing.

When using static tests to verify structural strength, subject the tow hitch to the limit load (per paragraph 126 a or b) in a rearward direction within the appropriate cone of displacement per figure 8.2. Testing to be done in accordance with the procedures of Chapter 1, paragraph 3, of this handbook.

b. Structural analysis.

If the local fuselage structure is not substantiated by static test for the proposed tow load, using a method that experience has shown to be reliable, subject the fuselage to engineering analysis to determine that the local structure is adequate. Use a fitting factor of 1.15 or greater in the loads for this analysis.

128. ATTACHMENT POINTS.

Tow-hitch mechanisms are characteristically attached to, or at, tiedown points or tailwheel brackets on the airframe where the inherent load bearing qualities can be adapted to towing loads. Keep the length of the hitch assembly arm from the airframe attachment point to the tow hook to a minimum as the loads on the attachment bolts are multiplied by increases in the moment arm.

129. ANGLES OF TOW.

Tests should be conducted on the system at various tow angles to insure that:

a. There is no interference with the tailwheel or adjacent structure.
b. The towline clears all fixed and movable surfaces at the maximum cone of displacement and full surface travel.
c. The mechanism does not significantly decrease the clearance from the tailwheel to the rudder.
d. The tow hitch does not swivel. Experience has shown swiveling could result in fouling both the release line and towline during operations by the towplane.
e. The opened jaw of the hitch does not strike any portion of the aircraft.

130. PLACARDS.

A placard should be installed in a conspicuous place in the cockpit to notify the pilot of the structural design limits of the tow system.

The following are examples of placards to be installed:

a. For glider tow - "Glider towline assembly breaking strength not to exceed ____*____ pounds."
b. For banner tow - "Tow hitch limited to banner maximum weight of ____**____ pounds."

* Value established per paragraph 126 a (1) or (2).
** Banner hitch limitations are one-half the load applied per paragraph 126 b.

131. WEIGHT AND BALANCE.

In most cases, the weight of the tow-hitch assembly will affect the fully loaded aft c.g. location. To assure that the possibility of an adverse effect caused by the installation has not been ignored, enter all pertinent computations in the aircraft weight & balance records. (In accordance with the provisions contained in FAR 43.5(a)(4).)

132. TOW RELEASE MECHANISM.

a. Release lever.

A placard indicating the direction of operation should be installed to allay the possibility of confusion or inadvertent operation,
and the design of the release lever should provide the following:

(1) Convenience in operation.
(2) Smooth and positive release operation.
(3) Positioned so as to permit the pilot to exert a straight pull on the release handle.
(4) Sufficient handle travel to allow for normal slack and stretch of the release cable.
(5) A sufficient handle/lever ration to assure adequate release force when the towline is under high loads. (See figure 8.3)
(6) Protection of cables from hazards such as:

(a) Wear and abrasion during normal operation.
(b) Binding where cables pass through fairleads, pulleys, etc.
(c) Accidental release.
(d) Interference by other aircraft components.
(e) Freezing and moisture accumulation when fixed or flexible tubing guides are used.

b. Test of the release.

A test of the release and hook for proper operation through all angles of critical loading should be made using the design load for the glider or banner.

c. Release cable.

Representative size and strength characteristics of steel release cable are as shown in figure 8.4; however, it is recommended that all internally installed release cables be 1/16 inch or larger.
 

133. [DELETED] Change 1

(DELETED) - Change 2

{See Figure 8.6.} {See Figure 8.7.} {See Figure 8.8.} {See Figure 8.9.}

134. - 145. [RESERVED]