1. INTRODUCTION

Almost everybody loves the ugly, old Piper Pawnee. A relatively low cost tow plane, its ready availability, plenty of towing power, and ease of maintenance all contribute to making the Pawnee popular with both tow pilots and the glider pilots who follow them. And they are fun to fly. As a result, the Pawnee has become nearly ubiquitous in American soaring.

With so many Pawnees around, it's natural that any Airworthiness Directive can have a big impact on soaring, both to club and commercial operations. AD 95-12-01, affecting the main wing spar attach points, has costly and repetitive requirements: you pay to inspect the airframe, and then you pay every two years. This led the Harris Hill Soaring Corporation (HHSC) and the Schweizer Soaring School, both heavily dependent on Pawnees for towing, to try to develop a one-time modification that would eliminate the costly repeat inspections required by the AD.

After reading the AD, Clarence See of HHSC talked with Les Schweizer (Schweizer). Both felt it should be easy and cost-effective to provide a permanent fix to the Pawnees. Schweizer offered to provide the engineering services of Schweizer Aircraft Company (Schweitzer Aircraft) as a gift to the soaring community, and See agreed to undertake the repairs to the first aircraft.

2. REVIEW OF AIRWORTHINESS DIRECTIVE 95-12-01

AD 95-12-01 applies to all Piper Pawnee models PA-25, PA-25-235 and PA-25-260, and requires the removal of the wings of the Pawnee. The wing main spar attach cluster and carry-through tube then must be inspected by eye, by dye penetrant tests, and by ultrasound procedures. If any corrosion past stated limits or any cracks are found, the forward spar fuselage tubular attach clusters and/or carry-through tube must be replaced. This inspection has to be done every two years.

If the clusters and carry-through tube have been replaced, the first inspection is not required for five years. The AD also allows replacement of the clusters within the confines of the original design every five years to fulfill its requirements. It is my understanding that the Soaring Society of America's input during the comment period resulted in the inclusion of the last two items. The result of the inspection, if any, must be sent to the FAA per paragraph (i) of the AD.

Obviously, this inspection is going to be expensive for any soaring operation. Both Clarence See and Les Schweizer felt that a permanent fix could be accomplished for less than the cost of two complete inspections.

A significant contributing factor in cost of the inspection is found in paragraph (d) (2), which details the requirements of the ultrasound inspector. The paragraph reads;

The inspection procedures in the Appendix of this AD, except for the dye penetrant inspection procedures, must be accomplished by a Level 2 inspector certified using the guidelines established by the American Society for Non-Destructive Testing, or MIL-S TD-410. A mechanic with at least an Airframe license may perform the dye penetrant inspection. [Emphasis added.]

After searching all over the USA, Schweizer could find only a few ultrasound inspectors who met these requirements. Obviously, they would expect to be well compensated for their inspection efforts, and would likely require travel expenses to the inspect ion site. See and Schweizer decided the best solution to the problem of high cost of fulfilling the requirements of AD 95-12-01 would be to develop a replacement design of the forward fuselage spar carry-through tube and clusters, and submit it for approval under paragraph (k) of the AD.

3. ORIGINAL CLUSTER / CARRY-THROUGH DESIGN

See removed the wings of one HHSC Pawnee, and inspected the forward wing attach points, finding several interesting items. Although the carry-through tube had been sealed with epoxy, there was a small amount of corrosion between the two attach fittings. The wing and landing gear attach fittings ("dogs") were welded to the cluster and not continuous into, much less past, the cluster [See
Figure 1]. When the wing and landing gear attach bolts are tightened, a side load is put on the "dogs". Also, any load produced by either the wing or landing gear must travel through the weld that attaches each "dog" to the airplane's tubular structure. See and Schweizer felt that any replacement design should address these points of potential weakness. Also, given that the FAA had issued an AD for this area of the airplane structure, they felt that the replacement parts and analysis should be substantially stronger than the original design.

4. DESIGN OF REPLACEMENT CLUSTER / SPAR CARRY-THROUGH

To avoid concentrating stress in welds and to provide overall strength, the attach fittings for both the landing gear and the wings were consolidated in one piece [See Figure 2]. These pieces of 1/4" thick chrome-molybdenum steel extend past the plane o f the fuselage side and are welded along the
carry-through tube, also chrome-moly steel approximately 1/8" in wall thickness. This arrangement transfers the loads produced by the wing and landing gear to the carry-through tube and provides a longer weld between the tube and the attach fittings.

The 1/8" gusset plate is notched to fit over the two attach plates and is welded to both the attach plates and the carry-through tube. The tubes that form the side of the fuselage are slotted and the gusset plate fits into these slots. Once again, a longer weld than the original one carries the load,
reducing the stress in the weld.

To avoid binding the attach plates when torquing the landing gear and wing attach bolts, See and Schweizer specified a bushing. Any force produced by the tightening of the bolt/nut combination is borne by the bushing and provides no side load on the attach plates.

5. OVERVIEW OF STC NO. SA01073AT

STC No. SA01073AT covers the Pawnees with 1/2" forward wing attach bolts. Another STC has been submitted for approval for those Pawnees with 7/16" bolts.

The Supplemental Type certificate is held by Schweizer Aircraft, as they are the quality control agency. Sales of the STCs are through Clarence See. Current cost is $1450 per kit. Installation of the STC requires an experienced aircraft welder using appropriate Tungsten Inert Gas welding equipment.

STC NO. SA01073AT adds 6 pounds of weight to the aircraft at 87.25" aft of datum.

An overview of the steps required to install the STC follows:

1. Completely remove all fuel from the aircraft.
2. Remove wings, cowling and all panels from nose to trailing edge of wing.
3. Build two fixtures from materials in the List of Materials Not Furnished. These fixtures attach to the fuselage at the forward and rear spar attach points and the fuselage strut attach point, one fixture per fuselage side. They support the fuselage structure the original cluster and carry-through tube are removed.
4. After preparatory work, lift the aircraft fuselage and remove the landing gear. The STC currently suggests that the aircraft be lifted by the engine lift ring. I recommend lifting the aircraft b
y the engine mounts if a hoist is used. See is in the process of adding this method to future STC instructions. I see no reason that the fuselage can not be appropriately supported in some other manner.
5. Install the fixtures and cut out the carry-through tube and fuselage wing attach points.
6. Install the new carry-through piece.
7. Paint the repair with a light color automotive enamel to help detect cracks and stresses in the repair area by cracks in the enamel.
8. Reinstall the landing gear and wings.
9. Fill the new carry-through tube with linseed oil.
10. Appropriately complete FAA Form 337, and record the installation/major repair in the airframe logbook. The Instructions for Continued Airworthiness become part of the Airplane Flight Manual, Inspection Guidelines Section and are attached to the Piper Aircraft Inspection Report Form.

The Instructions for Continued Airworthiness require visual inspection and removal of any corrosion of the new assembly every 100 hours. At the annual inspection, the same inspection is required. In addition, the linseed oil level must be checked and replenished if need be, and the wing attach bolts,
bushings and bolt holes inspected for wear and condition. At every sixth annual, the attach fittings must be inspected by dye penetrants. If the aircraft is used in an agricultural or corrosive environment, this must be done every third annual inspection. If any cracks are found, Schweizer Aircraft must be contacted.

6. REVIEW AND COMMENTS

Anecdotal experience has told us that the ultrasound inspection is not always accurate, and may produce erroneous readings. See knows of one aircraft that failed the ultrasound inspection, but when the carry-through tube was cut from the aircraft and inspected, no corrosion, damage or weakness in the structure could be found.

Generally, it seems that the STC can be installed in about 40 hours of work, but this will vary from aircraft to aircraft.

After removing the wings, it would be good practice to check that the forward wing and landing gear attach fittings are in the same plane. Also, check that the rear fittings are in the same plane. Some installers have encountered previous repairs in these areas that have distorted the position of the
fittings.

Originally, the replacement carry-through tube was to have a square cross-section with 0.095" wall thickness, which would have provided 50% greater, due to the lack of available materials, the kit has a square-sectioned tube of 0.120" wall thickness. This led to an increase in strength of 80% over the
original Piper design.

The FAA required a static test of the carry-through modification kit, with a required load of approximately 19,500 pounds. The test specimen failed at 29,300 pounds, 50% beyond the FAA requirement.

Clarence See has an economic interest in the two STCs. He sells the kits and provides as much technical support as possible to the installation. He has donated one kit to the US Team heading to St. Auban, France, in 1997. Les Schweizer has donated his time and Schweizer Aircraft's engineering and test facility capabilities. Stephen Garner has no economic interest in this
process.

Harris Hill Soaring Corporation donated the use of a Pawnee as a testbed aircraft. The necessary original Form 337 was filed and approved for this aircraft.

7. TOOLING REQUIRED

The following tools are required.

1. Lifting device.
2. 1/4" electric or air drill motor.
3. 1/8" or #30 drill bit.
4. Electric or air die grinder.
5. 1-1/4" X 1/16" grinding discs (Northern Hydraulics order #913306 or equivalent)
6. 1" or 2" Wide #80 grit emery cloth
7. Milwaukee Sawsall and/or band hack saw
8. 4-1/2" disc grinder
9. Oxy-acetylene welding outfit
10. Miller Syncrowave 250 TIG welder or equivalent

8. CONTACT

For information and sales of STC No. SA01073AT, please contact Clarence See;
PO Box 737 Tiffany Road; Whitney Point, NY 13862; 607/692-4582 (voice). The
FAA contact person for AD 95-12-01 is Christina Marsh, Aerospace Engineer; FAA,
Atlanta Aircraft Certification Office, Campus Building; 1701 Columbia Avenue,
Suite 2-160; College Park, Georgia 30337-2748; 404/305-7362 (voice)
404/305-7348 (fax). Stephen Garner can be reached at PO Box 321; Big Flats, NY