By Anthony Smith
Chair Airworthiness department
BACKGROUND
The Piper PA-25 Pawnee was first designed in the early 1950s as an agricultural aircraft. Prior to this most agricultural aircraft were converted from existing civil or military designs. Several versions of the Pawnee were made that increased engine power in later models. Typical of the certification at that time, a fatigue life was not published and neither were life extension surveys.
The aircraft was popular with crop dusting / spraying operators in Australia in the 1960s and 1970s. The type was largely replaced in agricultural work in the 1980s by the larger and more powerful Air Tractor.
Pawnees became increasingly popular as aerotow aircraft at gliding clubs in the mid 1980s as they became available in the second-hand aircraft market. Without the agricultural equipment or weight of fertiliser / pesticide, the type has an excellent power to weight ratio which results in a very good climb rate while towing a glider. It is highly suitable for operations off unsealed airstrips and paddocks.
Piper ceased production of the Pawnee in 1981. On 15 April 1988, Piper Aircraft sold the design to Latino Americana de Aviación S.A (Laviasa) in Argentina. This resulted in Laviasa being the Type Certificate Holder and responsible for maintaining the ongoing airworthiness of the type.
Aviación Civil Argentina (ANAC) is the national aviation regulator of Argentina (the equivalent of CASA). Argentina became the State of Design when Laviasa became the Type Certificate Holder. Under ICAO rules, any Airworthiness Directive (AD) issued by the national regulator of the State of Design is accepted by CASA and applicable to Australian aircraft.
AD 2024-05-01
Aviación Civil Argentina (ANAC) issued Airworthiness Directive RA 2024-05-01 on 18 July 2024 as a result of three crashes involving failure of the front spar of the PA-25 Pawnee. This AD requires inspections using two Service Bulletins from Laviasa. The inspections are of the front spar in key areas for corrosion (most likely from exposure to super phosphate), misdrilled holes (potential crack sites) and cracks.
The first Service Bulletin, SB 25-57-09, is available in English. However, the second Service Bulletin, SB 25-57-11, was published in Spanish. The AD also requires an eddy current inspection (a form of non-destructive testing) of the spar flanges. Normally there is a published procedure for the eddy current inspection that tells the inspector how to set up the equipment. This procedure was not published by ANAC.
The AD is not well written and perhaps some of intent has become confused in the translation to English, in particular regarding the time at which the inspections using SB 25-57-11 were due, with two interpretations possible.
The initial inspections are warranted, given the crashes, and ongoing inspections are a prudent safety measure. However, the requirements of the AD that allow ongoing operation of the aircraft are very restrictive. The AD requires ongoing inspections at a much higher frequency than would normally be expected.
WHAT HAS GLIDING AUSTRALIA DONE?
Gliding Australia contacted Laviasa and purchased a complete set of Service bulletins. These are available from the EMA, Dennis Stacey if required.
An unofficial English version of SB 25-57-11 was translated from the original Spanish.
An e-mail forum for Pawnee operators was created. This has since attracted Pawnee tug operators from around the world.
An Alternate Means of Compliance was drafted and approved by CASA to use smaller round inspection holes under the wings, rather than the larger, rectangular inspection panels required by SB 25-57-11.
Due to strong advocacy from Gliding Australia, CASA has published a clarification on the AD which allows visual inspection of the spar flanges until cracks are found.
Subsequent to initial communications with both ANAC and Laviasa, all responses from Argentina have ceased. Gliding Australia has resorted to requesting embassy staff to attempt to contact both ANAC and Laviasa and re-establish communications.
HAVE ANY CRACKS BEEN FOUND?
There are 44 Pawnees being used as towplanes in Australia. Out of those that have been inspected, 11 are presently grounded by the AD (around 33% of aircraft inspected). This is significantly better than anticipated. When the AD was published, we expected up to 66% of aircraft to be grounded. The majority of the groundings are due to extra holes in the spar flange, holes too close to the edge of the spar flange, or holes in the spar flange that are too close together.
The remainder of the aircraft have been grounded for corrosion. This is a little disappointing as the corrosion perhaps should have been found and treated as part of the ongoing annual inspections. Fortunately, no cracks have been found to date.
WHAT CAN BE DONE IN THE FUTURE?
Aviacion Civil Argentina is likely to have only considered the much higher fatigue accumulation rate of agricultural flying. Aerotow operations have a much lower fatigue accumulation rate and could have less severe ongoing inspections. Performing a stress analysis of the spar and a fatigue analysis comparing agricultural use to aerotow operations and having it approved by CASA may enable the reinspection interval to be substantially increased.
Gliding Australia has initiated discussions with CASA about the potential benefit of transferring aircraft that are only used for sailplane towing over to the 'Limited' category (same as warbirds) under Part 132. We already have the e-tugs - Pawnees fitted with the LS1 automotive engine - operating in this category. In the same conversation, we are starting discussion on what it would take to approve Gliding Australia as a Part 132 organisation, like Warbirds Australia. This may give Gliding Australia better autonomy on tow plane operations, airworthiness and maintenance for tugs.
I have received two independent proposals to manufacture composite wings for Pawnees. These would have the advantage of being Australian certified wings and not being beholden to interesting decisions on the wings from Argentina in the future. The certification may be made a little easier if we were to move the tugs over to Part 132 (see above). Manufacture of composite wings from scratch is not a short term solution. It carries some risks, for example, problems with certification, but it is a possibility for the future.
Gliding Australia has received advice from CASA that they will approve repairs for the spars for defects found during the AD, as opposed to insisting that everything needs to be approved by Argentine authoritities. I have had preliminary discussion with a Part 21 engineer about possible repair schemes using VH-WGC as an example. There are a couple of possibilities for repairs moving forward and these will be developed in the coming weeks.