On 30 April 2005, the pilot of a Beech Aircraft Corporation V35A
Bonanza aircraft, registered VH-FWE, was conducting a private
flight from Lilydale, Vic. to Temora, NSW. The pilot reported that
while cruising at 7,500 ft, there was a loss of aileron control.
Initially the aircraft tended to drift to the right, which he
corrected by rolling the aircraft to the left. He then felt
something break and the right wing dropped. He turned the aileron
control yoke to the left, until it was almost upside down, but the
aircraft continued rolling to the right and entered a progressively
steeper descent. The pilot broadcast a PAN1advising air traffic control that he had an
aileron control problem and that he would attempt to land the
aircraft on a local glider field. He reported that he arrested the
roll by extending the landing gear, adjusting engine power and
applying full left rudder. The pilot subsequently landed the
aircraft without the use of wing flaps.
Aircraft
The aircraft was manufactured in 1969 and had recorded 6,154.8
hours in service at the time of the incident. It was maintained in
accordance with the applicable maintenance requirements and had a
valid Maintenance Release. It had flown approximately 22.9 hours
since the last periodic inspection completed in March 2005.
Aileron control examination
When examined, the aircraft's left aileron was found deflected
to the fully down position and the right aileron fully up. To
return the ailerons to their neutral position, a force was required
to overcome the tension of the rudder interconnect bungee spring.
Once the force was removed, both ailerons returned to the fully
deflected positions. The examination of the aileron control cables
revealed that the right aileron 'up' cable terminal, located in the
rear spar carry through structure, had failed.
The aircraft manufacturer advised that since the aileron control
cables are connected to the rudder interconnect bungee spring, the
separation of the right aileron up cable would result in that
spring forcing the left aileron down and the right aileron up
(Figure 1).
Figure 1: Aileron control system
Terminal examination
The failed control cable terminal was sent to the Australian
Transport Safety Bureau (ATSB) for examination. The terminal shaft
that is screwed into the turnbuckle had fractured close to the
locking wire attachment point (Figure 2). The examination revealed
that the fracture was initiated by stress corrosion cracking2 that had propagated under the surface of
the shaft and weakened it to the point of failure.
Figure 2: Failed control cable terminal
The cable terminal was a standard swaged fitting designated AN669.
Chemical analysis of the material showed that its composition
closely matched that of SAE-AISI 303 stainless steel. A recent US
National Transportation Safety Board (NTSB) Safety
Recommendation3 identified SAE-AISI
303 stainless steel as being susceptible to stress corrosion
cracking when used in a corrosive environment. The Safety
Recommendation mentioned that cracking propagates as a function of
the time a component is exposed to the corrosive environment rather
than its actual time in service and that 'about 18 to 20 years is
required for terminals exposed to the most damaging environment to
reach their fracture point'.
Terminal inspection
During routine aircraft maintenance inspection of control system
cables, corrosion pits on the surface of the cable terminal shaft
may be the only visual indication of a potential problem. With the
shaft area being typically wrapped with safety wire, the shaft can
be difficult to inspect.
In August 2001, The Civil Aviation Safety Authority (CASA)
issued Airworthiness Bulletin 27-1 Issue 1, Control Cable
Terminal Inspection that was also published on the CASA web
site www.casa.gov.au. The Airworthiness Bulletin provided
information regarding the susceptibility of control cable terminals
made of SAE-AISI 303 stainless steel to failure due to stress
corrosion and highlighted the 'importance of meticulous inspection
of the terminals'. It recommended that aircraft older than 15
years, and using terminals constructed of SAE-AISI 303 stainless
steel, should have their control cable terminals visually inspected
on an annual basis.
A review of the aircraft's log books found no evidence of the
aileron controls having been subjected to any specific inspections
to detect corrosion, including the removal of lock wire, within the
previous 15 years. Routine maintenance inspections had been
conducted during that period.
Both the ATSB and CASA databases contained four reports of
similar control cable terminal failures in the period between 1995
and 2004. The NTSB Safety Recommendation mentioned 10 instances of
aircraft that were found having fractured or cracked control cable
terminals.
- PAN is a radio code indicating
uncertainty or alert. - A cracking process that requires the
simultaneous action of a corrosive environment, such as a
chlorine-rich atmosphere in moist coastal areas, and sustained
tensile stress. - US National Transportation Safety
Board Safety Recommendation A-01-6 through -8 of April 16,
2001.