Heads Up

The Consequences of Not Following Procedures

 
SERVICE DIFFICULTY REPORT
(SDR) # 20110526008

Canadian Repair Station Service Difficulty Report (SDR) text (abbreviation):

"We recently received a Multi-Function Control Display Unit (MCDU) (part number 100-601702-002) for repair from a European Operator. While conducting maintenance on the product, we noted that during a previous repair, a fuse (F1 on the 28 volt input line of the lightning protection circuit board) had been replaced with a solid wire.

The product is being repaired and brought back to conformity and drawing specification."

Airline’s response:

“The MCDU was looked at by our Avionics shop for troubleshooting before being sent for repair.

The technician saw that the power supply fuse F1 was unserviceable. He tried with an ammeter to see if the power was ok. The power was correct, and he has installed the wire to continue with “comfort” the troubleshooting[sic]. When it was discovered that the repair could not be done at home, the MCDU was assembled but the “comfort” wire was forgotten. The wire was used only for troubleshooting purposes.”

A Multi-Function Control Display Unit with a solid wire in the fuse mounting location

* Wire located in fuse mounting location

Transport Canada Comments:
Maintenance, overhaul and standard practice manuals give detailed instruction on trouble shooting and repair of the products they support. They also list approved tools for specific tasks. Deviation from them puts the maintainer in a dangerous area where simple mistakes could end up creating major problems. In the above example, a latent condition could have been created resulting in serious failure or fire.

Nose Landing Gear Collapse

AEROSPATIALE, ATR 42 300
SDR # 20101108015

While at the ramp and with the right engine running in hotel mode (ground idle, feathered and propeller brake ON), the left engine was started. As the left engine propeller was coming out of feather, the nose gear slowly retracted, collapsing the aeroplane’s nose to the ground. Prior to the collapse, the parking brake had been on and the landing gear select lever was in the gear down position, where indications showed all three gears down and locked. After the nose gear collapsed, the nose gear indication turned red and signified that it was unlocked.

The aeroplane’s nose was jacked and the nose gear was repositioned manually to its down locked position where several retraction/extensions were completed with no fault found.

As a precaution, the decision was made to replace the nose gear assembly, drag brace, retract actuator, gear selector valve and gear control lever. All removed parts were forwarded for teardown investigation.

After the replacement of the removed parts, the aeroplane performed all functional and operational required tests and returned to service where no further problems with gear operations have been noted to date.

The ATR 42’s gear and retraction system is designed as such that while on ground, weight-on-wheels (WOW); gear cycling operations are inhibited in-order to prevent inadvertent activation. Transport category aeroplanes are designed using this requirement which is stated in CAR part V, Airworthiness Manual chapter 525.

With the starting of the left engine, which in turn pressurized the #1 main hydraulic system, the aeroplanes nose landing gear (NLG) unintentionally unlocked and cycled towards retraction collapsing the nose of the aeroplane to the ground.

Through the shop bench test and tear-down report of the gear selector valve, the following was confirmed;

  • the unit failed an internal pressure drop test
  • the unit used an unknown type of lockwire for the end-cap plugs
  • the lockwire installation was “sloppy” with long pigtails
  • upon the valves disassembly, the spring cap for the spool shuttle valve of the “UP” solenoid side was found missing

The standard operation of the selector valve when the gear is not in motion or “static”, fully extended or retracted, with available hydraulic system #1 pressure is portrayed in figure 1. The spool shuttle valve within the selector valve will be in its relaxed or central position, removing all hydraulic pressure from the gear retract actuators.

Figure 1 - Cross section of selector valve normal condition
Figure 1 - Cross section of selector valve normal condition

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♦♦♦
♦♦♦♦
Figure 1 - Cross section of selector valve normal condition
Up solenoid
Down solenoid
Endcap plug
Spool shuttle valve
Spring cap found missing from the failed valve
Return port
Down port
Pressure in port
Up port
Return port

It is believed that due to the missing spring cap, the spool shuttle valves last set position was such as to allow the inadvertent direction of hydraulic pressure to the “UP port” or retract side of the three gear retract actuators, portrayed in figure 2.

Figure 2 - Cross section of selector valve with the spring cap removed
Figure 2 - Cross section of selector valve with the spring cap removed

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**
***
****
*****
******

♦♦
♦♦♦
♦♦♦♦
Figure 2 - Cross section of selector valve with the spring cap removed
Up solenoid
Down solenoid
Endcap plug
Spool shuttle valve position
Spring cap removed
Return port
Down port
Pressure in port
Up port
Return port

With this event, the NLG was the only gear to retract because of the fully loaded aeroplane weight to which the main gear actuators could not over-come.

An aeroplane product or part, such as this selector valve, carries its own unit type certification where any form of maintenance or repair work done to it, can only be accomplished by an approved Aircraft Maintenance Organization (AMO) rated for that specific product.

As defined in CAR 571.11 subsection (3);

“no person shall sign a maintenance release in respect of maintenance performed on an aircraft operated under Part IV or VII, or on parts intended to be installed on the aircraft, unless;

(a) the person is authorized to sign in accordance with a maintenance policy manual (MPM) established by the holder of an AMO certificate issued under section 573.02 with a rating of a category appropriate to the work performed.

With this incident, it is suspected that this gear selector valve had been tampered with by an unapproved individual or facility due to the shop bench test and teardown findings.

In a situation where a unit or part of an aircraft is suspected of being tampered with or “non-compliant”, then this part is to then be designated as a “Suspected Unapproved Part” (SUP) as defined in CAR Standard 571.13, information note:

A person who has reasonable grounds to believe that a part installed or intended for installation in a type certified aeronautical product that was not manufactured or certified in accordance with the applicable regulations of the state of production, or that is improperly marked, or that is documented in such a manner as to mislead with regard to the origin, identity or condition of the part shall submit to the Minister a report of the suspected unapproved part, using the service difficulty reporting system set out in section 521.401 of the CARs.

Transport Canada Civil Aviation would like to advise all maintainers and operators of the noted CARs and the importance to always favor the side of safety, as this operator did, when addressing a serious incident.

Fan Cowl Departures In Flight

 CANADAIR CL6002B19 (CRJ SERIES)
SDR # 20110617004

Upon landing, the right-hand engine lower fan cowl fell off the aeroplane and onto the runway. The aeroplane then taxied to the gate without incident. A maintenance investigation revealed that an ‘A’-check had been accomplished the night before where the cowls were removed. This was the second landing of the day when the cowl departed the aeroplane. Inspection of the cowl and nacelle shows evidence that not all of the fasteners were secured, specifically, the inboard fasteners appeared to be unlocked. The damaged cowls were replaced and the aeroplane returned to service.

Right-hand engine with a lower fan cowl missing

Transport Canada Comments:
This is not the first instance of an engine fan cowl departing an aeroplane in flight. If certain fasteners are not properly secured, air loads in flight are sufficient to deform the cowl and overcome the remaining fasteners. The critical area appears to be in the forward inboard location. The issue seems to be the fastener design and location. As the spring in the fastener and cams in the receptacle wear, its locking ability is compromised. Even with an apparently full rotation, one can get the impression of a fully engaged device. Compounding this problem is the proximity of the fastener row to the pylon structure. As seen in the photograph, there is little room to get a straight push on the tool. A short screwdriver is required making it even more difficult to apply adequate torque and pressure.

Extra vigilance is required when installing these units. Special attention should be paid to fastener wear including the receptacles, cross pins and springs.

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