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Flight History and Incident Scenario…
On the 9th of Sept 2007 the Scandinavian Airlines Bombardier Dash 8-Q400, aircraft (LN-RDK), flight 1209, had a domestic flight from Copenhagen Airport (EKCH) to Aalborg Airport (EKYT). Up to approaching phase, it was a normal flight with no mentioned incidents, however during the approach to EKYT airport, and when the flight crew selected the landing gear down, the nose landing gear and the left main landing gear (MLG) indicated down and locked while the right MLG indicated “Transit” (not down and locked i.e. Unsafe condition to land). So the flight crew decided to perform a “go-around” to verify this abnormal condition. During the go-around, the flight crew revise the Quick Reference Handbook (QRH) for a checklist concerning an unsafe landing gear, but they did not find any checklist with that header, and the only suitable checklist -as they thought- was “ALTERNATE LANDING GEAR EXTINTION”, and it was about performing an alternate landing gear extension… So they did, but nothing changed. The flight crew subsequently tried to make a normal gear up, the nose and left MLG were retracted normally, while the right MLG indication remained in “Transit”. A second attempt to use the alternate landing gear extension was performed, but without any changes to the right MLG indication. After this point the flight crew declared the need for an emergency landing, so they inform the airport about the situation and then the aircraft entered in a holding pattern in order to reduce the amount of fuel, and to acquaint the passengers about the emergency situation. Before landing, the cabin crew decided to evacuate the seats at rows 6, 7, and 8 (the seats in parallel with the right engine propellers area). [Hcl.dk, 2007]
During the landing, the left MLG touched down the runway first, followed by the right MLG, and approximately two seconds after touchdown the right MLG collapsed, the propeller blades of the -running- right engine struck the ground, then a blade splintered from the engine and wedged in the fuselage, the aircraft then veered to the right and came to rest away from the runway. Fortunately the passengers and the crew were evacuated safely with a total of seven minor injuries amongst the four crew and 69 passengers on board. The figure 1 shows the aircraft after the incident.
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It is a postulate that most of accidents occurs because of several (human) errors –in different forms- have been cumulated one over the other, awaiting for the proper chance to start an accident. This assignment will analyse the causal factors of the above incident, using the Human Factor Analysis and Classification System (HFACS), which based on the well-known error model “SWISS CHEES”, to identify the human errors were stand behind this incident, and then -after describe each factor separately- it will tie them together in a clear analogy to clarify the relationship and the error causation chain of the four levels of the latent and active human failure/errors, including the; Organisational Influences, Unsafe Supervision, Preconditions for Unsafe Acts and the Unsafe acts. The figure below shows these four error levels and their subcategories, and how they linked to each other as an accident causation chain.
Figure 2: The HFACS Error analysis framework.
When the pilot selected the landing gear lever to the down position, the retraction/extension actuator rod end pulled out from its place in the piston rod due to severe corrosion in the threaded connection of the internal piston. Thereby, the landing gear became free to fall, but in uncontrolled movement (un damped), this in turn (the free falling) generated sufficient kinetic energy to cause failure to the stabilizer brace joint lugs, which is responsible to lock the landing gear in the down position, thus losing the ability to lock the landing gear down (indication of “Transit” i.e. unsafe gear). So this unsafe landing gear condition led the right MLG to collapse, just two seconds after the touch down. The figure below shows the retraction/ extension actuator and its separated rod end, and the broken stabilizer brace joint lugs after the incident. [Hcl.dk, 2007]
Figure 3: Damaged landing gear components after the accident. [By Hcl.dk, 2007]
The Errors causal factors attributed to the manufacturers, the operators and the competent authorities. And they divided into three categories:
Figure 4: organizational influences categories.
The manufacturer in this incident had the biggest cut of this incident’s causation factors. Starting from the active failure which triggered the incident, (i.e. the separation of the rod end from the actuator piston) the causal factor of this failure, refer to use the landing gear’s manufacturer dissimilar materials in manufacturing the actuator’s piston and the rod end. The actuator’s piston and rod end were made of noble martensitic stainless steel and the less noble (4340) steel material respectively. [Hcl.dk, 2007]
Therefore, and based on the scientific fact that the dissimilar metals have different electrode potentials, so when two or more different materials come in contact (in existence of electrolyte of proper medium), an electrochemical process occurring -galvanic corrosion- causes the less noble metal to corrode. [Corrosion-doctors.org, 2012] So, by applying this fact on this case study, it led the piston connection threads to deteriorate gradually until it reached a point where it could not withstand the working load, so when the pilot selected the landing gear down, it split-off. [Hcl.dk, 2007]
Consequently after this, when the right MLG indicator indicated in “Transit” (I.e. Unsafe condition to land) Actually, the landing gear at that time was down but not down and locked, because when the rod end separated from the actuator piston, it caused the landing gear to free fall in un damped manner, thus revealed another manufacturing flaw, which is the weak construction of the stabilizer brace joints, as it was broken after the free falling, therefore it lost the chance to lock the landing gear in down position. And this Latent manufacturing Failure had appeared after the first Active Failure happened (hierarchy chain). [Hcl.dk, 2007]
After that, another manufacturing Latent failure was popped out, when the flight crew referred to the QRH to find the proper procedure to follow in this case, but the only thing they was able to find, was the use of alternative landing gear extension system. However the investigation readouts mentioned that the crew were not able to find a checklist in QRH for “unsafe landing gear”, because both the QRH and Aircraft Flight Manual (AFM) assumed that the alternative extension of the landing gear will be successful. Neither the QRH nor the AFM referred to back up checklist if the procedure was unsuccessful. [Hcl.dk, 2007]
Another Latent failure attributed to the manufacturer, and led to increase the loses of this incident; neither the QRH nor the AFM mentioned any procedure to shut down the engines in case of landing with unsafe landing gear, which would if applied -as in this case- reduces the damages and risks of the incident. (E.g. The broken blade wedged in the fuselage…etc.) [Hcl.dk, 2007]
N.B the manufacturer issued the QRH base on the AFM. The QRH is an extract of the AFM procedures and checklists, to help the pilot to operate the airplane in normal and abnormal situations.
At the end, these four organisational-attributed influences opened plenty of windows of opportunities toward this incident, awaiting other deficiencies to strike.
Unsafe Supervision (Latent Failures)
Supervision is to direct or oversight the work or actions. However supervision could lead to a disaster if the one or more of the underneath four factors committed by supervisors take place. [Aviationknowledge.wikidot.com, 2011]
Figure 5: unsafe supervision categories.
Every aircraft should be maintained in accordance with an approved maintenance programme (approved by EASA and the competent authority). The maintenance programme should be based on the Maintenance Review Board reports (MRB) where applicable. Refer to Article M.A.302 of Annex 1. Moreover, the MRB reports and the maintenance programme are developed by the manufacturer, approved by the competent Authority and implemented by the aircraft operator. [Easa.europa.eu, 2010] But in this incident case, neither the MRB’s reports nor the Maintenance Requirement Manual specified any task to inspect the MLG retraction/extension actuator, as well as no overhaul requirement for it. As a result of this, the actuator was installed in the aircraft at the time of manufacturing the aircraft (year 2000) and till the time of incident the aircraft had completed 14,795 flight cycle, with a maintenance records were verified to be in compliance with the established maintenance program, (which contains no specific inspection tasks for the actuator). However there was a restoration requirement at the interval of 22,400 flight cycles for the actuator (I.e. after moreover 7605 flight cycles!!). This requirement did call for replacement of the actuator rod end. And this requirement was the only direct specified requirement to be performed on the actuator and the rod end... Briefly, the unsafe supervision category here was; the “Planned inappropriate operation” of the actuator as the manufacturer overestimated the life limit of this actuator without suitably ascertain of its proper life limit (this point will explained thoroughly in the next paragraph). So, the lack of supervision of this point by the manufacturer and the competent authority (which gave the approval for the maintenance requirement manual), led to create a suitable environment for the corrosion to deteriorate the actuator piston gradually, thus create another window of chance toward the incident. [Hcl.dk, 2007] The unsafe supervision here lay under “Inadequate supervision” and Inappropriate planned operation categories.
In order to maintain the aircraft airworthy, the aircraft should be inspected for any hidden failures and abnormalities in operation at regular basis (regular unscheduled inspection), to optimize aircraft utilization and minimize aircraft downtime, as well as to solve any malfunction (if existed) at early stage. And this is the main objective of the inspection programme, which is an important part in the maintenance programme. But it seemed that the inspectors were ineligible or the followed unscheduled-inspection programme was too weak, as an inspection made after the incident for the left MLG retraction/extension actuator piston rod and the rod end connection showed corrosion and mechanical wear!! Also, before the incident on the 7th of June 2007 on “line check” session, the inspectors found the right MLG Actuator rod end loose!! And the action taken was, only retightening it without tracing the causal factors behind this looseness i.e. inadequate supervision (as it will be described later in the Unsafe Acts section).
As well as, the operator’s maintenance organization had made an inspection on the whole -Bombardier Dash 8-Q400- fleet and found that 26 aircrafts out of 40, MLG retraction/extension actuator rod ends had loose jam nuts due to corrosion deterioration!! So all these results revealed how did the ineligible inspectors or the operator’s weak inspection programme indirectly contributed in this incident and opened a window of opportunity/ latent failure. [Hcl.dk, 2007]
Preconditions for unsafe acts
Preconditions for unsafe acts are; those factors resulting in human error or unsafe situation, and they are divided into three main categories as shown in the figure below. [Skybrary.aero, 2014]
Figure 6: Preconditions for unsafe acts categories and subcategories.
The use of dissimilar materials in manufacturing the retraction/extension actuator piston and the rod end opened a suitable window of opportunity for a galvanic corrosion to occur, however, there was a need for a catalytic element to start this chemical process. And this element was found in the accumulated Moisture (i.e. electrolyte) inside the connection between the threaded piston rod and the rod end of the right MLG actuator, accumulated by temperature and pressure variation. So this physical environment acted as a precondition to cause this unsafe chemical galvanic corrosion. I.e. unsafe environmental factor [Hcl.dk, 2007]
During approaching phase and due to unsafe landing gear status, the ground proximity warning system (GPWS) started a continuous warning horn after selecting flaps 10°, and lasted after the aircraft descended below 1000 feet. This warning made communication difficulty in the cockpit and created unnecessary stress and distraction among the flight crew. However the information about which circuit breakers to pull to silent this warning was given in the QRH under “Emergency Landing” checklist, which hadn’t been used by the flight crew!! So this stress and distraction (precondition for unsafe acts) affected the mental state of the flight crew, and as a result affected their capability to make a common sense decisions such as; shutting down the engines before landing to reduce damages and risks. [Hcl.dk, 2007]
The unsafe acts divided into two main categories; Errors and violations. Errors are unintentional mistakes while, the Violations are intentional ones or are attributed to lake of experience. [Aviationknowledge.wikidot.com, 2011]
Figure 7: Unsafe acts categories and sub categories.
Errors: Due to the distraction and stress generated from the (GPWS) continuous warning horn or simply because they cannot found any other suitable checklist, the flight crew made a “Decision Error” by utilizing the improper checklist -to follow it- before the landing. The proper checklist to use should be “EMERGENCY LANDING checklist”, which contains information about which circuit breakers to pull in order to silent the continuous GPWS warnings during the approach and landing, also has information concerning reseating the passengers away from the propeller area and another safety aspects to take during emergency landing. On the other hand the flight crew had utilized the “ALTERNATE LANDING GEAR EXTENSION checklist” and they did not use any other, even after it failed in changing the condition of the right MLG indication (unsafe landing gear). Moreover, this checklist doesn’t have any safety aspects regarding the emergency landing.
It is clear to see that this incident was unavoidable, however it losses was able to be minimized if the pilot employed the common sense and shutdown the engine on the effected side (the R/H engine) upon landing, this in turn could reduce the danger of splintered blades (as a splintered blade wedged into the fuselage and made severe damage in it), as well as it could reduce the damages to the engine itself, as the investigation revealed that the engine was mainly damaged because of the sudden halt of the engine’s shaft (high moment of inertia), when the blades struck the runway while the engine was running. So, obviously this skill-based error contributed in increasing the incident losses.
Violation: An investigation of the maintenance records performed prior to the incident revealed that during “Line check” on June 7th, 2007, the actuator’s rod end nut on the RH MLG found loose in piston end. And the action taken was according to Aircraft Maintenance Manual (AMM); 32-00-710-801. “Nut fastened and operational test of main landing gear extension and retraction was made”. However, the maintenance technician had not measure the torque he applied to the jam nut, all what he did was; using a big wrench on the nut until it was not possible to tighten it more!! So, doing this violation surely affected the connection threads between the rod end and the piston end (as applying too much torque could ruin them and applying less than the specified torque would leave a looseness which eventually lead to deteriorate the threads and leaving room for moistures). So this unsafe act by this technician opened -by time- a suitable window of opportunity in this incident’s hierarchy causation chain.
How to prevent such incidents from occurring in future
The Accident Investigation Board, Denmark issues the following recommendations to EASA based on the investigation results:
• To review the designation of the main landing gear and the retraction/extension actuator i.e. to review the used material in manufacturing the actuator’s piston and the rod end and at the same time the effect of the un damped free fall on the stabilizer strut lugs.
• To review the certification of the maintenance programme of the MLG retraction/extension actuator and the rod end i.e. to specify inspectional tasks and overhaul requirements to the actuator at reasonable periods.
• To review the procedures in the Airplane Flight Manual and Quick Reference Handbook, regarding an emergency landing resulted by unsafe landing gear. [Hcl.dk, 2007]
Responding to the above recommendation, EASA issued the airworthiness directives AD No. 2007-0272 on 16th of October, 2007 and Transport Canada Airworthiness directive CF-2007-20R1 comprises directives and safety recommendations regards this accident and addresses the above recommendations in consideration. [Hcl.dk, 2007]
Analysis & Conclusion
The causal factors of this incident are split down into two main categories:
1] Factors with direct contribution in this incident:
Starting from the point of using the manufacturer dissimilar materials in manufacturing the retraction/extension actuator, by doing this, the manufacturer opened the first window of opportunity toward the incident (i.e. create a suitable environment for the galvanic corrosion). Subsequently, when the maintenance requirement manual did not specify any task to inspect the MLG retraction/extension actuator, as well as no overhaul requirement since the time of manufacturing the actuator till the time of the incident, that opened another window of opportunity aligned with the first one toward the incident i.e. by leaving the corrosion to spread and to deteriorate the metal freely. However, there should be a defence wall to prevent such incident happening shall the first two errors occur. This defence wall should be available in any aircraft operator organisation, which is the “unscheduled-inspection programme” (Preventative inspection/maintenance), but it was clearly appeared that this inspection programme was really flimsy (inadequate supervision), as inspections made after the incident revealed that; amongst the whole fleet there were been 26 aircraft out of 40 suffering from galvanic corrosion on the same actuator!!