In the last few weeks, we saw two major aircraft incidents with many losing their lives.
The first incident was a Boeing 747 cargo plane landing at the Hong Kong International Airport. As it touched down, it suddenly veered to the left at our new third runway and hit a patrol vehicle before it skidded off into nearby waters. It resulted in two runway patrol officers losing their lives as their vehicle was pushed into the sea.
Initial findings revealed that there were no clear messages from the voice recorder, pointing out that the pilot did not see anything out of the ordinary as he proceeded to land the aircraft. But the black box showed the fourth engine, on the far-right side of the plane, suddenly turned on to full forward thrust, instead of reverse thrust that the other three engines were operating.
In the cockpit of an aircraft, each engine is controlled by a separate throttle lever. When pushed fully forward, they enable the engines to give full power and therefore maximum thrust, required when taking off. After the plane has climbed a certain altitude with sufficient forward velocity, the throttle levers are pulled back gently for cruising at high altitude, where engine power can be significantly reduced as the air resistance is low and there are no climbing needs.
To obtain maximum reverse thrust, as it is required after landing to gain maximum deceleration that will allow an airplane to slow down enough to negotiate turns from the runway to taxiway, another set of levers are used. These levers have inherent detent interlocks to ensure that they are not casually operated, for obvious reasons.
The preliminary report revealed that the reverse thrust lever for the No 4 engine was faulty, but it should not affect flight operation as only the two inner engines would be required to run at reverse thrust at normal landing moves. Applying reverse thrust only on one outside engine alone could cause the aircraft to swerve.
How the No 4 engine suddenly turned to full thrust forward at landing will remain a mystery, but mechanical failure can be ruled out as it requires a deliberate action on the part of the pilot to initiate it.
This sudden forward thrust on one engine, coupled with the three engines on reverse thrust, could be the reason why the airplane suddenly veered left upon touch down.
It may be too early to blame the pilot or co-pilot for inattention, but I hope the full report will address this.
The other incident involved an MD-11 airplane taking off at Louisville Muhammad Ali Airport. Within a minute, the aircraft suddenly lost speed and crashed into a nearby building.
Initial findings indicated that the left-hand side engine was dislodged from the aircraft wing upon taking off. As this engine assembly was still running at full blast, it continued in its forward path but at a different direction to the crippled aircraft, and with a full tank of fuel it caused a major fire as it hit a building.
At cruising speed, the momentum of the flying aircraft together with the thrust from the other engine could have allowed the airplane to continue on its path with adequate control for the pilot to bring it back to ground in reasonable safety. But at take-off, with low speed, it could not hold the aircraft to a controlled course. Hence, the incident of the stricken aircraft crashing into the pilot training school near the airport caused many casualties.
Investigations revealed a lug holding the bolt supporting the heavy engine has sheared off from its mounting. Whether this was due to metal fatigue from repeated vibration of the 34-year-old airplane, or whether it was a weak point of the structure, inadequately designed for the purpose, will be the subject of further investigation. The United States Federal Aviation Administration has since grounded all similar aircraft, pending further investigations.
Experts in that field pointed to maintenance activity for overhauling spherical bearings in front of the engine pylon mountings. This could have led to overloading of the lugs at the rear mountings, causing premature cracking and failure. This maintenance requirement was suggested by the aircraft manufacturer to rectify a weakness of the bearing some 30 years ago, but the method of holding heavy engine assembly while it is dismantled could have caused undue stress to other mounting points.
It is likely that, for both of these incidents, the cause could be attributed to human error, either in operation or maintenance.
Human life is most precious and must be protected at all costs. Mechanical systems are designed to be fail-safe to protect us, but for certain critical equipment, they require continuous monitoring and maintenance to prevent mishaps. Incidents involving road vehicles may not always cause death or serious injuries, as speeds are lower and faulty engines will bring the vehicle to a stop. But for aircraft, no operational or maintenance failure can be tolerated as the airplane would fall to the ground, with disastrous results.
It is sad to see how transport operators sometimes do not strictly follow set procedures. Pilots neglecting training and operation procedures, and maintenance technicians not adhering to set procedures, could have been the reasons for these incidents.
Veteran engineer Edmund Leung Kwong-ho casts an expert eye over features of modern life
