Background
Flight 6526, an L188 aircraft, departed Cologne at 0300 hours and routed directly to Dublin. After a delay of over one hour due to a late freight truck, the aircraft departed for Shannon Airport at 0816 hours. The crew comprised the First Officer (handling pilot), the Captain (non-handling pilot), and a Flight Engineer. The takeoff gross weight was estimated at 83,701 lbs, well below the maximum takeoff weight of 116,000 lbs, and the center of gravity was within limits. The landing weight was estimated at 80,345 lbs, also below the maximum landing weight of 98,102 lbs. The total freight on board was 14,000 lbs, less than half the maximum permissible 33,000 lbs.
Approach and Warning
During the approach to Shannon Runway 24, the crew set flaps to 78% instead of the standard 100% due to weather forecasts. At an altitude of 700 ft, the Ground Proximity Warning System (GPWS) horn sounded. The Flight Engineer inhibited the GPWS. The crew did not carry out the "before landing" checks. Over the runway threshold, the landing gear warning horn sounded, followed by a scraping sound and severe vibrations. The crew realized the gear was not down and initiated a go-around.
Engine Failure and Electrical Loss
During the climb-out, at about 500/600 feet, the aircraft entered instrument meteorological conditions. Simultaneously, the propeller assembly and part of No. 3 engine fell to the ground. Electrical power was lost, and only the standby artificial horizon and wet compass remained serviceable. The Captain took control and, with the First Officer, struggled to maintain directional control using rudder pedals. Insufficient power prevented climbing to 3000 ft; less than 2000 ft was attained. By manipulating throttles and feathering No. 4 engine, directional control was recovered. The aircraft then flew with only No. 1 engine fully operative and No. 2 engine at half power with vibration.
Second Approach and Landing
The radar controller vectored the aircraft to approach Runway 24. The crew selected gear down; only two of three green landing lights illuminated, but the Captain proceeded with the landing, still with 78% flaps. The landing was successful, with all landing gear deploying correctly. The aircraft was evacuated while airport crash crews stood by.
Probable Cause
The official findings stated: "The primary responsibility for the safe conduct of a flight rests with the cockpit crew and, in this regard, they have the Aircraft Flight Manual (AFM), company Standard Operating Procedures (SOP's) and other technical manuals at their disposal in the cockpit. In particular, the company lays down the SOP's to be followed by each and every cockpit crew member in the interests of standardisation and flight safety. The implementation of these measures and procedures is carried out by the cockpit crew and they are aided in this process by artificial mechanical/electrical warning systems and audio alerts. In the L188 in question the two audio alert systems consist of a Ground Proximity Warning System (GPWS) and the landing gear warning horn. The GPWS system sounded at about 700' AGL and this should have been sufficient to warn the crew that the landing gear handle was not down and consequently that the undercarriage was not in the landing configuration. The Flight Engineer, however, reached up and inhibited the GPWS. He said he did this because he understood the aircraft to have 78% flap set for landing and that this was why the GPWS warning sounded. He obviously confused this warning with a warning which he would have got below 200 feet radio height when the flaps are set at less than 100% for landing. The fact that this particular aircraft had no FLAP OVERRIDE switch, as the other two similar aircraft in the fleet had, would probably have added to this confusion. Whether the engineer was instructed to inhibit the GPWS by other crew members is not clear from the CVR. In the debrief following the accident the other crew members agreed that the engineer would have been correct in inhibiting the GPWS as they were landing with 78% flaps configuration. They, therefore, also misinterpreted the GPWS warning. Having failed to carry out the landing checks and with the undercarriage not down, the normal undercarriage warning horn should have sounded when the throttles were retarded for landing. However, it is possible to silence this warning in the 78% flap configuration, which would not have been possible if the flaps were set in the normal (100%) configuration for landing. It appears that as the throttles were being retarded the Flight Engineer pressed the warning horn button to prevent the alarm from sounding. Whether he was so instructed is not clear as parts of the CVR tape were difficult to interpret due to its poor quality. During the final stage of landing the throttles were advanced again thus negating the warning cancellation. As the aircraft rounded out for landing the warning horn was again free to sound, and it did, as the throttles were retarded. However, at this stage, there was too little time to lower the undercarriage and five seconds later the propellers struck the runway surface."