Flight details
Date: 24 November, 2001
Operator: Crossair
Flight number: 3597
Aircraft type: Avro RJ100
Location: 3nm short of runway 28, at Zurich Airport [ZRH], Switzerland
Fatalities: 24 of 33
Accident type: controlled flight into terrain
Primary causes: Pilot error, managerial errors
(Aviation safety network, 20041)
Introduction
The following is a human factors analysis of the crash of Flight 3597; factual information of the sequence of events is drawn primarily from the official accident report composed by the AAIB (AAIB, 20042).
Sequence of events
Approach type
Events pertinent to the accident sequence commenced when Crossair Flight 3597 received clearance to commence an approach to runway 28, ZRH, at 20:58:50 UTC. Runway 28 did not have an ILS installed, so the pilots were forced to fly a VOR/DME approach. This lack of glideslope/localizer information required the pilots to maintain terrain clearance by reducing altitude in steps, as they flew closer to the airport as indicated by the DME. This is more difficult than a precision approach, and the captain had previously displayed very poor flying performance, and more specifically, approaches, particularly more difficult non-precision examples. In spite of this, the airline allowed the captain to continue flying without any significant remedial action. The approach chart that the pilots were using was missing some key terrain details, including the hill that the aircraft would eventually crash into.
Weather
While the weather at the time of the accident was poor, with low visibility and cloud base, runway 28 was still available. The weather conditions, particularly runway visual range, as measured by the airport were still above minimums. Unfortunately, this data was collected from a station distant from runway 28, and did not accurately reflect the conditions the pilots were about to fly into. A few moments later, another aircraft that had just landed on runway 28 advised of very poor weather conditions, the runway was sighted at about 2.2nm out, and that it was near minimums. Despite this, the pilots continued with their approach, seemingly unconcerned.
Descent through MDA
As the pilots continued their approach, they reached the minimum descent altitude [MDA] of 2,390 feet AMSL at 21:06:10. At this point, the captain advised the first officer that he had the ground in sight, and continued to descend below the MDA, which the first officer did not prevent. There was no suggestion that either pilot had the runway or approach lighting in sight, the requirements for descent below the MDA. It was determined from the cockpit voice recorder that both pilots, and in particular the captain, were trying to spot the runway and keep the ground in sight throughout the rest of the sequence.
RA callouts and impact
Twelve seconds later, at 21:06:21, an audible “500” alert was generated, indicating that the aircraft was 500 feet AGL, as measured by the radio altimeter [RA]. Eleven seconds later, at 21:06:32 a “minimums” aural alert were generated, as a result of a RA reading of 300 feet AGL. Two seconds later, at 21:06:34, the captain commenced a go-around. About one second later, a “100” alert was generated, and the CVR recorded initial impact sounds. The aircraft crashed into a wooded hill, still three nautical miles short of the runway 28 threshold. The time taken from the initial descent below the MDA to impact was about 25 seconds.
Lack of warnings
There were no warnings of imminent terrain impact apart from the limited RA callouts. Despite being equipped with a ground proximity warning system [GPWS], there were no urgent "terrain, terrain!" and "pull up!" warnings, as the aircraft was being flown in a profile conducive with landing, and therefore was outside of the flight envelope required for the warnings to be sounded. Furthermore, this approach was not equipped with a minimum safe altitude warning MSAW, which could have alerted the pilots when they descended below the MDA.
Errors committed
Active errors
Poor CRM
By not effectively dividing tasks and working as a team, the pilots' situational awareness was significantly degraded. In particular, the captain spent a significant amount of time looking out the window trying to spot the airport, when he should have been flying the aircraft. The authority gradient, or power distance was also too high. This occurred even though Crossair's CRM training met ICAO standards.
Hazardous attitudes
By deliberately descending through the MDA without visual contact with the runway, the captain displayed a strong anti-authority attitude. Furthermore, the fact that the crew continued the approach in an unconcerned manner even when being advised of poor weather near minimums shows both complacency and overconfidence.
Lack of situational awareness
The pilots did not recognise their proximity to the terrain until it was too late, they were flying 'behind the aircraft', in the sense that they did not comprehend its position on the approach.
Violation of MDA
The captain deliberately violated the MDA by descending below 2,390 feet AMSL without the runway in sight. This was the main cause of the accident, and happened even though Crossair's CRM training met ICAO standards.
First officer's actions
The 21-year-old, and relatively inexperienced, first officer did not speak up against the captain's violation, likely due to the cockpit culture, in particular the high authority gradient or power distance.
Latent errors
Missing terrain information
The hill with which Flight 3597 collided was not shown on the approach plate the crew was using. Although it likely would not have prevented the accident, if the hill was shown it would have improved the pilots' situational awareness.
Captain's performance
During the past several years, the captain had shown repeated major deficiencies in his ability to safely fly aircraft, in particular, his ability to fly complicated approaches and use more modern instrumentation.
Crossair's failings
Although Crossair was aware of the captain's poor performance, the airline did not take adequate steps to remedy the situation.
Inaccurate weather information
The airport's means of calculating weather minimums for runway 28 were inaccurate. The calculation of weather minimums used data from another site on the airport and did not accurately reflect the actual conditions on runway 28. Had the calculation been accurate, the pilots may have been forced to discontinue the approach.
Other considerations
Lack of MSAW
The approach to runway 28 was not fitted with a MSAW; if fitted, this system would have alerted to the pilots that they were descending below the MDA, and likely would have resulted in a go-around.
GPWS
The Avro RJ100 was only fitted with basic GPWS, which did not generate "terrain!" or "pull up!" warnings as the aircraft was outside the envelope for these alerts to be generated, if fitted with EGPWS which was available at the time, the pilots would have received "terrain!" or "pull up!" warnings about the impending collision in enough time to avoid the impact with terrain
SVS
The development of synthetic vision systems would almost certainly have prevented the crash, as it would have given the pilots a clear view of the terrain below.
Conclusion
This accident was primarily caused by the captain deciding to violate the MDA, and the first officer's lack of preventative action. It was facilitated by Crossair's oversight of the captain's flying ability, and missing defence layers, primarily EGPWS and MSAW.
Want to know more?
Dramatisation of Flight 3597 on Aviation Knowledge
AAIB page on Flight 3597, including final report
Flight 3597 on Wikipedia
