About the chain of events in the air crash on the island of Tenerife and the error chain in it you can read here:
1. The Chain of Dramatic Circumstances
1.1. An Explosion on the island of Gran Canaria
On Sunday, the 27th of March 1977, a bomb exploded at the airport of Gran Canaria. Police closed the airport and searched it for further explosive devices. Large airliners, among them two Boeing 747s were diverted to the airport Los Rodeos on the island of Tenerife.
A KLM jumbo jet, flight number 4805, came from Amsterdam. The other one with the flight number 1736 belonged to Pan Am and was on its way from Los Angeles to the Canary Islands after a stopover in New York.
The explosion and the following diversion of the flights are the first two links in our chain of events.
1.2. We find More Links of the Chain of Events at Los Rodeos Airport
Los Rodeos is a small airport located 632 metres above sea level. As it lies at the foot of the mighty volcano Pico del Teide, it is often coated in fog which descends from the mountain.
At this airport, which normally only handles small, regional aeroplanes, there was no ground radar at that point in time. In addition, the runway centreline lighting was out of service.
In the tower, there were only two air traffic controllers on duty as was the norm on Sundays. They were not used to heavy air traffic, not to mention jumbo jets.
The flight crews of the aircraft arriving on this day were instructed to park on the taxiway, because the air traffic controllers had no other choice.
Between the taxiway and the runway there are four access lanes, C1 to C4. Number 1 and 2 were not accessible any more due to the aircraft which were sitting on the tarmac. Number 3 and 4 are shown on the accident sketch in the ICAO Circular 153-AN/56 on page 57.
2. The Catenation Continued
2.1. A Wait Lasting for Hours
The KLM airliner was sitting in front of the Pan Am jet at the edge of the taxiway. The crew from the Netherlands (Captain Jacob Veldhuyzen van Zanten, First Officer Klaas Meurs, flight engineer Willem Schreuder) decided to take on 55 tonnes of fuel here already in order to save time in Gran Canaria. After all, they still had their return flight to Amsterdam ahead of them. The crew was concerned that they might exceed their maximum time of duty.
The Pan Am crew (Captain Victor Grubbs, First Officer Robert Bragg, flight engineer George Warns) descended from the cockpit to find out whether they would be able to get past the KLM airliner and onto the runway. However, 3.7 metres of space were lacking, which is little considering the wingspan of 59.6 metres of both planes.
2.2. Heading Towards the Take-off Position in Thick Fog
By the time the possibility to fly on to Gran Canaria finally had arrived, Los Rodeos Airport was shrouded in thick fog.
With visibility ranges partially below 300 metres, the aircraft were sent to the end of the runway one after another. To reach this position, they taxied along the runway and not along the taxiway, because of the planes which were parked there.
The pilots were instructed to follow one another directly. There was no waiting until an aircraft had taken off safely.
The KLM team was to perform a 180 degree turn in order to position their airliner for take-off. The Pan Am team was to use exit C3, which branches off the runway at an acute angle.
2.3. Captain van Zanten Released His Brakes, the Pan Am Team Was Looking for Access Lane C3
Captain van Zanten was ready for take-off, his first officer told air traffic control: "We are now at take-off". The sentence might also have been "we are now - uh - takin´ off." This could not be distinguished clearly by listening to the existing recordings.
In the first case, the airliner would have been in its take-off position, in the latter, it would have been in the process of taking off. The air traffic controller seemed to be puzzled, as well. He replies: "OK (pause), stand by for take-off, I will call you."
The pilots of the Pan Am, whilst they were looking for exit C3, answered immediately via radio that they were still on the runway. ATC asked them to report runway clear as soon as they would have left it.
ATC never issued take-off clearance to the KLM crew. Nonetheless, van Zanten released his brakes and started to accelerate the massive aircraft as his first officer was still communicating with ATC.
Willem Schreuder, the flight engineer, however, heard the radio message of the Pan Am and dared to speak up. He asks Captain van Zanten whether the Pan Am might not have left the runway yet. The captain replied emphatically: "Oh yeah!" Having been rejected in this manner, the flight engineer did not dare to insist.
3. The Collision on the Runway
The Pan Am copilot was the first to see the KLM jumbo jet as it emerged from the fog. Upon his yelled "get off, get off, get off", captain Grubbs turned left sharply and opened the throttles fully at the same time. In the cabin, the passengers and personnel were shaken wildly.
At that moment, van Zanten realised that the Pan Am airliner was right in front of him on the runway. He yanked the control column towards him so sharply that during that attempt to take off the tail section of the KLM aeroplane scratched the runway.
The front wheels could be lifted over the top of the Pan Am, but not the outer engine on the right, which stroke the fuselage of the Pan Am. Also due to the 55 tonnes of extra fuel, the airliner was too heavy for that.
Subsequently, the KLM jet hit the runway and burst into flames immediately. The Pan Am airliner caught fire, as well. In addition, its engines were still running and started throwing pieces of metal around. 61 passengers and nine crew members of the Pan Am managed to escape the burning wreckage. Unfortunately, nine of these passengers succumbed to their injuries later.
4. The Investigation Identified Technical and Human Failure
Experts from Spain, the Netherlands , and the United States carried out the air crash investigation.
4.1. The Heterodyne
The examination of the cockpit voice recorder showed that there had been a heterodyne when ATC had been talking to the KLM team while the first officer of the Pan Am had been announcing simultaneously that they were still taxiing down the runway.
Hence, the KLM Team could only understand the "OK" of the sentence "OK, stand by for take-off, I will call you", whereas the rest of the sentence became inaudible due to a garbled noise.
4.2. Missed Exit as Another Link in the Chain of Events
The wreckage of the Pan Am jumbo jet was found between the exits C3 and C4, which clearly leads to the conclusion that the crew had missed their exit. The reasons for this additional link in the chain of events may have been the thick fog and distraction due to the radio message of the KLM crew, as well as the fact that the team was not familiar with the airport layout. Furthermore, the access lanes were not clearly marked.
4.3. Possible Training Syndrome
Captain van Zanten had mainly worked as a trainer in the simulator recently. There, the concentration lies totally on the flying manoeuvre which is being practised. Therefore, his situational awareness during the real flight, in which all pieces of information from the surroundings must be taken into account, may have been compromised.
4.4. The Exit C3 at an Acute Angle May Have Been Inaccessible for Jumbo Jets
Exit C3 branches off the runway at an acute angle. As experts stated after the investigation, it might be almost impossible for pilots of jumbo jets to make a turn at that angle. As the air traffic controllers had no experience with such large aircraft, they may not have thought about that.
Taken together, we can see that a chain of circumstances, which had partially already existed before the landings at Los Rodeos Airport, technical failures and human factors contributed to the catastrophy. You can find them summarised in the figure below.
5. A Short Repetition of the Error Chain
As explained in the last articles from my blog , the examination of more than 300 incidents and accidents in aviation has led to the model of the error chain with its eleven links. In this model, the eleven links of the error chain are clearly defined and refer to human factors.
A chain of events, however, as we have seen above, contains also other factors, such as the fog and the heterodyne in this example. Now, after a short repetition of the error chain, using the figure provided again below, we will explore, which of the links we can detect in this chain of events.
6. Finding the Links of the Error Chain in the Chain of Events
Look at the chain of events under item 4. again.
Which links of the error chain can you recognise?
6.1. Distraction/Preoccupation
The Pan Am crew was distracted by the radio message of the KLM whilst taxiing down the runway. This was one of the reasons why they missed their exit.
Due to their concerns about exceeding their maximum time of duty, the concentration of the KLM crew may have been compromised.
6.2. There was Much Confusion in This Chain of Events
We can only try to put ourselves in the position of the teams which were in the middle of the events. The initial situation alone with so many aircraft at Los Rodeos and the bomb explosion, the long wait and the uncertainty about when and how things would progress, has certainly contributed to the confusion.
Air Traffic Control
The two air traffic controllers had to coordinate too many flights at a time and too large planes without ground radar in thick fog in an unusual situation.
The misleading radio message of the KLM copilot: "We are now at take-off/we are now - uh - takin´ off" certainly created another moment of confusion.
Pan Am Team
The Pan Am team was confused by radio message from the KLM crew, as well. The copilot reacted immediately. As we can see here, distraction can additionally confuse.
We can only try to imagine, what the passengers and cabin crew experienced in terms of confusion during the entire event.
6.3. Using Any Undocumented Procedure
As this is an unprecedented event, much of what happened at the airport refers to this link of the error chain:
- aircraft were parked on the taxiway
- the pilots were told to use the runway to taxi into position
- both the jumbo jets taxied almost directly one after another to their take-off position
- one jumbo jet was required to make a turn at an acute angle
6.4. Unresolved Discrepancies
After the flight engineer´s speak up no clarifying took place.
6.5. Failure to Meet Targets in Various Links of the Chain of Events
The KLM team ran the risk of exceeding its maximum time on duty.
On the whole, all the deviated flights were delayed for hours.
The Pan Am crew missed exit C3.
In all these cases, no deliberation can be detected. Thus, there was no violation of existing rules on purpose.
6.6. Incomplete Communication
The sentence „we are now at take-off“/“we are now – uh – takin´ off“ was misleading (ICAO Circular 153-AN/56, page 62 and following).
Due to the heterodyne, an important part of the answer was lost.
The speak up did not lead to a reevaluation of the situation. If a team does not resolve a discrepancy using appropriate communication, communication remains incomplete in such a case, as well.
6.7. Violating Limitations/Regulations
A violation of limitations/regulations can be discussed concerning the take-off without take-off clearance. However, this take-off was also caused by non-standardised phraseology and the heterodyne, which is why we should be cautious with our interpretation.
7. Conclucion Chain of Events/Error Chain and What No-one Had Expected
We can recognise some of the links of the error chain easily. Partially, it is not possible to clarify in hindsight, what was really going on within the teams. Nevertheless, we are able to detect more than three links of the error chain in the accident.
On the basis of the links of the error chain identified, we can see that several members of the respective teams required or carried out actions which contributed to the crash. These are intertwined with the chain of events.
That no-one had reckoned with the following, certainly contributed to these actions:
- ATC may not have expected that the Pan Am crew missed the exit
- the Pan Am team had definitely not expected the sudden take-off of the KLM
- ATC had not expected that take-off without take-off clearance either
- all three teams had not expected the heterodyne and the loss of information
8. Resulting Improvements of Safety in Flight
During the lengthy and meticulous air crash investigation light was shed on all its facets. From these, various institutions and authorities derived measures to maximise aviation safety.
Overview of the measures taken:
- a new airport on the island of Tenerife at sea level
- ground radar at Los Rodeos Airport
- standardised communication on the radio
- first generation of CRM: flattened hierarchies in the cockpit and speak up
- a blocking device for airliners to prevent heterodynes, not mandatory yet
In rememberance of the 583 victims, but also of those who were severely injured and mentally traumatised.
9. In My Next Blog Article You Can Read About:
The example showed us impressively that the speak up was not followed by an according reaction. Furthermore, it was not repeated assertively in the culture which prevailed at that time. What has changed since then? Moreover, what do we mean by "just culture" in CRM?
Please feel free to browse earlier blog articles.
Author: Eva-Maria Schottdorf
Date: September 18th 2021