Tenerife airport disaster

On March 27, 1977, two Boeing 747s, KLM Flight 4805 and Pan Am Flight 1736 collided on the runway at Los Rodeos Airport (now Tenerife North Airport). All 248 passengers on the KLM flight alongside 335 out of the 396 people on the Pan Am died, with 61 survivors. With 583 fatalities, it is the deadliest accident in aviation history. Both aircraft were destroyed by the post-crash fire.

The investigation by Dutch and Spanish authorities concluded that the KLM captain had mistook ATC clearance for take off clearance due to only recently returning to flying, and that his super-star status prevented other crew members from intervening. KLM ultimately claimed responsibility for the accident and agreed to pay relatives compensation.

The accident had a everlasting effect on the aviation industry, with many airliners establishing crew resource management and radio communications being improved.

Background
Pan Am Flight 1736 was a chartered flight that took off from Los Angeles International Airport (LAX) at evening on March 26, 1977 with an immediate stop in John F. Kennedy International Airport (JFK), before leaving for Gran Canaria International Airport (also known as Las Palmas).

The Pan Am was a Boeing 747-121 with the registration N736PA and was nicknamed Clipper Victor. It first entered service on January 22, 1970, making it the first 747 to enter service and also became the first 747 to be hijacked and land in Cuba which occurred on August 2, 1970.

The KLM flight was also a chartered flight that took off from Amsterdam Airport Schiphol, Netherlands with the Holland Travel Group. The aircraft was a Boeing 747-206B with the registration PH-BUF and was nicknamed Rijn (Rhine). The captain was chief flight instructor Jacob Veldhuyzen van Zanten, who at the time had a "superstar" status at the airline.

Las Palmas Bomb Explosion
At 13:15 on March 27, a bomb planted by the Canary Islands Independence Movement exploded in the flower shop of Gran Canaria injuring 8. There had been a phone call warning of the bomb 15 minutes earlier, which allowed the airport to be evacuated.

Minutes later another phone call to the police warned them of a hidden second bomb in the check-in counters. As another attack occured in January of that year against South African Airways, authorities closed the airport. It was eventually discovered there was no second bomb.

Diversion to Los Rodeos
By the time the bomb exploded, multiple aircraft include 4805 and 1736 were already approaching the island. Without mentioning the reason, ATC told all aircraft to divert to Los Rodeos 70km away in Tenerife. The message disappointed the Pan Am crew, who asked if they could circle the airport due to having enough fuel for 2 hours, but were ordered to divert and informed of the bombing.

Los Rodeos airport was a regional airport with only one runway, one major taxiway and several smaller taxiways connecting them. As a result, the airport couldn't easily handle the congestion of traffic which included 5 large aircraft. The KLM landed at 13:38 followed by the Pan Am at 14:15, alongside 9 other aircraft. Departing planes had to use the runway as a taxiway as the main apron was full.

The KLM flight was parked in a small accessory apron, next to a Braathens Boeing 737, a Sterling Airlines Boeing 727 and a Douglas DC-8 from SATA Air Acores. The Pan Am was slightly behind the KLM.

Refueling
At 15:00, Gran Canaria and Victor Grubbs ordered all passengers to reboard. However the KLM was still refueling causing Victor to call Zanten:
 * "How long will it take you to refuel?"

Zanten's answer in a annoyed tone was:
 * "About 35 minutes."

After his answer, Zanten abruptly hanged up. The Pan Am crew did discuss whether to taxi past the KLM, but decided not to due to the lack of a 3.7 meters (12 ft) clearance. Despite Zanten's response, refueling took over an hour to finish with it finishing at 16:24. Zanten called all of his passengers to reboard, but Dutch tour guide Robina van Lanschot who lived on the island at Puerto de la Cruz decided to not reboard.

Taxiing and Weather Conditions
Reboarding was delayed further by a missing Dutch family who had to be reboarded by a airport operator. Once the doors were closed, KLM asked for taxi clearance at 16:58. Since the taxiway was blocked, the KLM was asked taxi down the entire runway and perform a 180-degree turn, now as a "backtaxi". Due to working on the checklist, the ATC's order to copy ATC clearance was delayed.

By this time weather conditions were deteriorating due to the airport's height above sea (610 meters/2001 ft), and the fog was starting to set in (something that was regular in Los Rodeos), At 17:02, Pan Am was ordered to taxi and exit at C-3 (or E-3), but had trouble understanding the request due to the Spanish's poor English pronunciation.
 * ATC - "Affirmative, taxi into the runway and -ah leave the runway third, third to your left."
 * Pan Am F/O - "Third to the left, O.K."
 * Pan Am F/E - "Third he said."
 * Pan Am Captain - "I think he said first."
 * ATC - "-third one to your left."
 * Pan Am F/O - "I'll ask him again."

The Pan Am crew successfully identified the C-1 and C-2 exits, but when they arrived at the C-3 exit, skipped it. A conversation confirms they sighted it in the fog, but also discussed about the turn angle. The exit required two 148-degree turns, which made them fear they would end-up in the dirt. Additionally, the ATC only informed them of their exit after they already passed C-1, which created further confusion.

As a result, the Pan Am continued to exit C-4, which only required two 35-degree turns. At the same time, the KLM finished backtracking with Zanten forwarding the throttles. The first officer spoke up about them not having ATC clearance.
 * KLM F/O - "Wait a minute, we don't have ATC clearance."
 * KLM Captain - "No, I know that, go ahead, ask"

The first officer contacted ATC who gave them ATC clearance and orders after take-off, but did not explicitly say they were cleared for take-off. The KLM co-pilot repeated the newly received ATC instructions, and added:
 * "We are now at take-off." or "Uh...taking off."

The control tower was confused about this transmission and asked if it meant the plane stopped or if it was actually taking off. However the Pan Am crew also says at the same time:
 * ATC - "Ok, stand by for take off. I will call you."
 * ATC - "And we're still taxiing down the runway, the Clipper one seven three six."

This meant it created a 3 second "whistling" noise called a heterodyne, meaning the only word heard was "OK". As a result, the KLM crew continued down the runway at speed. The tower then asked Pan Am to report the runway clear, and upon hearing this the KLM flight engineer expressed his worry about the Pan Am not being clear of the runway.
 * KLM F/E - "Is he not clear then?"
 * KLM Captain - "What do you say?"
 * KLM F/E - "Is he not clear, that Pan American?"
 * KLM Captain (empathetic) - "Oh yes."

Collision
At 17:06:40, the Pan Am noticed the KLM barrelling down the runway and increased thrust to maximum while trying to exit using C-4. At the same time, the KLM attempted to get off the runway but was still below take-off speed. As a result, it created a major tailstrike resulting and only lifted the plane up a few meters.

The lower part of the KLM fuselage struck the upper deck of the Pan Am fuselage. It remained in the air for a few moments before crashing back down onto the runway 300 meters away.

Investigation
The accident was investigated by Spain's Comisión de Investigación de Accidentes e Incidentes de Aviación Civil (CIAIAC). There were roughly 70 personal including the United States, Netherlands, and the two airliners. KLM initially wanted their captain Zanten to investigate, before learning he was involved.

Probable Cause
The investigation concluded that the main cause of the accident was Zanten's attempted take-off without clearance, which the investigators suggested was one due to new KLM policies. Pilots can be fired if they exceed their flight time. Other contributing factors were:
 * Fog resulting in decreased visibility.
 * Radio interference resulting in both possible warnings being unheard.
 * Non-standard phrases such as "We're at take-off".
 * Pan Am not leaving at its intended exit. It was discovered this was because the turn would be "virtually impossible" for a 747 to make.

Dutch Response
The Dutch were initially reluctant to accept the Spanish report and noted some other contributing causes:
 * The cockpit voice recorder recorded sounds of a Soccer match suggesting the ATC was watching the March 27, 1977 match between Spain and Hungary.
 * The ATC's instructions were vague and could've been interpreted as take-off clearance.
 * They also noted the Pan Am not leaving at its intended exit, but instead said it could be feasible.

Speculations
As this was one of the first accident investigations to add human factors as contributions. This includes:
 * The KLM captain had not flown for 10 weeks.
 * The KLM captain's seniority and super-star status made the flight engineer reluctant to challenge him.