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Mark ở vũ trụ khác · Another Universe Anatomy of a Disaster · Aviation · Communication

Tenerife Runway Collision

"OK"

KLM 4805 and Pan Am 1736 · 27 March 1977 · Los Rodeos airport, Tenerife, Spain

What happenedtwo Boeing 747s collided on a runway in fog
Loss583 dead, 61 survivors
Root causea take-off without clearance, after a chain of misheard words
Main sourceSpanish report, 1978; Dutch comments
Re-read in 1 minute

Essence

One runway, two 747sThick fog. The tower could not see the runway.
One ambiguous sentence"We are now at take-off": the first officer meant "taking off"; the tower heard "in position, waiting".
One "OK" and 3 seconds of squealTwo warnings went out at the same moment and blocked each other on the radio.
One question brushed asideThe flight engineer asked, "Is he not clear, that Pan American?" The captain: "Oh, yes." Fifteen seconds later, impact.
Not just one manThe report names the KLM captain, but lists six contributing factors. The Dutch did not fully agree.

Source: Spanish report (Subsecretaría de Aviación Civil, 1978), sections 1.2, 1.7, 2.1 and conclusions; FAA, "Lessons Learned", PH-BUF/N736PA.

Part 1

The question

The best-known captain, a full crew, a flight engineer who spoke up: why did the 747 still roll into the fog? Because everyone heard what they expected to hear, and in that cockpit a junior's question was not heavy enough to stop a senior.

midday, 27 March 1977DiversionA bomb goes off at Las Palmas airport. Flights pile into the small Los Rodeos airport.
→
16:50–17:05FogVisibility drops from 2–3 km to 300 m. Two 747s taxi on the same runway.
→
17:06:47CollisionKLM starts its take-off without clearance and hits Pan Am, still on the runway.

Source: Spanish report (1978), sections 1.1 and 1.7 (times, visibility).

Part 2

What happened

Times come from the cockpit voice recorder (CVR) and tower tapes, as printed in the Spanish report.

Diagram of Los Rodeos airport, not to scale. The runway runs left to right, with threshold 12 on the left near the apron and threshold 30 on the right. Exits C-1 and C-2 are blocked by parked aircraft. KLM taxis to the end, turns 180 degrees and starts its take-off run to the left. Pan Am taxis behind it, is told to leave at the third exit, C-3, a sharp turn of about 148 degrees, misses it, and is still on the runway between C-3 and C-4 when it is hit. apron packed with aircraft C-1C-2C-3C-4 threshold 12threshold 30 fog · visibility ~300 m KLM: taxi to the end,turn 180°, take-off run ← Pan Am: taxi behind,"leave at the third exit" (C-3) C-3 needs a ~148° turn, almost a U-turn 17:06:47
Illustrative diagram, not to scale. The runway is 3,400 m long and 45 m wide. The Pan Am wreck lay about 1,385 m from threshold 30. At that point the tower could not see the aircraft on the runway.Source: Spanish report, sections 1.1, 1.7, 1.14.2 and 2.1; the 148° turn at C-3 is from the ALPA study, cited via Wikipedia (secondary).
The last minute. 17:05:41, first officer: wait, we don't have an ATC clearance. 17:05:53, the tower gives the route clearance, not a take-off clearance. 17:06:09, the first officer reads back and says "We are now at take-off". 17:06:11, brakes released, captain: "we're going". 17:06:18, tower: "OK". 17:06:19 to 17:06:22, a squeal covers the tower's "stand by for take-off" and Pan Am's "still taxiing down the runway". 17:06:32, the flight engineer asks whether Pan Am is clear. 17:06:35, captain: "Oh, yes". 17:06:47, collision. 17:06:09 · KLM first officer"We are now at take-off" 17:06:32 · flight engineer"Is he not clear?" squeal ~3 s 17:05:53 · towerroute clearance,not a take-off clearance 17:06:18 · tower: "OK" 17:06:35 · captain"Oh, yes" 17:06:11 · brakes off: "we're going" 17:05:41 · first officer"wait, no ATC clearance" 17:06:47 · collision time scale: seconds, 17:05:40 to 17:06:50 (recorder time)
From brake release to impact: about 36 seconds. The tower's "OK" came about 6.5 seconds after KLM had begun its take-off roll. Two people in the KLM cockpit spoke up, but only with a question or a mild reminder.Source: Spanish report, sections 1.1 and 2.1 (times, words); conclusions as printed in ICAO Circular 153-AN/56 (6.5 seconds); PBS NOVA CVR transcript, taken from the Spanish report ("we're going", Dutch "We gaan"). The 36 seconds is Mark's calculation from 17:06:11 and 17:06:47.
13:38 and 14:15 · Landing at Los RodeosA bomb at Las Palmas closed that airport. KLM arrived first; Pan Am arrived later and parked behind it.
While waiting · KLM loads 55,500 litres of fuelIt took about 30 minutes. Pan Am was ready but could not get past the KLM jet.
Duty-time pressureSince December 1976, Dutch rules no longer let a captain extend the duty limit. KLM said a departure before a certain time looked fine; otherwise the crew would have to stop.
17:02 · Visibility 300 mLow cloud drifted across the field. The runway centreline lights were out of service.
17:05:27 · KLM turns at the end of the runwayPan Am is still taxiing on the runway behind it, in the fog.
17:06:47 · CollisionKLM had already lifted its nose; its tail scraped the runway for about 20 m before it hit Pan Am. The fire burned until 3:30 the next morning.

Source: Spanish report, sections 1.1 (landing times), 1.7 (weather, centreline lights), 1.14.2 (tail scrape, fire), 1.16 and 2.1 (fuel, duty time).

248people on KLM, no survivors (14 crew, 234 passengers)
396people on Pan Am
335Pan Am dead, including 9 who died later of their injuries
61survivors, all on Pan Am

Source: Spanish report, section 1.2; FAA, "Lessons Learned" (583 dead, 61 survivors).

Part 1

What happened

fogthe last minute583 people"Oh, yes"
Part 2

Why

ambiguous wordsblocked radiocockpit hierarchytwo reports
Part 3

Work lessons

say it clearlyread it backdare to interruptCRM
Part 3

Why: five layers of cause

The circumstances: holes that lined up

chain of eventsSpanish report, conclusions
Eight barriers, each with a hole, and one arrow passing through them all: the Las Palmas bomb, diversion to a small airport, a packed apron, fuel and duty time, fog, ambiguous words, blocked radio, cockpit hierarchy. Then the collision. im-pact bomb atLas Palmas diverted tosmall field packedapron fuel,duty time fog300 m ambiguouswords blockedradio cockpithierarchy left to right: from midday to the last moment in the cockpit
No single layer caused the accident. The Spanish report states that the bomb, the refuelling and the reduced-power take-off were not direct factors. The contributing factors it lists are duty time, weather, blocked radio, language, Pan Am not leaving at the third exit, and airport congestion.Source: Spanish report, conclusions, as printed in ICAO Circular 153-AN/56, pp. 22–68. The "barriers with holes" drawing is Mark's illustration.

The words: "at take-off" and "OK"

ambiguous phraseologySpanish report, 2.1 and conclusions
What was said and what was heard. The KLM first officer said "We are now at take-off", meaning they were taking off; the tower understood "at the take-off position, waiting". The tower said "OK", and only two seconds later "stand by for take-off, I will call you". To KLM, "OK" sounded like permission. the wordswhat the speaker meantwhat the listener heard KLM: "We are nowat take-off" "we are takingoff" tower: "in take-offposition, waiting" tower: "OK" … 2 s …"stand by for take-off" "route clearancereceived, wait" KLM: "OK" sounds likepermission; rest squealed
Nobody knows for sure whether the first officer said "we are at take-off" or "we are taking off". The Spanish report calls both this sentence and the tower's "OK" inadequate language, and recommends avoiding the word "take-off" in the route clearance.Source: Spanish report, section 2.1, conclusions and recommendations 3.2–3.3; the 2 seconds run from 17:06:18.19 ("OK") to 17:06:20.08 ("stand by").

The radio: two warnings cancel each other

heterodyne · blocked transmissionSpanish report, 1.1
Five seconds on the radio frequency. The tower transmits from 17:06:18 to 17:06:22: "OK… stand by for take-off, I will call you". Pan Am transmits at the same time: "we're still taxiing down the runway". From 17:06:19.4 to 17:06:22.1 the signals overlap and the KLM cockpit hears only a squeal. :17:18:19:20:21:22 tower: "OK" … "stand by for take-off, I will call you" Pan Am: "still taxiing down the runway" KLM hears only a squeal (19.4 → 22.1 s) towerPan Am
A radio channel lets one person talk at a time. When two stations transmit together, the signals interfere and become a squeal; neither is heard clearly. The Spanish report logs the squeal from 17:06:19.39 to 17:06:22.06. Other summaries say "about 3 seconds" or "almost 4 seconds".Source: Spanish report, sections 1.1 and 2.1; ICAO Circular 153-AN/56 ("about three seconds"); PBS NOVA ("almost four seconds"). Illustrative diagram; the end of Pan Am's sentence is not timed in the report.

The cockpit: the authority gradient

authority gradientSpanish report, 1.5 and 1.16
Three people in the KLM cockpit. The captain: 11,700 flight hours, more than 10 years as an instructor, great prestige. The first officer: 95 hours on the Boeing 747. The flight engineer asks a question: "Is he not clear, that Pan American?" The captain answers firmly: "Oh, yes." Nobody asks again. flight engineer"Is he not clear?"asks, does not assert first officer95 hours on the 747"wait, no clearance" captain11,700 flight hours>10 years instructor "Oh, yes." No one asks again. the authority slope in the KLM cockpit
The person who was right had the least power. The Spanish report says the captain's great prestige may have made the others reluctant to object, and calls the first officer still green on the type. When asked, the captain answered firmly, and the question ended there.Source: Spanish report, sections 1.5.1 (11,700 hours, 1,545 on the 747), 1.16 (instructor for more than 10 years, "gran prestigio", first officer 95 hours on the 747) and 2.1.

The inquiry: two countries, two stories

Spanish report vs Dutch comments1978–1979
Spain (official report, 1978)Netherlands (comments, 1978–1979)
Main causeThe KLM captain took off without clearance and did not abort when Pan Am reported it was still on the runwayA radio-communication misunderstanding was the main cause; the captain took off "prematurely" but should not be blamed alone
The tower's "OK"Not a take-off clearance; inadequate languageKLM and the Dutch civil aviation authority argued the crew had reason to read it as a clearance
Pan AmContributing factor: did not leave at the third exitStressed: had Pan Am turned at C-3, there would have been no collision
Air traffic controlNot a causeRaised the possibility that the tower was distracted by a football broadcast (not confirmed)

Source: Spain column: official report, conclusions. Netherlands column: "Dutch comments on the Spanish report" and the Raad voor de Luchtvaart report (1979). Mark could not read the originals; content via Wikipedia "Tenerife airport disaster" and aviacrash.nl "Rijn" (secondary).

Part 4

Fact check: truths and myths

13 things often said about Tenerife, checked against the original record. Tap a line to see why.

SOLIDKLM began its take-off run without a take-off clearance

Fundamental cause no. 1 in the Spanish report. At 17:05:53 the tower gave only the route clearance; "OK… stand by for take-off" came after KLM had released its brakes.

SOLIDThe KLM flight engineer asked whether Pan Am was clear of the runway

CVR, 17:06:32: "Is he not clear, that Pan American?" The captain answered emphatically that it was (Spanish report, section 2.1).

SOLIDThe KLM captain was under duty-time pressure

Dutch rules changed in December 1976; captains could no longer extend the duty limit. The report calls the uncertainty a significant psychological factor and lists it as a contributing factor.

SOLID583 dead, the deadliest accident in aviation history

248 on KLM and 335 on Pan Am (Spanish report, section 1.2). The FAA calls it the deadliest accident in aviation history.

CAUTION"It was only the KLM captain's fault"

The Spanish report names him as the fundamental cause, but lists six contributing factors: duty time, weather, blocked radio, non-standard language (including the tower's "OK"), Pan Am not leaving at the third exit, and an overloaded airport. The Dutch did not accept blame on him alone. He made the final decision, but he was not the only hole.

CAUTION"Pan Am got lost and ignored its exit"

Pan Am read back "the third", but C-3 required a turn of about 148° and then another, very hard for a 747 (ALPA, via Wikipedia). The crew was still debating which exit to count. The Spanish report still lists it as a contributing factor: they should have checked.

CAUTION"The controllers were listening to football"

A Dutch contention, based on background sound on the tower tape, worded as "may have been distracted". The official report makes no such finding.

CAUTION"CRM was born from Tenerife"

The name "Cockpit Resource Management" was coined at a NASA workshop in 1979, and the first comprehensive CRM programme was United Airlines' in 1981. Helmreich and colleagues' history of CRM (1999) cites a 1978 United crash, not Tenerife. Tenerife is the classic teaching case for CRM, but not its only origin.

CAUTION"After Tenerife, English became mandatory in aviation"

Both sides in this accident spoke English on the radio. ICAO's English proficiency requirements were adopted on 5 March 2003 and applied from 5 March 2008, 26 years later. The ICAO/IFALPA document does not mention Tenerife.

CAUTION"The extra fuel made KLM too heavy to clear Pan Am"

KLM loaded 55,500 litres and did get airborne, scraping its tail. The Spanish report classes the refuelling as an indirect factor. No official calculation was found showing a lighter KLM would have cleared Pan Am. That the fuel made the fire worse is widely reported.

FALSE"The KLM captain was an inexperienced pilot"

He had 11,700 flight hours and more than 10 years as an instructor (Spanish report, sections 1.5.1 and 1.16). That reputation was part of the problem: the report says his great prestige may have made others reluctant to object.

FALSE"The bomb caused the disaster"

The bomb started the chain by pushing flights to a small airport. But the Spanish report states clearly that it was not a direct factor.

FALSE"Nobody in the KLM cockpit spoke up"

The first officer said "wait a minute, we don't have an ATC clearance" at 17:05:41, and the flight engineer asked about Pan Am at 17:06:32. They did speak, but only with a mild reminder and a question, never a demand to stop.

Source: Spanish report (1978), sections 1.2, 1.5, 1.16, 2.1 and conclusions; FAA "Lessons Learned"; Helmreich, Merritt and Wilhelm (1999); IFALPA briefing leaflet 08ATSBL03 on ICAO language requirements; the Dutch position and the C-3 angle via Wikipedia (secondary).

Part 5

Assessment

Scores are Mark's judgement based on the sources in Part 8, not data from a study.

Avoidable
5/5
Warning in time
4/5
Transparent inquiry
3/5
Lesson retained
4/5
Done right afterwards
  • Standard wording: "take-off" used only to clear a take-off
  • Crew teamwork training (CRM) spread across the industry
  • Tenerife got ground radar (per Wikipedia)
Not done right
  • Two countries published two explanations, with no shared picture
  • A long blame fight crowded out the system lessons
  • Runway collisions still happen decades later

A disaster of communication: everything needed to stop was said out loud, but not heard in time.

More: why "Lesson retained" is 4, not 5

The lessons on wording and teamwork were adopted worldwide. But in 2007, 30 years later, the FAA still had to launch a "Call to Action" on runway incursions, and the FAA lists Tenerife as part of the reason.

Source: FAA, "Lessons Learned", PH-BUF/N736PA. Some sources say CRM became mandatory in 2006, but Mark found no primary source for that date and does not use it.

Part 6

Work lesson: say it clearly, read it back, and dare to interrupt the boss

For any decision that travels by voice or chat: shift handovers, code-review approvals, an "OK" in the team chat.

Four habits: reserve the key word for the decision, read back in full instead of saying OK, say a stop sentence instead of a soft question, and the person in charge asks before speaking. 1 · reserve the key word"take-off" only to clear"done" only when done 2 · read it backno bare "OK"repeat what you understood 3 · say "stop""Stop. Pan Am is not clear."not "is he not clear?" 4 · the boss asks first"Anyone see a problem?"then give your view A junior's question is often a warning said quietly.
Each habit patches one failure of 27 March. "Take-off" was used loosely, "OK" replaced a read-back, a question replaced a stop, and the most senior person answered before hearing everything.
One word, one meaningAviation now says "take-off" only to clear or cancel a take-off, and "departure" otherwise. Which words in your team, like "done", "agreed" or "ship", carry two meanings?
Close the loop: say, read back, confirmFor anything important, the receiver repeats it in their own words. The sender listens and corrects at once.
Agree on a "stop phrase" in advanceA phrase anyone may use, including the newest person. When it is said, everyone stops and checks before arguing.
Treat a question as a signalWhen a junior asks "are we sure?", the person in charge should pause and check, not answer straight away with confidence.

Source: the "take-off"/"departure" rule per SKYbrary, "Immediate Takeoff Clearances", citing ICAO Doc 9432 and Doc 4444, chapter 12; recommendations 3.2–3.3 of the Spanish report.

Flight engineer: "Is he not clear, that Pan American?" Captain: "Oh, yes."KLM cockpit voice recorder, 17:06:32–17:06:35, Spanish report, section 2.1.
Part 7

Further reading

Karl E. Weick, "The Vulnerable System: An Analysis of the Tenerife Air Disaster", Journal of Management (1990)The classic analysis of how stress degrades a team's communication.
Helmreich, Merritt and Wilhelm, "The Evolution of Crew Resource Management Training" (1999)A short history of CRM across five generations, hosted as a PDF by the FAA.
FAA, "Lessons Learned from Transport Airplane Accidents": TenerifeA summary page with the timeline, causes and the English translation of the Spanish report.
Anatomy of a Disaster #04: Chernobyl (1986)One accident, two IAEA reports with different conclusions: blame the people or fix the system.
Part 8

Sources

Original documents first. Where only a secondary source exists, it is marked.

  1. Subsecretaría de Aviación Civil (Spain), accident report of 27 March 1977, PH-BUF and N736PA, files A-102/1977 and A-103/1977, 1978. Spanish text: SKYbrary library. English translation: faa.gov
  2. ICAO, Aircraft Accident Digest, Circular 153-AN/56, pp. 22–68 (conclusions of the Spanish report). faa.gov
  3. FAA, "Lessons Learned from Transport Airplane Accidents": Boeing 747-206B and 747-121, Tenerife. faa.gov
  4. Netherlands Department of Civil Aviation, "Dutch comments on the Spanish report" (1978), and Raad voor de Luchtvaart, Final report and comments of the Netherlands Aviation Safety Board (1979). Mark could not read the originals; content via Wikipedia "Tenerife airport disaster" and aviacrash.nl "Rijn" (saved copy).
  5. PBS NOVA, "The Final Eight Minutes", CVR transcript from the Spanish report. pbs.org
  6. Helmreich, Merritt and Wilhelm, "The Evolution of Crew Resource Management Training in Commercial Aviation", International Journal of Aviation Psychology 9(1), 1999. faa.gov
  7. Cooper, White and Lauber, Resource Management on the Flight Deck, NASA CP-2120, 1980 (workshop of 26–28 June 1979). ntrs.nasa.gov
  8. IFALPA, briefing leaflet 08ATSBL03 on ICAO language requirements. ifalpa.org
  9. SKYbrary, "Immediate Takeoff Clearances". skybrary.aero