The International Civil Aviation Organization (ICAO),is a specialized agency of the United Nations to promote the safe and orderly development of international civil aviation throughout the world. Its sets standards and regulations necessary for aviation safety,security,efficiency and regularity.The organization serves as the forum for cooperation in all fields of civil aviation among its 191 Member States. ICAO has issued documents and recommended practices,annexe and amendments on various aviation subjects. The process of communication phraseology is perhaps the most important document (Annexe 10  Vol.II and ICAO Doc 9432)because it enables Pilot and Air Traffic Controller to communicate quickly and effectively despite difference in language and reduces the opportunity for misunderstanding. Ambiguous or no-standard phraseology is a frequent and casual or contributory factor in aircraft accident or incident.

On Sunday,27th March,1977, the Tenerife Airport Disaster occurred when two Boeing 747 passenger aircraft collided on the runway of Los Rodeos Airport of Spanish Island of Tenerife.  The crash is the deadliest accident in aviation history with  total 583 fatalities and only 61 survived injuries. Aircraft involved were KLM 4805 and PANAM 1736.Both were diverted to Tenerife due to closure of Gran Canaria airport and had taxied out for take off from RWY 30 after back tracking RWY 30. KLM was at take off point and PANAM was was still back tracking on runway. Visibility was very poor for both aircraft as well as for Air Traffic Controller. At time 1705:36.7 KLM was ready for departure and at 1706.50 crashed with PANAM on  take off run – in just 1 min & 14 seconds tragedy occurred !!

Investigators from Spain, USA and two airlines were involved. Fundamental cause was attributed to Captain of KLM who took off without “take off clearance”. Other major factors were, limited visibility and simultaneous radio transmissions.  Contributory cause was “USE OF AMBIGUOUS AND NON-STANDARD PHRASEOLOGY BY KLM CO-PILOT AND TENERIFE CONTROL TOWER.  KLM co-pilot had said “We are at take off”. Ambiguousn phraseology. It should have been “Ready for departure”. In response Tower had said “OKAY” which is non-standard.He should have said “WAIT,PANAM back tracking on runway.” Correct usage of standard phraseology is must for air traffic controllers so that air traffic clearances are clear,concise and complete in all respect in the interest of accident/incident free aircraft operations as well as  for prestige of high profile profession “Air Traffic Controller”.

Detailed information on this incident along with extract of cockpit voice recorder and tapes from Tenerife Tower is available on http://en.wikipedia.org/wiki/Tenerife_airport_disaster.  In fact this should be material for case study at all flying clubs/academy,airline training centers, all airports and Civil Aviation Training College, Allahabad, if not already included.