Description

On 2 October 1996, a Boeing 757-200 being operated by Aero Peru on a scheduled passenger flight at night from Lima to Santiago experienced control difficulties because of incorrect flight instrument readings. A return to Lima was attempted but when control was lost the aircraft impacted the sea and all 70 occupants were killed.

Investigation

An Investigation was carried out by an Accident Investigation Board made up of personnel from the Peruvian General Directorate of Air Transport (DGTA). Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) data was recovered to assist the Investigation. After establishing that the apparently malfunctioning altimeters and airspeed indicators (ASIs) had been serviceable, it was quickly concluded that the problem with these instruments had been the result of a failure at any time before the night departure to see that all the static ports were still taped-over after release for the flight from maintenance.

The CVR confirmed that the First Officer had acted as PF for the departure. It was noted that the flight crew were experienced in proportion to age and that neither were new to the aircraft type. Post accident testing confirmed that the radio altimeters and relevant aircraft alerting systems including the Stall Protection and GPWS/TAWS had been in normal working order.

It was established that immediately after take off from runway 15, the flight crew had identified a lack of credible altimeter indications. Three minutes after becoming airborne, an emergency was declared to TWR ATC and radar vectors were requested. ATC APP began providing vectors for an Instrument Landing System (ILS) to runway 15 and were immediately asked to also provide the speed of the aircraft and told that the crew were “having problems with the controls”. Two minutes later ATC were advised that the aircraft was at FL120 and flying out to sea on heading 205° and ATC suggested a heading of 350° would be appropriate in order to route towards the ILS LLZ. As vectoring continued, the crew repeated their need for assistance with altitudes and speeds due to “trouble reading their instruments” and advised ATC that “they had cut the engines and were still accelerating”. Soon afterwards, they advised that their speed remained high and asked to be “rescued”, requesting the support of another aircraft, which was arranged.

When asked two minutes later about the malfunctions being experienced, the First Officer stated that none of the altimeters, airspeed indicators or vertical speed indicators were functioning properly and that, after an over speed warning, although thrust on both engines had been cut, the aircraft was still not slowing down. A GPWS alert was then advised and ATC advised that on radar, the aircraft was showing FL105 and making a turn to the west 40nm from the airport (i.e. over the sea). Shortly afterwards, the crew advised ATC that their indicated speed was 370 knots to which the controller responded that ground speed was 220 knots and that the aircraft was on radar at 50nm from the airport heading 270 at an indicated FL 100, which corresponded to the reading on the aircraft altimeter. ‘Too Low terrain’ GPWS Warnings began to be annunciated. Two minutes later, after a total time airborne of 29 minutes, FDR data showed that the aircraft had hit the sea surface at a speed of 250 knots in a near level attitude and bounced into a climb of about 300 feet before a second terminal impact 18 seconds later.

The TWR controller reported noting visually that after becoming airborne, the aircraft had levelled out over the long runway abruptly at a height of approximately 300 feet, before continuing the climb into the 800 feet base of a 2200 feet thick stratus layer present in the vicinity of the airport. The height of 300 feet was noted to correspond to the time when the crew had first advised of altimeter indication faults and when FDR data also showed that failure of both airspeed and altimeter indications had begun to trigger false system warnings.

A review of the CVR data showed that overall both pilots had been similarly confused by the corrupt instrument readings but that the First Officer had voiced more doubts about the wisdom of descent against persistent GPWS warnings. In the presence of almost continuous audible alarms, including the (valid) stick shaker due to the low thrust set because of over speed warnings, the Investigation concluded that the aircraft commander “falters in his commands” and noted that the GPWS Warnings were interpreted by him as false because they contradicted the (erroneous) altimeter indication on the basis that the aircraft reading was several times corroborated by the radar controller.

A significant problem during the whole flight was the persistence of rudder ratio and mach speed trim warnings, which were a distraction and adding to the problem of multiple other alarms and warnings which it was concluded created “chaos which they do not manage to control, neglecting the flight and not paying attention to those alarms which (were) genuine”.

As to the origin and purpose of the tape over the static ports, it was noted that prior to the flight, the aircraft had been parked in the maintenance area where two fan blades of one of the engines had been replaced because of bird strike damage. The hydraulic pump of the right hand engine had also been changed. It was concluded that the maintenance staff did not remove the protective adhesive tape from the static ports after completing their work. This tape was then not detected during the various phases of the release of the aircraft to the line mechanic, in the course of its transfer to the passenger boarding apron or during the pre flight external inspection carried out by the accident aircraft commander.

It was determined that the Principal Probable Cause was:

• Error of the maintenance staff and the flight crew in not detecting the obstruction of the static ports

Contributing Causes were determined to have been:

• Failure of the aircraft commander to ensure an appropriate response to GPWS Warnings or observe and react to the readings on the correctly functioning radio altimeters

• Failure of the First Officer to be “more insistent, assertive and convincing in alerting the pilot-in-command much more emphatically to the ground proximity alarms”.

It was also noted in respect of rescue coordination that in respect of liaison between ATC and the Coast Guard after the accident outcome, there had been a complete lack of coordination in the emergency procedures. It had taken six hours to identify a coast guard search aircraft by which time a Navy aircraft had already found the remains of the aircraft.

A total of 34 Safety Recommendations were made as a result of the Investigation:

• That Operator Aero Peru familiarise crews with specific emergencies involving erroneous speed indications and design a procedure for flying with erroneous or no altitude indications.

• That Operator Aero Peru conduct practical sessions in a specific simulator, and also refresher courses including simulated flight with the erroneous speed indication failures, using the specific tables for the procedure in the various flight phases, and follow United Airlines' advanced manoeuvre programme.

• That Operator Aero Peru design eye-catching covers for protecting the static ports when maintenance and polishing work is done on an aircraft.

• That Operator Aero Peru make the crew aware that it is mandatory to follow the evasive procedures in response to GPWS terrain alarms and conduct practical sessions in flight simulators.

• That Operator Aero Peru must (ensure) better use and observation of, and reliance on, the radio altimeter.

• That Operator Aero Peru establish special regulations and procedures for flights experiencing problems with the indications of instruments receiving information from the ADCs, and for the interpretation and appropriate use of alternative means.

• That Operator Aero Peru comply strictly with the procedure designed by the Flight Safety Directorate for the documentation of an aircraft's release from when it leaves maintenance to its acceptance by the crew assigned to the flight, in order to inculcate safer and more efficient operational discipline.

• That Aero Peru Maintenance Service implement a better quality control system.

• That Aero Peru Maintenance Service carry out better documented pre-flight checks (at present the static ports are not specifically mentioned).

• That Aero Peru Maintenance Service select higher quality technical staff, with continuing training and the creation of incentives for staff to perform more effectively in the interests of operational safety.

• That Aero Peru Maintenance Service monitor the manufacturer's standards and recommendations, and comply strictly with the future recommendations issued as a consequence of this accident.

• That Aero Peru Maintenance Service implement regulations for flights after maintenance in relation to polishing, painting or other similar work.

• That ANSP Corpac raise the level of operational technical knowledge in practical terms for controllers, putting them in situations in which they play the role of the pilot, so that they can analyse the type of information which could cause confusion, since the pilot relies on the controller's correct information.

• That ANSP Corpac assess controllers with a view to effective selection, rejecting staff with insufficient English and aeronautical culture.

• That ANSP Corpac retrain the rejected staff for a second practical operational assessment.

• That ANSP Corpac carry out coordination exercises (simulations) with SEI staff. Work with the SEI staff and evaluate the minimum reaction time needed to resolve the emergency. Make resolution and implementation plans for all types of emergency.

• That ANSP Corpac give controller resource management (CRM) courses so that controllers have better situation awareness and decision-making abilities in emergency cases, training controllers in human factors and problem-solving.

• That ANSP Corpac recommend team integration work, and advice and support for the recognition of emergency situations in radio communications, with the use of English and the correct application of phraseology as soon as the emergency is declared.

• That ANSP Corpac implement the international airport's terminal and area radar system as a matter of urgency and priority for practical, safe and expeditious air traffic operations.

• That ANSP Corpac implement an automatic terminal information service (ATIS) in accordance with the importance of the workstations.

• That the DGTA create a communication system through the publication of technical bulletins, safety circulars, flyers and specific documents on operational matters, in conjunction with the airlines' representatives on topics related to safe operations and accident prevention programmes.

• That the DGTA plan seminars, workshops and conferences relating to operational safety, human factors, accident prevention and specific matters such as operational integration through CRM or similar programmes.

• That the DGTA be more demanding and drastic in the penalties against operators with a view to their complying with the established operations and airworthiness procedures.

• That the FAA immediately release and communicate technical information issued by the National Transportation Safety Board (USA) (NTSB), such as FSIBs (Flight Standards Information Bulletins). Such information is not always accepted by the FAA but is very important for the operator as essential information obtained from accident investigations. It must be circulated given the importance of the safety elements it involves, which are useful to the operator and manufacturer, without prejudice to the FAA's opinion.

• That the DGTA work with manufacturers on the improvement of procedures relating to the interfaces between crews and automated cockpits and the reliability limit to produce a guide on how to investigate the factors contributing to an error involving contradictory alarms.

• That the DGTA determine the contributing factors regardless of their origin, whether they result from errors associated with the pilot, mechanic, dispatcher, air traffic controller or any other participant in the operational system.

• That the DGTA, in order to obtain positive results, the must communicate the conclusions (of the Investigation) immediately to all staff who were a contributing factor, so that appropriate and effective measures can be taken in accordance with the recommendations.

• That Boeing give more importance to flight training based on attitude and power in the various operational phases, which are not adequately covered in the training programmes or in the manufacturer's specific manuals.

• That Boeing implement systems which avoid conflicting or contradictory alarms, such as overspeed and stick shaker being activated at the same time.

• That Boeing introduce a "caution" alert when the speed and altitude are not reliable on the EICAS screen.

• That Boeing design a procedure with all the steps and actions to be followed in the event of a total failure of the dynamic and static instruments (to be included in the QRH).

• That Boeing advise airlines for the establishment of specific guides to the problem of static port blockages.

• That the Directorate of Search and Rescue Coordination Operations reorganise the search and rescue system in accordance with the norms and methods recommended in ICAO Annex 12, "Search and Rescue", incorporating it into the COSPAS-SARSAT system.

• That the Directorate of Search and Rescue Coordination Operations develop a specific manual for our situation, tailored to the Peruvian geosystem and topography, using as a basis ICAO Doc. 7333-AN/859, "Search and Rescue Manual", and ICAO Circular 185, "Satellite-aided Search and Rescue".

The Final Report of the Investigation was issued in December 1996.

Editors Note: The Final Report is an unofficial translation made from the published original report in the interests of safety improvement with no objection from the Peruvian authorities.

Further Reading

• Flight Crew Pre Flight External Check
• Pitot Static System
• Maintenance Error
• Altimetry System Error
• Unreliable Airspeed Indications
• CFIT Precursors and Defences

The following events listed on SKYbrary involved unreliable airspeed indications:

• B712, en-route, Union Star MO USA, 2005
• B752, en-route, Northern Ghana, 2009
• A332, en-route, Atlantic Ocean, 2009
• B763, en-route North Bay Canada, 2009