Aeroperú Flight 603
On Tuesday, the 1st of October 1996, Aeroperú flight 603 departed Miami for a scheduled passenger flight to Santiago, Chile, and landed in Lima, Peru for a planned stop to refuel. The aircraft was a Boeing 757-200 with two flight crew and seven cabin crew. In the cabin were sixty-one passengers, mostly Chilean but also twenty British travellers and a scattering of other nationalities, who all wished to go to Santiago.
The flight crew had reported a bird strike when they landed at Jorge Chavez International airport, Peru’s main international and domestic airport. They had ingested a bird into the right engine. While on the ground, the right engine turbine blades were changed and the front of the fuselage was cleaned.
In order to clean the right side of the fuselage, it was necessary to protect the pitot and the static ports from contamination. The pitot measures the dynamic air pressure and the static ports measure the static air pressure: between them, they provide the altitude and the airspeed to the flight crew.
A line mechanic covered the static ports on both side of the aircraft with masking tape to protect them. He then cleaned and polished the fuselage, as per his instructions. He should have removed the tape when he finished but somehow, he forgot to do so and didn’t check his work. Worse, the quality control staff member who supervised the finishing of the job didn’t notice that the static ports were still covered with tape. The duty supervisor handed the aircraft to the flight crew without checking that the job had been completed correctly.
The final chance to spot the problem was the pre-flight checks, which specifically included a visual check that the static ports were clear. However, when the captain did his walk-around, he clearly didn’t spot the tape, a dull silver colour, covering the static ports high off the ground.
The flight departed Lima just past midnight, local time, with the first officer as the Pilot Flying and the captain in the role of Pilot Monitoring, although they wouldn’t have referred to their responsibilities as such at the time. As the Boeing 757 climbed away from the runway, the first officer asked for the landing gears to be retracted. Instinctively checking their height at this point, he realised that there was something wrong. The altimeters were stuck.
As he called out, he was interrupted by the wind shear alarm, which sounded three times. The captain told the first officer to keep the airspeed up and asked if he was descending, at the same time, the first officer said that he was climbing.
The captain attempted to call the Tower but he was interrupted by the rudder ratio alarm, which serves as a warning to the pilots not to use large or abrupt rudder inputs.
The flight displays told them that their airspeed was too low and that they were not climbing. They had no frame of reference looking out at the dark night. The air traffic control staff in the tower saw the aircraft level out briefly and then climb away, disappearing into the clouds.
The displays and the alarms in the cockpit made no sense and the flight crew could not work out what they needed to do. The captain was primarily concerned about the altitude and that, as far as he could see, they weren’t climbing properly. The first officer was worried about the low indicated airspeed and thus did not want to increase his rate of climb for fear of stalling the aircraft.
As the captain attempted to find out why the rudder ratio alarm was sounding, the first officer checked the source selector, clearly suspecting a blocked static port. The source selector collects the air data from a different static port. However, it made no difference as all of the static ports had been taped up. The back-ups exhibited the same failure as the primary systems.
The Mach trim warning sounded while the rudder ratio alarm was still blaring. The captain called for basic instruments, attempting to limit their information to the most trustworthy data. The first officer called ATC to declare an emergency. At this stage, they were flying through a low layer of cloud at night with no visual references and they were unsure of their height, their speed, the state of their rudder and the state of their stabiliser.
Jorge Chavez International Airport had a radar surveillance system which was being tested, so the controller asked the flight crew to switch frequencies so that ATC could offer them radar advice and support.
The captain took control of the Boeing 757 while the first officer spoke to the controller to coordinate a return to the airport using the Instrument Landing System (ILS). Meanwhile, they both tried to resolve the alarms in the cockpit and get a handle on what, exactly, was wrong.
They flew out to sea so that they did not have to worry about terrain: the foothills of the Andes are just north of Lima. Here was the safest space for them to troubleshoot the problem and safely return to the airport.
The first officer asked the air traffic controller for confirmation of their flight level and the controller responded with 4,000 feet.
This must have been a great relief to the flight crew, as it corresponded to what the altimeter was showing. They focused again on the rudder ratio and their airspeed, which still appeared to be low.
But the controller was using secondary radar, which means that he was picking up the altitude information from the aircraft’s transponder. He’s not confirmed their height. All he has done is repeat the same wrong information that their altimeter was showing them in the cockpit. The partially blocked static ports are not able to measure the air pressure correctly and the height information is not to be trusted. Worse, the crew now believe they have independent verification that their altitude, at least, is correct, so now they can focus on solving the problem.
As the Boeing 757 continued to climb, the first officer asked the controller to confirm their airspeed and altitude. Those are the two parameters needed for safe instrument flight. The controller responded that they were currently climbing through 6,000 feet. The first officer said that their altimeter showed 7,000 feet and the controller confirmed that was correct: Now reading seven zero.
It didn’t occur to anyone that they were reading the same faulty data back to each other.
The first officer told the controller that they had control problems. The aircraft should have been handling fine, so he probably meant the rudder ratio and mach trim alarms. In the cockpit, the flight crew worked through what seemed most likely to be the relevant checklists while climbing to 12,000 feet out to sea. The first officer checked the alternate air source again, clearly suspicious of the air data. But again, as all of the static ports were blocked, it did not help.
Earlier that same year, another Boeing 757 suffered these same inexplicable warnings. The NTSB investigation concluded that the pitot-static system had failed because of a wasp’s nest built in the pitot tube, obstructing it. They recommended that the 757 flight manual include the information that if the rudder ratio and the mach trim alarms go off at the same time, it’s a symptom of an airspeed discrepancy and likely a failure of the air data. However, as it had only happened once, this information was not disseminated as a matter of urgency. The Aeroperú flight crew had no idea.
All they could do was to attempt to solve the problem, if possible, and then return to the airport and intercept the localiser on the ILS, the Instrument Landing System.
The Instrument Landing System is based on airport runway information being transmitted from the ground and received in the cockpit, so the data is not affected by the faulty systems on the aircraft. It consists of a localiser and a glide slope. If they could just get onto the localiser, they could simply follow the glide slope to bring them down.
The flight crew asked again for assistance with altitudes and speed until they could be guided to the localiser. It was clear to them that their instruments couldn’t be trusted.
Air Traffic Control gave them a new heading to turn back and followed this with full instructions, in case they lost radio communications. They needed to turn right to intercept the localiser to complete the ILS with a descent of up to 4,000 feet.
The flight crew followed the vectors from ATC and attempted to set up for a landing in the dark with no auto-pilot, no Flight Director and no auto-throttle. As they entered the descent, the displayed airspeed started to increase rapidly. The overspeed warning sounded, another alert which needs immediate and attention and action. If the aircraft were overspeeding, the engines could flame out or even be destroyed, taking Aeroperú 603s situation from bad to catastrophic.
The flight crew reduced the engine throttle immediately. The rudder ratio alarm sounded again. Both pilots agreed that the instruments and at least some of the warnings had to be wrong but what was their actual status? They had no idea.
ATC confirmed that they were 31 miles west of the airport at just over 10,000 feet with a speed of 270 knots over the ground. This was the first time that ATC had given them their groundspeed. The cockpit display showed an airspeed of 350 knots.
The airspeed was wrong. So was their altitude. But as they continued their descent, the overspeed alarm sounded again and continued to sound, even after they reduced the thrust and activated the air brakes.
The stick shaker activated again, vibrating the control columns. This indicates a high angle of attack, which means that the aircraft is in danger of stalling. According to the multiple alarms going off in the cockpit, the Boeing 757 was both flying precariously slowly and hazardously quickly. Both situations are urgent an both require immediate action but which one? The wrong decision would make things worse.
The first officer asked ATC for another aircraft to join them. Someone in the air could confirm their altitude and determine their true airspeed, which was either dangerously high or dangerously low. Another aircraft might also have spotted that the Boeing 757 was in a gentle descent. The flight crew had no idea: the altimeter was not changing and the controller was mirroring the false information.
The stick shakers shook the columns and the alarms continued to sound. It must have been a nightmare trying to think straight. The first officer believed that they were stalling. The captain believed the overspeed warning. Both of them believed that they were at 9,500 feet.
Then the Ground Proximity Warning System kicked off with TOO LOW, TERRAIN, calling into question the one fact that they thought they knew. It sounded twenty-two times over 45 seconds. The captain panicked, thinking they must have flown over the coast and are heading into the mountains, the only sensible reason why their height over the ground could have changed so quickly. The first officer reported the warning to ATC, who confirmed that no, the Boeing 757 is definitely out to sea, flying northwest at FL100, 10,000 feet above sea level.
The Ground Proximity Warning System uses the radio altimeter, not the barometric altimeter. Here, finally, is information that they could trust. The barometric altimeter uses the faulty air data but the radio altimeter is independent, bouncing radio waves up to reflect off the ground. It is only useful at range but it gives the true distance between the aircraft antenna and the ground directly below it. The Ground Proximity Warning System was configured to sound if the radio altimeter showed that the ground was less than 2,500 feet away while the aircraft was not configured for landing.
This means that the flight crew have two pieces of data t hat they can rely on: their true height above the ground from the radio altimeter and their ground speed from the radar readings. At low level, knowing the wind direction from their take off only a few minutes before, they could calculate their approximate airspeed, at least close enough to know whether they were overspeeding or at risk of stalling.
However, they had been so bombarded with information that the cockpit resource management has fallen apart. They no longer are working as a team and they became unable to agree on any clear decision as to what their situation was or what they needed to do.
The aircraft entered a gentle climb, too subtle to notice in the dark over the water. As they ascended over 2,500 feet, t he terrain waning stopped. As they still believed that they were flying straight and level, this must have seemed like confirmation t hat the alarm was wrong. “It must be fictitious,” said the captain, dismissing the Ground Proximity Warning System as one of the clearly false alerts in the cockpit.
The aircraft then descended 700 feet and a SINK RATE alert sounded. The truth was that they were stalling and dangerously close to the dark sea. The aircraft’s height above the sea shifted repeatedly over this time: they climbed to 2,400 feet and then descended to 1,300 feet and then climbed to 4,000 feet and levelled out. Throughout this, the altimeter showed only minor changes and that they were consistently above 9,000 feet. The controller confirmed it. He had asked a Boeing 707 to depart the airport and help guide the 757.
The flight crew turned and entered their descent to 4,000 feet, ready to intercept the localiser for the Instrument Landing System. They reduced their their thrust to idle and yet the overspeed warning continued to sound. The Ground Proximity Warning System sounded again and kept sounding: TOO LOW, TERRAIN. TOO LOW, TERRAIN. The first officer checked their altitude and queried ATC again. Both the instruments and the radar showed that they were at 9,700 feet. They continued their descent.
The aircraft touched the water as the the first officer called the controller, who for the first time noticed the terrain warnings in the background. The captain shouted PULL UP, PULL UP as the controller called out the same, GO UP! GO UP IF IT INDICATES PULL UP! He knew that the Ground Proximity Warning System should be trusted over the radar information that he was passing on.
The captain pulled up and the Terrain warnings briefly stopped. But the Boeing 757’s speed was already precariously low and now it decayed to the point of a stall. The Captain realised he’d lost control of the aircraft; as the aircraft banked left, his last words were, “We are going to invert.” There was no chance of recovery.
As the Boeing 757 crashed into the sea, the captain’s airspeed showed on the display as 450 knots and the altimeter still read 9,700 feet above sea level.
Only at the very end would the passengers have been aware, as the previous climbs and descent were too gentle to be felt. They, like the flight crew, could see nothing out of the windows other than the black night and the lights of the wings reflecting off the low cloud.
The left wing had been badly damaged when it clipped the water and although the captain managed to get the aircraft airborne again, the aircraft inverted and impacted the water. There were no survivors. Only nine bodies were recovered.
Divers confirmed that all three static ports on the left side were covered with masking tape. They could not reach the right side but it seems reasonable to assume that the tape placed there had not been removed.
This was the third Boeing 757 crash in ten months. In the aftermath of the crash, Aeroperú filed for bankruptcy. Boeing paid compensation to the families of the victims, taking responsibility for failing to train pilots to deal with this circumstance but not for the disaster itself. The cause of the cash, the static ports being covered, was, they said, caused by careless maintenance and a negligent pre-flight inspection by the captain.
The NTSB recommended that conspicuous covers be used to cover static ports during aircraft cleaning operations in order to ensure that they were visible. These covers now also have warning flags in order to make it even more obvious if they are covered.
The line mechanic was convicted of negligent homicide.