Iberia Flight 610

The aircraft involved was a Boeing 727–256, powered by three Pratt & Whitney JT8D-9A turbofan engines capable of carrying 189 passengers and registered as EC-DDU. Manufactured in 1979, the aircraft was delivered to Iberia later in the same year and was named Alhambra de Granada, referencing the Alhambra in Andalusia. It had flown 13,400 hours at the time of the accident.[2]

The aircraft was carrying 141 passengers and 7 crew members. While majority of the passengers were Spaniards, authorities also confirmed that several foreigners were on board the aircraft. Among those confirmed were three Bolivians and three Americans. Two Britons, one Swiss and one Dutch were stated to be on board, however these reports could not be confirmed.[3]

Notable passengers include then-incumbent Bolivian Labor Minister Gonzalo Guzman Eguez, who was travelling to Spain to hold talks with authorities on the construction of La Paz - El Alto railway, and former Spanish Foreign Minister under Franco's regime, Gregorio López-Bravo.[3]

The commander of the flight was identified as 51-year-old Captain José Luis Patiño Arróspide. Spanish newspaper El Pais described Patiño as an aristocratic pilot after it was discovered that he was the cousin of the Marquises of Villanueva de las Torres, the Counts of Sástago and the Barons of Bétera. A senior pilot in the airline, he had been flying with Iberia for 19 years. He had accumulated a total flying experience of 13,600 hours, having previously flown with Spantax before joining Iberia.[4] His co-pilot was 38-year-old First Officer Emilio López Peña Ordóñez. He had accumulated a total flying experience of 5,548 hours, of which more than 2,800 were on the type. There was also a flight engineer on board, identified as 38-year-old Gregorio Arroba Martín Delgado.[5]

In 1984, following a disastrous financial year, Iberia decided to withhold the salaries of its pilots. Dozens of Iberia pilots went on strike, including Captain Patiño. The strike was regarded by the Spanish government as one of the most serious strikes in Iberia's history, as the company had lost a total of 2.4 billion pesetas as of July 19. After threatening Iberia that they would not fly flights that were crucial to Iberia's financial operation, Iberia issued multiple sanctions and fired many of the striking pilots, including Captain Patiño's contract, which was terminated on July 18. The government was forced to intervene, and Spanish ministers eventually ordered arbitration between Iberia and the Spanish pilots' union (SEPLA), which was agreed upon by both parties. Spanish judiciary eventually ruled in favor of the pilots and let them return to Iberia. Patiño eventually joined back in November. As Patiño had just been fired from Iberia, he had to be trained again to obtain his flying license.[6]

At 4,300 feet (1,300 m), the crew switched on the altitude alert system whilst flying through overcast and rainy weather. At 09:09, the crew was given permission to descend to 4,300 feet (1,300 m). Contact was then handed over to Bilbao, and at 09:16 First Officer López established contact with the tower of Bilbao Airport. Bilbao ATC then gave permission to the crew to continue their descent.[2]

The crew repeated the transmitted information, which was immediately followed by the controller's offer for a direct approach to the airport. Choosing the direct approach would have made the aircraft arrive faster, though the aircraft's descent profile would be significantly different from the standard approach. Usually, pilots prefer the direct approach than the standard approach to save time. However, on that particular day, Captain Patiño gestured to First Officer Lopez to decline the offer, apparently as a sign of protest to Iberia's management. By choosing the standard approach, the travel time would be longer, and eventually, this would deplete the fuel further. First Officer López then stated that they would fly the standard approach plan to Bilbao rather than the usual "shortcut" approach. Bilbao ATC accepted the transmission and asked the crew to report back when they had reached the VOR of the airport.[2]

The aircraft continued to descend, and the crew began to run through the descent checklist. The aircraft was configured accordingly, and the autopilot's altitude select system was set. The system then sounded its first tonal alarm, indicating that the aircraft was nearing its selected altitude. After the second tonal alarm, the autopilot then levelled the aircraft at the targeted altitude and the aircraft passed the VOR at 09:22. The crew reported on the VOR passage of Bilbao at an altitude of 7,000 feet (2,100 m), reaching the starting point of the landing approach. The crew received permission to descend to an altitude of 5,000 feet (1,500 m).[2]

This was the last radio transmission from Flight 610 to Bilbao ATC. Afterwards, the aircraft began to initiate a tight turn to the right heading towards the outbound leg and the pilots lowered the target altitude from 7,000 ft to 5,000 ft into the autopilot.[2]

EITB antennas in 2014

Approximately 900 feet from the target altitude of 5,000 ft, the first tonal alarm sounded. The aircraft reached 5,000 feet (1,500 m) at 09:25 and the second tonal alarm informed the crew that the aircraft had reached the intended altitude. The target altitude was lowered again, this time to 4,300 ft. The crew then commanded the autopilot to increase the rate of descent to 1,500 ft in order to quickly reach the intended altitude. The altitude alert went off and the crew continued the descent as the second tonal alarm had not sounded. The crew began to initiate the loop back towards the airport by making a tight turn to the left. During the turn, they would bypass Mount Oiz.[2]

The rate of descent was eventually decreased to 750 feet per minute. The pilots then fed the autopilot with the intended input and began to configure the flaps. The aircraft continued to descend. The weather conditions at that time were cloudy with fog, with 4 kilometres (2.5 mi) of visibility.[2]

At 08:27, after 57 seconds from the moment of descent to a safe altitude, flying 177 feet (54 m) above Mount Oiz (elevation 3,356 feet (1,023 m)) in a landing configuration, and at a speed of 239 miles per hour (208 kn; 385 km/h), the aircraft's left wing collided with an antenna of the Euskal Telebistarues (EITB) television tower at 3,356 feet (1,023 m). The tower struck right between the airframe and the wing, completely shearing the wing off from the airframe. Losing all possible lift from the left side, the aircraft immediately rolled to the left and dived towards the woods. It hit dozens of pine trees in the process, leaving swathes of cut trees for 930 meters before it crashed into a ravine in inverted condition. The wreckage slid for another 230 meters, ending in a tremendous explosion and instantly killing all 148 passengers and crew on board.[8][9]

The analysis of the flight recorders indicated that there was a discrepancy between the pilots' expected altitude and the actual altitude that they had been flying during the approach. The pilots apparently believed that they were at the correct altitude, even though the actual altitude showed that they were lower by about 300 meters. The error might have started before they conducted the type of approach to Bilbao.[2]

Since the airport was surrounded by mountains, the standard approach would have required the crew to take a detour before they could reach their destination. The crew would have needed to maintain their altitude at a minimum of 7,000 ft until they reached the airport's VOR, located on the west side of the aerodrome. Upon reaching it, they would then turn again, facing towards the approach fix point, which was located opposite of the airport's VOR, and descend to 5,000 ft before continuing their journey to reach said point. By then, they would need to loop back to line the aircraft up with the runway before they could begin the landing phase.[2]

Taking the total distance into account, the standard approach was long and complicated. The crew had to fly past the airport for approximately 13 nautical miles before directing their aircraft back towards the airport. To cut time, air traffic controllers sometimes offered a shortcut for pilots who were flying to Bilbao, in which they would be instructed to fly directly to the airport's approach fix. This action was not unusual and was considered acceptable as long as there was only light traffic in the area. If the pilots accepted the offer, then they would need to maintain their altitude at 7,000 ft before joining the airport's minimum safe altitude of 4,354 feet and looping back towards the airport.[2]

The recording of the cockpit voice recorder suggested that First Officer López had intended to accept the offer. However, he changed his mind after Captain Patiño, according to the investigation, made some kind of sign or gesture to refuse it and take the former option instead. The crew eventually elected to follow the standard approach. The investigation team noted that the types of approaches that they had taken differed greatly from each other. By taking the standard approach, they were allowed to descend to 5,000 ft while flying towards the approach fix. If they had taken the shortcut, they would have needed to maintain 7,000 ft before they reached the approach fix point. Since the aircraft had been maintaining 7,000 ft, by the time they reached the approach fix, the crew would have to make a rapid rate of descent to reach the airport's minimum safe altitude in a timely manner.[2]

However, even though both pilots had opted to follow the standard approach instead of the shortcut, the flight recorder showed that First Officer López, who was at the controls, somehow made an input for a shortcut approach by increasing the aircraft's rate of descent by a great deal. First Officer López suddenly entered a rate of descent input of 1,500 ft while the aircraft was descending through 5,600 ft. As the travel distance for the standard approach was much longer than the shortcut approach, the crew had enough time to slowly adjust their altitude, and the aircraft didn't need such a high rate of descent. First Officer López might have forgotten the type of approach that they had taken. As a result of his input, the aircraft quickly descended to the intended altitude.[2]

Despite the high rate of descent, the aircraft was equipped with an altitude select system. The system was equipped with an alarm, which it would activate to inform the crew that they would reach the intended altitude in less than 900 feet. The altitude that had been selected into the autopilot system was still within the safe limit for an approach to Bilbao, and if the altitude select system had worked properly, then the crew would have heard the tone and the aircraft would have stopped descending by the time it had reached its intended altitude.[2]

The altitude select system was integrated into the autopilot system of the aircraft. To use the feature, pilots must switch the system to altitude capture mode. That way, the autopilot would try to "capture" the intended altitude. After feeding the autopilot with the input, the autopilot would cause the aircraft to fly to the intended altitude. Before activating it, the pilots should decide on the altitude they would like to fly at by turning the altitude knob. Afterwards, the pilots should arm the autopilot by pressing the "ALT SEL" button. The autopilot would be activated, and the aircraft would fly accordingly. The system was also equipped with a horn that would sound whenever the aircraft approached the intended altitude. The two-second aural tone would sound when the aircraft was about 900 feet from its selected altitude. The alarm would also sound again at another time when the aircraft deviated from its target altitude by 300 feet. Other than the alarm, the sound would also be accompanied by the activation of the altitude lights. The altitude alert light would illuminate on the aircraft's control panel, which was located directly in front of the crew. The lights would stay on until the aircraft reached 300 feet from its intended altitude, where they would turn off.[2]

In Flight 610, both the altitude select alarm and lights had worked properly according to their designs. However, due to the way the system works, their activation unfortunately caused both pilots to misinterpret the situation. The incident happened while the aircraft was descending through 5,000 ft during which one of the pilots selected an altitude of 4,300 ft into the autopilot. At the time they made the input, the aircraft was flying approximately 700 ft above its intended altitude. Since the aircraft's altitude was already below 900 ft, the aural tone didn't turn on. Meanwhile, the altitude light immediately turned on. The crew didn't notice this since the alarm didn't sound. By the time the alarm blared, the crew thought that they were 900 ft above the selected altitude. The aircraft, in fact, was flying 300 ft below its intended altitude. As First Officer López, who was in control, thought that there were still 900 feet left to reach their target altitude, the aircraft continued to descend further.[2]

Even with such a mistake, the aircraft would have still leveled off at the intended altitude since the altitude capture mode had been engaged, and as such, the aircraft wouldn't have continued its descent. In Flight 610, instead of leveling off, the aircraft descended even further. The investigation eventually revealed that the crew had never used the altitude capture mode during the descent to 4,300 feet. Several tests were conducted, and it was eventually found that the system could be accidentally disengaged in several ways. Among those were manually changing the altitude while the aircraft was flying through the previously selected altitude, selecting an altitude that was too close to the present altitude, and disengaging and then re-engaging the autopilot. Investigators, however, concluded that the cause of the unengaged altitude select system was simply caused by either the pilots forgetting to press the "ALT SEL" button to engage the system or one of them accidentally disarming it by pushing the button for a second time. In any case, the unengaged autopilot allowed the aircraft to continue its descent past its target altitude and caused it to fly lower than the minimum safe altitude, enabling the collision to happen.[2]

Even with all the errors, the aircraft wouldn't have crashed had the pilots paid attention to the altimeter. However, investigators thought that the crew might have misread the reading, hence causing a misinterpretation. This might be attributed to the design of the altimeter that had been used in the Boeing 727.[2]

The involved aircraft was equipped with the drum and needle altimeter, a type of altimeter that has two different displays for the altitude. The circular dial would display the hundreds of feet, while the rotating drum would show the thousands. The type was extensively used in the Boeing 727 during the 20th century. However, several studies showed that the use of such an altimeter increased the risk of misinterpretation since it was hard for pilots to read the small window for the different altitudes in thousands and hundreds. NASA survey conducted by NASA revealed that more than 80% of the pilots that were sampled reportedly misread the altimeter, and around the same percentage repeated the same mistake more than once. This resulted in multiple accidents and led the FAA to discontinue the use of such an altimeter on U.S carriers. The policy was not implemented in Spain and therefore the design was still used in Spanish airliners.[2]

A schematic of a "drum-needle" altimeter

Due to the use of the drum and needle altimeter, pilots would normally glance a bit at the altimeter, overlook the displayed information. This was further corroborated by findings obtained from simulation flights and interviews that had been conducted by investigators, in which pilots had only glanced a little and eventually became uncertain of their aircraft's actual altitude. Many pilots eventually had to assume the altitude they were flying at. In Flight 610, investigators thought that the pilot who was at the controls at the time, First Officer López, had misread the altimeter and assumed that the aircraft was approximately 1,000 feet higher than the airport's minimum safe altitude, allowing the aircraft to descend even further.[2]

Despite the misreading, they should have been aware of it since there was a monitoring pilot inside the cockpit, who was responsible in monitoring the instruments. Captain Patiño, the pilot monitoring who had been with First Officer López throughout the flight, should have monitored the altimeter and conducted altitude callouts every 1,000 ft. The cockpit voice recording indicated that he was almost wholly absent from the flight and instead stayed mostly silent during the approach. He was also supposed to handle the communication between Flight 610 and the ATC in Bilbao, but during the approach the communication was conducted by First Officer López, who at the time was also at the controls.[2]

Other than the callout, the investigation also found several other deficiencies regarding the conduct of Captain Patiño that might have contributed to the cause of the crash. Despite being pilot monitoring, Captain Patiño was believed to have influenced First Officer López's decision during the flight. When the crew was offered the shortcut by the ATC, Captain Patiño seemed to give a negative response to First Officer López, which caught him off guard and caused him to momentarily forget that they were making a standard approach instead of the usual shortcut, causing the high rate of descent. While the aircraft was descending through 5,000 ft, one of the pilots made an input into the altitude select system and chose 4,300 ft, which was less than 900 ft from their present altitude and caused the altitude alarm not to sound. It was believed that Captain Patiño was the one who was responsible in changing the altitude. While making such input, he didn't inform First Officer López and eventually, this caused First Officer López to suffer memory lapse during the approach.[2]

Due to the lack of altitude callouts and monitoring on the instruments, the aircraft was allowed to descend even further. Captain Patiño's silent conduct during the entirety of the flight confounded First Officer López's performance, who had been controlling and handling the communication between Flight 610 and the airport. The aircraft descended below the intended altitude for 57 seconds and struck the antenna, severing the left wing and causing the aircraft to plunge into the ravine.[2]

Investigators were able to identify several other deficiencies in the crash, including the construction of a TV tower on Mount Oiz and the incomplete approach chart. With its height of approximately 1,026 meters (3,366 ft), Mount Oiz was not included in the approach chart that had been used by the crew. The approach chart also didn't include the TV tower. The tower was 28 meters high and exceeded the minimum safety altitude of the airport. Since it was not included, the construction of the tower posed a significant risk to the safety of airliners in the area.[2]

According to the investigation, the crew distrusted the autopilot during the descent phase, but it is likely that they forgot to turn on the ALT SEL (altitude select) button, accidentally disengaged it, or the system itself malfunctioned. As a result, after reaching their assigned altitude, the autopilot did not support it, which led to a further decline, and the pilots did not control it. When the altitude alert sounded, the crew most probably misinterpreted the alarm. Investigators also discovered that the TV tower installed on the mountain was not marked on the navigation maps available to the crew.[8]

Investigators determined the cause of the crash was pilot error due to the flight crew misinterpreting data and flying the aircraft below the safety altitude. The accident report stated:

"Their [i.e. the pilots'] confidence on the automatic capture performed by the Altitude Alert System, the misinterpretation of its warnings, as well as a probable misreading of the altimeter made the crew fly below the safety altitude, colliding into the television antennas' base, thus losing the left wing, falling to the ground with no possible control of the aircraft."

— Technical Report A-009/1985 – Accident that occurred on February 19, 1985 involving a Boeing 727-256 aircraft, registration EC-DDU, at Monte Oiz (Vizcaya) / CIAIAC[2]