Summary Faulty radio altimeter led to engine power reduction, speed loss, & stall. Crew error exacerbated the situation by not reacting to faulty data promptly. Systemic issues included subpar pilot training and Boeing design flaws & oversight.
On February 25, 2009, Turkish Airlines Flight 1951 (TC-JGE), a Boeing 737-800 model 8F2 named Tekirdağ, crashed while attempting to land at Amsterdam’s Schiphol Airport (AMS). The incident, now known as the 'Polderbaan Incident,' resulted in 9 fatalities and 120 reported injuries among 128 passengers and 7 crew members, making it one of the deadliest aviation accidents in recent Dutch history. The crash was the result of a complex web of technical failures, human errors, and systemic issues within the airline industry, with blame shared among multiple parties.
We’re taking a closer look at the factors that led to this tragic event. A technical failure: the faulty radio altimeter The primary cause of the crash was a malfunctioning radio altimeter. The radio altimeter is a critical component that measures the aircraft's altitude above the ground by bouncing a radio signal off of the ground and recording the response time, providing data to various automated systems, including the autothrottle.
On Flight 1951, the left-hand radio altimeter malfunctioned, incorrectly reporting the aircraft’s altitude as negative 8 feet repeatedly in the days leading up to the crash, and it malfunctioned again as the plane descended below 1,950 feet. This faulty reading had dire consequences. According to the investigation, the incorrect altitude reading caused the autothrottle to reduce engine power to idle, as if the aircraft was on the verge of landing.
This sudden and unwarranted power reduction led the aircraft to lose speed and altitude rapidly, ultimately resulting in a stall just short of the Polderbaan runway (18R). A closer look at the fascinating story of Schiphol's best-known landing strip. This crash became known as the Polderbaan Incident because it was the eponymous Polderbaan runway that Turkish Airlines flight 1951 came up short on.
Crew error: the response that came too late While the radio altimeter's failure initiated the sequence of events, the crew’s delayed reaction compounded the situation. The crew was aware of the malfunctioning radio altimeter before the final approach but did not disable the autothrottle, which continued to rely on the faulty data. As the aircraft began to lose speed, the first officer, who was flying the aircraft, noticed the decreasing speed but failed to take corrective action until it was too late.
According to the final investigation report by the Dutch Safety Board (DSB): The pilots did not respond appropriately to the series of stall warnings issued by the aircraft’s systems. When the crew finally attempted to recover from the stall by increasing engine power, it was already too late. The plane crashed into a field approximately 1.
5 kilometers (0.9 miles) from the runway threshold, breaking into three pieces upon impact, killing 9 and injuring nearly every one of the survivors. During the accident flight, while executing the approach by means of the instrument landing system with the right autopilot engaged, the left radio altimeter system showed an incorrect height of -8 feet on the left primary flight display.
This incorrect value of -8 feet resulted in activation of the 'retard flare' mode of the autothrottle, whereby the thrust of both engines was reduced to a minimal value (approach idle) in preparation for the last phase of the landing. Due to the approach heading and altitude provided to the crew by air traffic control, the localizer signal was intercepted at 5.5 NM from the runway threshold with the result that the glide slope had to be intercepted from above.
This obscured the fact that the autothrottle had entered the retard flare mode. In addition, it increased the crew's workload. When the aircraft passed 1000 feet, the approach was not stabilized, so the crew should have initiated a go around.
The right autopilot (using data from the right radio altimeter) followed the glide slope signal. As the airspeed continued to drop, the aircraft's pitch attitude kept increasing. The crew failed to recognize the airspeed decay and the pitch increase until the moment the stick shaker was activated.
Subsequently, the approach to stall recovery procedure was not executed properly, causing the aircraft to stall and crash. Source: Aviation Safety Network Systemic issues: training and design flaws Beyond the immediate technical and human errors, the crash highlighted deeper systemic issues within the aviation industry. Turkish Airlines was criticized for inadequate pilot training, particularly in handling automated flight systems and emergency situations.
The investigation revealed that the pilots were not sufficiently trained to handle the specific failure mode they encountered, contributing to their delayed and ineffective response. Additionally, Boeing faced scrutiny for the design of the 737-800’s automated systems. The investigation noted that the autothrottle was overly reliant on the radio altimeter data without sufficient redundancy or safeguards to prevent a single-point failure from leading to a catastrophic event.
Boeing later issued updates to the 737's systems and provided new guidance to airlines on handling similar failures. The airline operates five units of the twinjet. The investigation and the aftermath The Dutch Safety Board led the investigation into the crash, with contributions from Turkish authorities and Boeing.
The final report concluded that a combination of technical failure, crew error, and inadequate training led to the accident. The report also made several recommendations to improve flight safety, including changes to pilot training programs, updates to aircraft systems, and improvements in crew resource management (CRM) to ensure better communication and decision-making in the cockpit. Addressing the controversy: A 2020 investigation by The New York Times found that the Dutch investigation into the crash "either excluded or played down criticisms" of Boeing following pressure from Boeing and US federal safety officials, who instead "emphasized pilot error as a factor .
.. rather than design flaws.
" Following the crash, Turkish Airlines implemented significant changes to its training programs, particularly focusing on automated flight systems and emergency procedures. Boeing, too, responded by enhancing the reliability of the 737’s systems and improving the redundancy of critical components, but continues to be plagued by incidents like this one even today. Wrapping it up The Turkish Airlines Flight 1951 crash was a complex tragedy, shaped by a combination of technical malfunctions, human error, and systemic flaws within the aviation industry.
The incident revealed critical vulnerabilities in automated systems, underscored the need for more comprehensive pilot training, and prompted significant changes in both airline procedures and aircraft design. While improvements have been made in the years since, the lessons from the 'Polderbaan Incident' continue to be relevant for the ongoing discussions about aviation safety and the evolving role of technology in flight operations..
