Final Update on the RSAF's F-16 Crash at Tengah Air Base

The investigation on the Republic of Singapore Air Force (RSAF) F-16 crash at Tengah Air Base on 8 May 2024 has concluded. The investigation was led by the RSAF, and supported by Lockheed Martin and the Transport Safety Investigation Bureau.

During the investigation, data downloaded from the Flight Data Recorder and the Digital Flight Control Computer of the incident F-16 were reviewed. The incident pilot and aircraft engineers were interviewed. The maintenance records and procedures for the aircraft were also studied.

The Incident. The pilot had experienced a malfunction of his F-16's Digital Flight Control System during take-off (see Figure 1: Broad Description of the F-16's Flight Control System). As a result, he was not able to safely control the aircraft. After correctly determining that it was unsafe to take the aircraft further, the pilot responded in accordance with emergency procedures, and ejected within Tengah Air Base. He was thereafter evacuated by the RSAF RESCUE10 helicopter to the Singapore General Hospital, in line with established medical evacuation procedure.    

Figure 1: Broad Description of the F-16's Flight Control System.

Root Cause. The root cause of the malfunction has been attributed to degraded pitch rate gyroscopes^[1] , which form part of the motion sensors feeding inputs to the Digital Flight Control Computer. Specifically, two out of four pitch rate gyroscopes in the F-16 gave erroneous but similar inputs to the Digital Flight Control Computer. This resulted in the flight control logic accepting the similar erroneous inputs as "correct", and sequentially rejecting the inputs from each of the remaining functioning gyroscopes as "incorrect". Consequently, the Digital Flight Control Computer manoeuvred the aircraft in response to the erroneous pitch rate feedback signal from the two degraded gyroscopes, making the F-16 uncontrollable by the pilot (see Figure 2). According to Lockheed Martin, this is a rare occurrence and the first such failure reported to it since the F-16s first flew in 1974.

Figure 2: Summary of events leading to F-16 crash at TAB.

Maintenance Regime. The investigation also concluded that the F-16 aircraft was maintained in accordance with established protocols, and the RSAF had adhered to required maintenance inspections. The RSAF F-16s' gyroscopes are the same as those used by other F-16 operators around the world. Lockheed Martin does not stipulate any preventive maintenance for these gyroscopes. The gyroscopes are to be replaced when a fault is detected. The F-16's pre-flight built-in test did not detect any fault with the functioning of the aircraft's gyroscopes before take-off.  

Additional Preventive Measure Going Forward. It is assessed that the two pitch rate gyroscopes had degraded due to wear and tear and failed during take-off. To reduce the chance of a reoccurrence, the RSAF has put in place an additional preventive maintenance procedure for the gyroscopes, under which the RSAF engineers will periodically remove the F-16 gyroscope assemblies and test them using specialised equipment. This will increase the likelihood for engineers to detect early signs of degradation and pre-emptively replace the gyroscopes, before the gyroscopes reach the stage of accelerated degradation leading to failure. This measure is over and above the prescribed aircraft manufacturer's maintenance procedures. Before the RSAF resumed F-16 flights on 21 May 2024, each RSAF F-16 aircraft's flight control gyroscopes had been checked and cleared.

The investigation findings have been reviewed by the SAF's Inspector General Office. The External Review Panel on SAF Safety has also been updated on the investigation findings. The Panel concurred with the additional preventive procedure by the RSAF.

The RSAF will continue to ensure the highest standards of maintenance for the airworthiness of every aircraft.  With approximately 3,100 F-16s operating in 25 countries and over 19 million flight hours^[2] , the Ministry of Defence and the RSAF remain confident in the operational capability and reliability of the F-16 to defend Singapore's skies.

^[1] The gyroscope consists of a mechanical spinning rotor set within gimbals to help detect rotation rates. This is then converted to electrical signal readings, which are fed to the flight computer. The F-16 has four pitch rate gyroscopes with three in use during normal conditions and one as backup.

^[2] Statistics from Lockheed Martin’s F-16 Fighting Falcon Fast Facts, correct as of May 2024.