Fukushima Police Leonardo AW139 FTR LOC-I Accident: JTSB Final Report (JA139F)
On 1 February 2020 Leonardo HelicoptersAW139JA139Fof theFukushima Prefectural Police Aviation Unit crashed near Mihota, Koriyama City, Fukushima Prefecture while transferring a human heart for a transplant operation. All seven occupants were injured, four seriously.
Montage of Eyewitness Imagery of Fukushima Police Leonardo AW139 JA139F after LOC-I/Aircraft Upset (Credit: JTSB as are all images unless otherwise specified)
This was an accident that Aerossurance first covered just 28 days later. when we translated the initial Japan Transport Safety Board (JTSB) release about the accident. The JTSB published its final safety investigation report into this accident earlier this year.
The Accident Flight
The captain had 1400 hours total experience, 450 on type. The co-pilot had 3800, 350 on type. Both had accumulated their AW139 experience relatively slowly over c 5 years.
- 07:09 JST: JA139F takes off from the Fukushima Prefectural Police Heliport destined for Aizu Chuo Hospital where it was to collect the organs for transplant in Fukushima.
- 07:24 JST: The helicopter passes over the vicinity of the accident site at c 3,000 ft and an indicated airspeed (IAS) of c 120 kt.
- 07:28 JST: The captain receives information about a 20 minutes delay at Aizu Chuo Hospital before they will be ready with the organsvia police radio.
- 07:29 JST: The captain and co-pilot discuss flying at a higher altitude on the return trip due to strong winds (about 50 kt from the northwest) observed over Lake Inawashiro.
- 07:56 JST: The helicopter lands at the Aizu Chuo Hospital Temporary Operation Site.
- 08:00 JST: The helicopter takes off from Aizu Operation Site with the organs and medical passengers.
- 08:05 JST: After climbing up to an altitude of about 5,700 ft over Lake Inawashiro, the helicopter started descending gradually. The captain was conscious that they were flying with a strong tailwind.
- 08:07:31 JST: While flying over the Ou Mountain range at IAS 152 k, ground speed (GS) 198 kt and c 4,300 ft, the helicopter started rolling to the right at a rotational speed of 100 º/s or more after the IAS increased rapidly to 188 kt, and made right rolling motion exceeding 360º. The captain pressed the Force Trim Release (FTR) button on the cyclic stick (discussed more below and relevant to another AW139 accident we have discussed). Shortly after we see an aircraft upset evident in flight data:
- 08:07:38 JST: The main rotor blades contact the tail drive shaft.
During the roll and sideslip manoeuvre that occurred the left hand cockpit window fell from the aircraft and the forward right cabin window fell into the cabin, resulting in a high cabin noise. This happened too in AW139 accidents in Spain and Nigeria which we have discussed previously. The captain also banged his head and lost his headset. Crew coordination was therefore difficult.
The captain decided to attempt an autorotative landing.
- 08:07:49 JST: The captain operated the gear lever to extend the landing gears.
- 08:08:06 JST: The co-pilot reported by police radio that the helicopter would make a forced landing.
- 08:08:20 JST: The co-pilot set the AP switch to OFF in an attempt to reset the Autopilot (AP).
- 08:08:55 JST: The helicopter impacts a paddy field and rolls over. According to the Flight Data Recorder (FDR), touchdown was at 45 ft/s, a right yaw rate was 87 º/s, and the ground speed < 20 kt. Fortuitously the landing gear absorbed much of the load with a deceleration of 1.86 G at the center of the fuselage and 6.8 G at the nose. The fuselage deformed to absorb more energy.
Wreckage of Fukushima Prefectural Police Aviation Unit Leonardo Helicopters AW139 JA139F (Credit: JTSB)
JTSB Safety Investigation
On 25 February 2020 the Japan Transport Safety Board (JTSB) issued anupdate(in Japanese) that elaborated on the damage.Our translation (which we first shared 3 days later) is below:
In their final report JTSB provided greater detail on the damage to the fuselage:
Inside the cabin the occupied seats stroked to absorb the impact energy. This accident helps to demonstrate the impressive survivability possible with modern helicopter types in a vertical impact.
A study for the JTSB by the Atmosphere and Ocean Research Institute, University of Tokyo, using Japan Meteorological Agency (JMA) data, indicated the helicopter would have flown through strong downdrafts and updrafts passing over the mountain range.
JTSB report that:
In order to make a major change in the aircraft attitude and the speed change, the captain had never used the Cyclic Stick’s Beep Trim [but] instead used to depress the Force Trim Release Switch (FTR) [which changes ATT Attitude Hold mode to SAS Stability Augmentation System mode] and operated by adjusting the pitch attitude while using the distance between the airframe equipment located on the central window frame and the horizon as a guide.
[When] encountering turbulence, the captain used to prioritize manual control by pushing the FTR of cyclic stick by SAS mode while looking at the horizon as thinking that it would be able to stabilize the helicopter’s attitude earlier than reducing speed or operating in the ATT mode.
The normal procedure in the AW139 Flight Manual is described by JTSB as follows:
“The force trim release button (FTR) on the cyclic stick should be kept depressed during all large stick movements”, on the other hand, “For small attitude adjustments in forward flight (± 2 to 3 kt) the system beep trim mode (TRIM) can be used”, and “The trim method most commonly used is a combination of trim release and beep trim”.
JTSB comment:
Especially during high-speed flight, the reaction to flight control operations becomes more sensitive. For this reason, the AW 139 is designed so as to make the pitch angle change rate of the beep trim small. Therefore, it is more likely necessary for high-speed flight to limit cyclic inputs or use the beep trim.
Both pilots had undertaken a type rating course at Leonardo in Italy from November to December 2014. Significantly, they had not undertaken any AW139 simulator training subsequently, only annual training in an AS350B3 FTD. The JTSB hypothesise this may have introduced negative learning.
JTSB Findings
JSTB present their investigative findings graphically:
The pink numbers are described as follows:
(1) A strong vertical flow was generated over the mountainous terrain on the southeast side of Lake Inawashiro due to strong northwester[ly wind], and the helicopter flying at high speed with a tailwind probably encountered an area where the air currents drastically changed in a short period of time. It is probably that the flight control of the helicopter was strongly affected by the sudden decrease in horizontal wind speed.
(2) Regarding the sudden increase in IAS and the start of the right rolling motion, on the east side of the Ou Mountains, while flying in a northwest wind with a tailwind of about 50 kt, the helicopter encountered a strong downdraft. And when the horizontal wind speed of the tailwind suddenly decreased, the ground speed did not change significantly due to the inertial force, resulting in an increase in the IAS. When the IAS suddenly increased, pressing FTR stopped the attitude retention function, causing a left roll attitude and a nose-down attitude. It is probable that the right rolling motion exceeding 360° started when the cyclic stick was largely manipulated rearward to the right to counteract this attitude change.
(3) During the right rolling motion, the main rotor blades contacted and the tail drive shaft was severed, which resulted in the failure of the tail rotor drive. At the time when the contact noise was recorded, the helicopter had an abnormal attitude with a nose-up pitch angle of 19°, the right bank angle of 108° and the roll rate of the right rolling motion at 100 deg/sec. And at a vertical acceleration of (-) 1.4G, when the collective pitch lever was operated up, the main rotor blades were flapping largely toward the fuselage, contacted with the tail drive shaft, and cut it off.
(4) It is probable that after falling into tail rotor drive failure condition, during the descent, the helicopter flew straight due to the IAS increase, repeatedly rotated to the right due to the deceleration. It is highly probable that at the time of the forced landing, while the helicopter continued to turn to the right, its attitude was being pulled up just before the touchdown and landed in a state of tilting to the right.
(5) During the descent, the engines were switched to the manual back-up mode and the engine overspeed prevention function activated eight times, but the engines did not shut down until the forced landing. In the sector where the engine output was recovered, it is probable that the descent rate was reduced, leading to guiding to the forced landing site and alleviating the impact at touchdown.
(6) When the IAS suddenly increased, the captain operated the collective pitch lever and cyclic stick to control the pitch attitude but operated the cyclic stick farther to the right rear, which is the opposite direction of the aircraft attitude. Since this operation was performed while pressing FTR, it is possible that the amount of correction has become excessive.
(7) It was more likely extremely effective from the impact-resistance point of view for the captain to have made a forced landing with the landing gears extended. In addition, it was probably appropriate for the captain to make a forced landing while avoiding landing in a village under a difficult condition of controlling the helicopter after the right rolling motion started and the tail rotor shut down.
(8) It is probable that the reason that the casualties were limited to injuries was that when the helicopter attempted a forced landing, it made a hard landing while turning to the right, but its attitude was almost horizontal when it touched down, and each part of the aircraft absorbed a large amount of energy, it is most likely that the impact-resistant airframe design worked well.
JTSB Conclusions
The reason why the main rotor blades severed the tail drive shaft is most likely because when the helicopter encountered a strong downdraft while flying at a high speed over mountain regions in strong winds, it started a right rolling motion exceeding 360° after the rapid increase airspeed, and the main rotor blades were largely flapping toward the fuselage.
In addition, regarding the fact that the helicopter became a right rolling motion, it was probably affected by the captain’s large stick movement when encountering a downdraft.
Our Safety Observations
This non-fatal accident highlights the benefits of modern crashworthiness design features and certification requirements.
The availability and use of appropriate simulators is important.
Flight Data Monitoring (FDM: not discussed by JTSB) is a vital way to understand what we could call Flying As Done vs Flying As Imagined (FAD vs FAI) and provide a feedback loop to procedures & training and to crews as safety promotion.
Safety Resources
The European Safety Promotion Network Rotorcraft (ESPN-R) hasahelicopter safety discussion group on LinkedIn. You may also find theseAerossurance articlesof interest:
- Retreating Blade Stall Incident: HEMS BK117B2 VH-VSA
- Loss of Control, Twice, by Offshore Helicopter off Nova Scotia
- Dramatic AW139 Accident at Houma: Skillful Recovery from a Latent Production Defect
- Night Offshore Take-Off Loss of Control Incident Norway
- SAR AW139 LOC-I During Positioning Flight
- AAIB Report on 2013 Sumburgh Helicopter Accident
- Offshore Night Near Miss: Marine Pilot Transfer Unintended Descent
- Night Offshore Windfarm HEMS Winch Training CFIT
- Italian HEMS AW139 Inadvertent IMC Accident
- Fatal Night-time UK AW139 Accident Highlights Business Aviation Safety Lessons
- ADA AW139 A6-AWN Ditching off UAE, 29 April 2017: Final Report
- Night Deck Landing Accident While Training off Brazil
- Tail Rotor Lightning Strip Damages AW139 Main Rotor
- Ungreased Japanese AS332L Tail Rotor Fatally Failed
- Vortex Ring State: Virginia State Police Bell 407 Fatal Accident
- AW139 Brownout Accident with the Nigerian VP Aboard
- NTSB Investigation into AW139 Bahamas Night Take Off Accident
- Swedish SAR AW139 Damaged in Aborted Take-off Training Exercise
- Blinded by Light, Spanish Customs AS365 Crashed During Night-time Hot Pursuit
- SAR AW139 LOC-I During Positioning Flight
- Fatal USCG SAR Training Flight: Inadvertent IMC
