T-Bolt Trouble: Unrecorded Maintenance on US HEMS BK117C2/H145 and Loss of Tail Rotor Pitch Control (Metro Aviation N191LL)
On 6 March 2024 Metro Aviation Airbus BK117C2 / H145 helicopter air ambulance N191LL was damaged at Purdue University Airport (LAF), West Lafayette, Indiana. The three occupants were uninjured.
Metro Aviation Airbus BK117C2 N191LL (Credit: via Redditt)
On 24 April 2025 the US National Transportation Safety Board (NTSB) released their safety investigation report, in a creditable 14 months.
The Accident Flight
The pilot reported that, while in a hover taxi to accelerate for takeoff, he felt a force against his feet from the pedals. The helicopter yawed to the right, so he applied full left pedal, but when the pedal was depressed, there was no resistance and no effect on the helicopter’s yaw.
The helicopter landed hard came to rest and rest upright, with damage to “the fuselage, tailboom, vertical fin, horizontal stabilizer, tail rotor assembly, and one main rotor blade”.
The Safety Investigation – The Accident Sequence
The T-bolt and its attachment bolts, which connect the pitch change bellcrank to the pitch change slider was found to have become disconnected.
The installation of the BK117 T-bolt and associated attachment hardware onto the pitch change slider (Credit: Airbus / modified by NTSB).
Investigators concluded that the T-bolt attachment bolts had been installed but not torqued and lock wired, allowing them to back out while rotors running.
Once the attachment bolts backed out, the T-bolt also backed out resulting in a loss of tail rotor pitch control.
The T-bolt then likely impacted and damaged a tail rotor blade. The subsequent imbalance led to the overload separation of the upper vertical fin. One attachment bolt was found on the ramp after the accident, and the other was “lodged into a honeycomb panel at the aft-lower area of the fuselage, adjacent to the fuel cell.”
The Safety Investigation – The Maintenance History
During maintenance a few days prior, the T-bolt and its attachment bolts were removed by a mechanic at the direction of a lead mechanic to facilitate troubleshooting of adjacent components for the tail rotor control system.
As the T-bolt’s removal was temporary and quick, the mechanic crucially choose not to record the removal in the Work Order’s Discrepancy Sheet.
The T-bolt attachment bolts were then subsequently temporarily installed “finger-tight’ by a second mechanic to assist a third mechanic who was installing the tail rotor blade mounting forks and pitch change links.
According to the NTSB “the first mechanic [sic] was tasked to another company helicopter shortly after”. The NTSB state that “while he stated he relayed to the other mechanics that the T-bolt attachment bolts were finger-tight, no one followed up on the installation of the T-bolt attachment bolts”.
This part of the NTSB report is rather ambiguous but it seems likely the NTSB mean the mechanic tasked to another job was the second mechanic, who reinstalled the bolts finger tight, rather than the first mechanic who did the unrecorded removal.
Without a maintenance discrepancy entry for the removal of the T-bolt, there was no open task to verify T-bolt installation was complete.
Furthermore, no one identified that lock wire was missing from these bolts during the final checks before the helicopter was released to service.
NTSB Probable Cause
The failure of maintenance personnel to properly install the tail rotor pitch change slider attachment hardware (T-bolt), which led to the disconnection of the pitch change slider, a loss of tail rotor control, and subsequent hard landing.
Contributing to the accident was maintenance personnel’s failure to complete a maintenance discrepancy entry on the work order for the removal of the T-bolt.
Our Safety Observations
Disappointingly the maintenance human factors are not examined in any depth, no safety recommendations are made and no safety actions are recorded. There is also no NTSB Public Docket released at this time where some depth of supporting evidence would have been expected.
The NTSB do highlight a 2015 safety alert: Mechanics Manage Risk and Follow Procedures (SA-022)
In November 2015 we wrote about a November 2013 accident where EC135P1 N911KB, crashed during a post-maintenance check flight at another Metro maintenance facility. The NTSB determined the probable cause in that case to be:
The mechanic’s improper installation of the antitorque pedalss, which resulted in an in-flight loss of helicopter control
Ironically in that article we wrote that:
The concept of a failure to learn has been raised in relation to BP after the 2005 Texas City refinery explosion, prior to the 2010 Macondo / Deepwater Horizon disaster. It is also the subject of a book of the same title by Australian National University Emeritus Professor Andrew Hopkins and our own article on a Dutch refinery explosion (Shell Moerdijk Explosion: “Failure to Learn”).
We have also previously discussed Metro BK117C2 N263MH that had a hard landing after FOD due to loose airfield fencing.
Safety Resources
Aerossurance worked with the Flight Safety Foundation (FSF) to create a Maintenance Observation Program (MOP) requirement for their contractible BARSOHO offshore helicopter Safety Performance Requirements to help learning about routine maintenance and then to initiate safety improvements:
Aerossurance can provide practice guidance and specialist support to successfully implement a MOP. We have written several general articles related to maintenance safety:
- Professor James Reason’s 12 Principles of Error Management
- How To Develop Your Organisation’s Safety Culture
- Aircraft Maintenance: Going for Gold?
- Critical Maintenance Tasks: EASA Part-M & -145 Change
You might find these safety / human factors resources of interest:
- James Reason’s 12 Principles of Error Management
- Back to the Future: Error Management
- This 2006 review of the book Resilience Engineering by Hollnagel, Woods and Leveson, presented to the RAeS by Aerossurance’s Andy Evans: Resilience Engineering – A Review and this book review of Dekker’s The Field Guide to Understanding Human Error: The Field Guide to Understanding Human Error – A Review
You may also find these Aerossurance articles of interest too:
We have previously looked at several accident which featured relevant maintenance human performance issues:
- Fatal $16 Million Maintenance Errors where fasteners were missed from rotor system components and the NTSB made significant maintenance human factors recommendations
- Loose B-Nut: Accident During Helicopter Maintenance Check Flight where a disconnected fuel coupling was left under-torqued
- USAF RC-135V Rivet Joint Oxygen Fire where disconnected oxygen couplings were left under-torqued
- The Missing Igniters: Fatigue & Management of Change Shortcomings were engine igniters were left disconnected
- B1900D Emergency Landing: Maintenance Standards & Practices The TSB report posses many questions on the management and oversight of aircraft maintenance, competency and maintenance standards & practices. We look at opportunities for forward thinking MROs to improve their maintenance standards and practices.
- Crossed Cables: Colgan Air B1900D N240CJ Maintenance Error On 26 August 2003 a B1900D crashed on take off after errors during flying control maintenance. We look at the maintenance human factor safety lessons from this and another B1900 accident that year.
- Meeting Your Waterloo: Competence Assessment and Remembering the Lessons of Past Accidents: No one was injured in this low speed derailment in London after signal maintenance errors but investigators expressed concern that the lessons about maintenance errors from the fatal triple collision at Clapham in 1988 may have been forgotten.
- EC130B4 Accident: Incorrect TRDS Bearing Installation
- Ungreased Japanese AS332L Tail Rotor Fatally Failed
- R44 Ditched After Loss of TGB & TR: Improper Maintenance
- When Down Is Up: 747 Actuator Installation Incident
- Maintenance Human Factors in Finnish F406 Landing Gear Collapse
- Missing Cotter Pin Causes Fatal S-61N Accident
- Engine & Emergency Flotation Failures – Greenland B206L4 Ditching
- FAA Rules Applied: So Misrigged Flying Controls Undetected in an accident to a Cessna 172 in Bermuda
- SAR AS365N3 Flying Control Disconnect: BFU Investigation
- Lost in Translation: Misrigged Main Landing Gear
- Flying Control FOD: Screwdriver Found in C208 Controls
- BEA Point to Inadequate Maintenance Data and Possible Non-Conforming Fasteners in ATR 42 Door Loss
- BA A319 Double Cowling Loss and Fire – AAIB Report
- BA A319 Double Cowling Loss and Fire – AAIB Safety Recommendation Update
- ANSV Report on EasyJet A320 Fan Cowl Door Loss: Maintenance Human Factors
- Tiger A320 Fan Cowl Door Loss & Human Factors: Singapore TSIB Report
- Human Factors of Dash 8 Panel Loss
- Cessna 208 Forced Landing: Engine Failure Due To Re-Assembly Error
- AAIB Report on the Ditchings of EC225 G-REDW 10 May 2012 & G-CHCN 22 Oct 2012
- EC225 LN-OJF Norway Accident Investigation Timeline
- In-Flight Flying Control Failure: Indonesian Sikorsky S-76C+ PK-FUP
- AAR Bell 214ST Accident in Afghanistan in 2012: NTSB Report
- Fuel Tube Installation Trouble
- How One Missing Washer Burnt Out a Boeing 737
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn.
Recent Comments