US HEMS EC135P1 Dual Engine Failure: 7 July 2018 (UPDATED: 9 November 2019)
On the evening of 7 July 2018, at about 21:23 Local Time, Airbus Helicopters EC135P1 N312SA, powered by two Pratt & Whitney Canada (PWC) PW206B turboshafts, impacted the ground hard after an autorotation following a dual engine failure over Chicago, Illinois. The helicopter operated by Pentastar Aviation Charter as a Part 135 helicopter air ambulance (HAA/HEMS) flight for Superior Ambulance.
The pilot and paramedic sustained minor injuries, the flight nurse sustained serious injuries, and the patient was not injured during the accident.
The helicopter sustained substantial damage to the fuselage, tailboom, and main rotor blades.
The Accident Flight
The NTSB report that:
…satellite tracking and air traffic control information revealed the helicopter was traveling northwest from the St. Mary Medical Center on a direct route to Advocate Christ Medical Center about 1,000 ft above ground level. About 5 miles southeast of Advocate Christ Medical Center, the helicopter turned to the right after the pilot requested to return to the Gary, Indiana, airport. About 50 seconds later, the pilot declared a “mayday” and stated the helicopter was going down into a field. The helicopter came to rest upright in a grass area between the Interstate 94 and Interstate 57 interchange.
Surveillance video from a Chicago Transit Authority rail platform located adjacent to the accident site depicted the helicopter during the final phase of the autorotation and impact with terrain. The video showed a fire near the number 2 (right) engine during the autorotation. A[n] explosion was observed after the impact with terrain.
Examination of the Wreckage
Investigators say that:
….the initial impact was consistent with the fenestron skid cap contacting the terrain first, followed by the landing gear skids and fuselage. The left landing gear skid was separated and came to rest near the ground scar consistent with the fuselage. The fuselage was crushed upward, and the fenestron assembly was separated at the tailboom attachment location.
The pilot seat, paramedic seat, and flight nurse seat were found fully attenuated.
Thermal damage was noted on the right engine and main transmission cowling.
Both engines power turbine wheel blades were missing the outer halves of the blades. Multiple impact dents, consistent with the fractured turbine blades, were noted inside the exhaust stubs. The No. 1 engine had a 1/2″ by 1/2″ hole in the exhaust stub at the 2 o’clock position forward of the aft firewall, and the No. 2 engine had a 2″ by 1″ hole in the exhaust stub at the 11 o’clock position forward of the aft firewall.
Next Steps in the Investigation
The helicopter was equipped with Outerlink IRIS. This provides video, voice and flight data recording as well as satellite communications. The IRIS equipment was sent to the NTSB Vehicle Recorder Lab for analysis.
We will update this report as more information emerges.
NTSB Investigation Update (9 November 2019)
Although the NTSB are yet to issue their probable cause they have issued several revealing documents and a factual report update.
The Accident Flight
The NTSB report that:
On the evening of the accident, the pilot received a flight request, checked the weather, and performed a preflight inspection for the planned 12 to 13-minute flight.
After departure, the pilot climbed to 1,700 ft mean sea level, or about 1,000 ft agl. About 5 miles west of GYY [Gary International Airport], he contacted Chicago Midway International Airport (MDW), Chicago, Illinois, requesting entry into the airspace, and noticed a “Twist Grip” warning on the left engine 1 side warning panel. The pilot noticed a second indication but could not recall the specific warning. He grabbed each engine throttle twist grip individually to gently verify if he could feel they were in or out of position, and he did not notice any significant changes to the throttle position.
The pilot decided he did not have enough time to trouble shoot the emergency procedure before landing at the intended hospital destination, and he would not land at the hospital with a warning indication. He informed the medical crew they would divert to GYY and handed them the helicopter emergency checklist book to assist with locating the emergency checklist procedure(s).
As the pilot executed the turn to GYY, he noticed the No. 2 engine indication (N1 gas producer) no longer matched with the No. 1 engine; “it was lower and oscillating.”
Within about 1 minute of the turn toward GYY, the pilot “heard the low rotor RPM horn”, and he lowered the collective to maintain rotor speed. The pilot located a “dark spot” which would give him the best opportunity to complete a full autorotation with a flare to cushion the landing. The pilot determined he no longer could troubleshoot the problem and was doing his best to fly the helicopter. As he started a turn toward his intended landing location, he felt the tail oscillate to the right and back and heard increase and decrease in engine speed. About 200 ft agl, he thought he may land short of the intended location, and he made adjustments to the collective and cyclic to maintain rotor RPM and airspeed. The pilot then initiated a flare and landing. After the helicopter came to rest, the flight paramedic mentioned the helicopter was on fire, and the pilot noticed a fire near the No. 2 engine.
An NTSB CVR transcript summary is as follows:
2109:41.2 Pilot: Alright. Flight guarded – cautions -warnings – all good.
2110:13. 5 Medical Crew 1 (MCR 1): Caution warning lights?
2110:15.7 Pilot: Everything is out.
2110:34.9 Cockpit Area Microphone (CAM): [sound similar to helicopter transitioning to hover]
2114:25.0 Pilot: Unintelligible word/expletive [spoken under breath]
2114:50.0 CAM: [sound of increasing engine power]
2121:57.0 Pilot: Hey guys.
2122:03.2 Pilot: We gotta re-return to uh Gary you guys are gunna have to go by ground I got a manual twist grip here.
2122:10.3 Pilot: So I also – might need you guys to look – look it up.
2122:25.3 Pilot Radio: Yeah, I’d like to return to uh Gary airspace um medivac two sierra alpha.
2122:41.1 Pilot: I’mma hand this back to you.
2122:58.5 MC1?: Okay – what are we looking up?
2123:00.9 Pilot: Engine manual twist grip.
2123:20.9 CAM: [sound of electronic warning gong]
2123:26.0 CAM: [start of sound similar to high rotor RPM alarm begins and lasts until 2124:01.5]
2123:29.6 CAM: [sound similar to increase of rotor RPM]
2123:31.3 Pilot Radio: Mayday, mayday, mayday
2123:50.1 CAM: [sound similar to brief increase in engine or rotor RPM]
2123:51.1 CAM: [sound similar to decrease engine or rotor RPM]
2124:01.5 CAM: [sound of electronic warning gong]
2124:06.3 CAM: [sound similar to low rotor RPM alarm, continues until 2124:28.4]
2124:08.4 CAM: [sound similar to decrease in engine or rotor RPM noise]
2124:16.6 CAM: [sound of impact]
2124:58.0 Pilot Radio: Mayday, mayday, mayday – we’re on the ground – uhhh had an accident – three one two sierra alpha
The NTSB Investigation
The operator’s fleet was made up of two EC135P2+s and one older EC135P1. NTSB note that the pilot was hired by Pentastar in August 2016 and primarily flew the EC135P2+ which had a glass Center Panel Display System (CPDS) compared to the accident EC135P1′s an analogue Cockpit Display System (CDS) At the time of the accident, the pilot had flown c 319 flight hours in the EC135P2+ but only c 11 total hours in the EC135P1. His most recent Part 135 competency check was completed 31 March 2018, in a EC135P2+.
The pilot had never flown an EC135P1 simulator for the simple reason there were none worldwide. The NTSB note:
The pilot completed the Pentastar “RW EC-135P1 Differences Training”, which was an online self-study course, on February 18, 2018. The online course included, but was not limited to, the following differences: [Cockpit Display: CDS vs CPDS], analogue versus first limit indicator (FLI) all engines operative and one engine inoperative limits, and twist grip controls.
In addition, the pilot stated he completed some “hands-on” EC135P1 training with other company pilots, and familiarization flights.
The EC135P1′s engines were equipped with Electronic Engine Controls (EECs). The engine throttles or ‘twist grips’ are mounted on the collective. Normally the throttles are set in the neutral position, allowing the EECs to control the engines.
Examination of the engines revealed that:
The damages [sic] observed on both engines are characteristics of blade release due to a power turbine rotor overspeed condition. The physical evidence of the overspeed is consistent with what was found from the flight data, which showed that prior to impact the engines were in manual mode and the torque was increased using the twist grip, with the collective decreasing. The high rotor alarms at 106% and 112% NR were activated, after which the engine torque was further increased until the NPT signal went above 127% at which point the data held the last good value until the speed came back below 127% NPT. The faults recovered in the EEC show that both Q and NPT sensors recorded the NPT exceeding 127%.
Examination of the twist-grip throttles confirmed continuity from each throttle to the respective engines. The No. 1 engine twist grip throttle was unable to move, and the No. 2 engine twist grip throttle was free to rotate.
Flight conditions prevailing at the time of the accident flight were such that the main rotor speed (NR) and power turbine speeds should have been governing at 100% during the entire flight. At 2114:29.5, the No. 1 engine entered manual mode following a collective (CLP) input, and the engine remained in manual mode for the remainder of the flight. The No. 1 engine no longer responded to CLP inputs and appeared to increase in torque while the CLP was being lowered which can be explained by a rotation of the twist grip.
The No. 2 engine was operating in automatic mode up to time 2123:17.1, at which point the torque increased with a steady-state CLP (in automatic mode the torque should remain constant with a constant CLP), and the engine operated in manual mode.
As the torque was increased with the CLP decreasing, the engines’ NPT speeds and main rotor speeds increased.
According to the helicopter manufacturer, with one engine in manual mode, and the other under EEC control, a reduction in power (using the twist grip to reduce fuel flow) on the manual engine would result in a power increase on the engine under EEC control (in an effort to maintain the rotor speed), up to the predetermined limits. If power was increased on the engine in manual mode (using the twist grip), then the power could increase to the engine fuel control limits, and there would be a corresponding decrease in power on the engine under EEC control.
In other words, it appears that flight crew action resulted in overspeeding both engines while inadvertently in manual mode and the resulting power turbine blade shedding.
Previous Events and Available Risk Mitigation
A similar, albeit fatal, accident occurred to EC135P1 N601FH on 30 June 2006. Again this was the only CDS equipped EC135P1 in that operator’s fleet and again the differences training was trivial (1 hour). In that case the NTSB Probable Cause was:
The operator’s inadequate training program and the pilot’s failure to maintain control of the helicopter following his inadvertent disabling of the No. 1 and then the No. 2 engine full authority digital engine control system.
A modification was available that would have helped mitigate the risk:
According to Eurocopter Service Bulletin EC135-67-013 – Rotor Flight Control – Collective Control – Replacement of collective levers and introduction of a weight compensation, dated July 28, 2008, Eurocopter (Airbus) offers collective levers with grips ENG 1/ENG 2 that offer an increased mechanical protection against unintentional adjustment. In addition, the surface structure of the grips ENG 1/ENG 2 differs decisively so that they can be distinguished by tactual sensation.
As of the end of 2018, although there were 17 EC135P1s in the US only 2 SB EC135-67-013 kits had been purchased in the US.
We will update this report when the NTSB probable cause is issued.
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