BK117B2 Air Ambulance Flameout: Fuel Transfer Pumps OFF, Caution Lights Invisible in NVG Modified Cockpit

Air Methods Airbus BK117B2 HEMS Air Ambulance N238BK Flameout: Fuel Transfer Pumps OFF, Caution Lights Invisible in NVG Modified Cockpit

Just after sunset on 1 July 2017 an Air Methods Corporation Airbus Helicopters BK117B2, N238BK, conducting a Part 135 inter-hospital patient transfer air ambulance flight, landed hard and rolled over during a forced landing in a field outside Perryville, Missouri.

Wreckage of Air Methods Airbus Helicopters BK117B2 Air Ambulance N238BK in Perryville, MO (Credit: NTSB)

Wreckage of Air Methods Airbus Helicopters BK117B2 Air Ambulance N238BK in Perryville, MO (Credit: NTSB)

The pilot, the three medical personnel and the 5 year old patient all received minor injuries.

History of the Accident Flight

The aircraft was established in cruise at 120 kts and an altitude of 1600 ft AMSL (approximately 1200 ft AGL).  According to the US National Transportation Safety Board (NTSB) investigation report, the pilot stated that…

…about 17 minutes after takeoff while in cruise flight at dusk, the helicopter “experienced a sharp change in attitude yawing to the left with a hard-upward bump,” followed by a change in engine noise. He saw that the engine gas generator speed (N1) gauges for each engine were indicating below 40% and decreasing and that the No. 1 engine low warning light, the No. 1 generator light, and the battery discharge warning lights were illuminated.

…the helicopter suddenly “pitched nose up and rolled to the right” and that he then heard the rotor speed begin to deteriorate. He entered an autorotation by applying right forward cyclic and lowering the collective to full down.

During the autorotative descent, he saw power lines and a ditch, which required him to change the helicopter’s flightpath and land on the far side of the ditch. He flared the helicopter about 100 ft above ground level, and the rotor speed began to decay rapidly.  [T]he helicopter landed right skid low and then skidded for about 100 ft. The main rotor blades hit the ground as the helicopter rolled onto its right side.

Wreckage of Air Methods Airbus Helicopters BK117B2 Air Ambulance N238BK in Perryville, MO (Credit: NTSB)

Wreckage of Air Methods Airbus Helicopters BK117B2 Air Ambulance N238BK in Perryville, MO (Credit: NTSB)

Safety Investigation

The NTSB comment that the pilot’s recollection was consistent with a dual-engine loss of power and rotor system damage was consistent with no power at the time of impact.

The fuel transfer system between the main tanks and the supply tanks and from the supply tanks to their respective engines and the fuel delivery system functioned normally during operational testing. No residual fuel was found within the engine fuel filter bowl, indicative of no fuel reaching the engines. Based on the pilot’s statement that he saw a steady stream of fuel leaking from a fuel vent port on the helicopter’s belly shortly after the accident, fuel was likely present within the main fuel tanks.

Therefore, based on the evidence, it is likely that the pilot did not activate the fuel transfer pumps, which resulted in no fuel transferring between the main fuel tanks and the supply tanks and led to eventual fuel starvation. Thus, when the engines consumed all available fuel from their respective supply tanks, the dual-engine loss of power occurred.

The helicopter’s warning and caution annunciator panel functioned normally during post-accident testing.  However, it was found that the dimming function was activated.

Fuel Controls and Cockpit Lighting Dimming Switches (Credit: FAA via NTSB)

Fuel Controls and Cockpit Lighting Dimming Switches (Credit: FAA via NTSB)

The pilot confirmed he had activated this feature before take-off.

The annunciator panel contains caution lights for when the fuel transfer pumps are off and for when the fuel quantity in each supply tank is low. Illumination of these caution lights leads to the illumination of the master warning light but generates no aural tones.

A night vision goggle (NVG) compatible interior lighting system, manufactured by Ahlers Aerospace, Inc., was installed under FAA Supplemental Type Certificate (STC) No. SR09523RC. The night vision imaging system (NVIS) kit uses infrared filters that are installed externally to the faces of instruments and displays to reduce or eliminate infrared glare. An infrared filter is also placed over the advisory, caution, and warning annunciator panel. STC No. SR09523RC does not affect the instrument lighting controls originally installed on the helicopter.

Undimmed (Credit: AMC via NTSB)

Undimmed (Credit: AMC via NTSB)

When the annunciator lights were dimmed [during post-accident tests] the following was observed:

  1. The “ENG I LOW”, “ROTOR RPM”, “ENG II LOW”, and “XMSN OIL PRESS” lights were dimmed but still visible.
  2. The “ENG II CHIP” light was at full brightness and visible.
  3. The “BAT DISCH”, “EXT PWR”, “CSAS ROLL”, and “CSAS YAW” lights were dimmed and partially visible.
  4. The remaining advisory, caution, and warning lights on the annunciator panel were not visible.
  5. The master warning light had dimmed to a level where its illumination was not readily distinguishable.
Dimmed (Credit: AMC via NTSB)

Dimmed (Credit: AMC via NTSB)

The ambient light at the time of the flight and the pilot’s activation of the dimming function in conjunction with the night vision imaging system filters likely precluded the pilot from being able to see the illuminated caution lights on the annunciator panel and an illuminated master warning light.

The dimming function is not mentioned in Air Methods recurrent training included by the NTSB in the public docket and there is no advice in relation to it when using NVGs.

Note: there is no indication in the NTSB report that NVGs were actually in use on this flight.

The NTSB make no mention of certification requirements for NVG integration or how this STC was undertaken in their final report.  There is brief mention of daylight readability testing the HF Specialists report but no mention of dimming.

The pilot, age 59,…had 6,237 hours of total flight time with 308 hours of flight time in the make and model of the accident helicopter.

BK117 Fuel System

The helicopter contains four flexible fuel tank bladders: an 80 kg fuel tank, forward main tank, left and right prime (supply) tank, and a rear main tank. The 80 kg fuel tank is connected to the forward main tank via one interconnecting tube. The forward main tank is connected to each supply tank via overflow tubes. The left supply tank supplies fuel to the No. 1 engine and the right supply tank supplies fuel to the No. 2 engine.

BK117B2 Fuel System (Credit: Airbus Helicopters via NTSB)

BK117B2 Fuel System (Credit: Airbus Helicopters via NTSB)

 

The total usable fuel capacity of all fuel tanks is about 184.2 gallons. The 80 kg fuel tank holds about 26.3 gallons, the forward and aft main fuel tanks combined holds about 132.4 gallons, and both supply tanks combined holds about 25.5 gallons.

There are four fuel pumps within the helicopter fuel system: two fuel prime pumps and two fuel transfer pumps. Each supply tank contains a prime pump that delivers fuel to the engines via feed lines during engine start. Once the engines are running, the engine-driven fuel pumps draw fuel from the supply tanks and the prime pumps can be turned off.

BK117B2 Fuel System Fuel Flow Schematic (Credit: Airbus Helicopters via NTSB)

BK117B2 Fuel System Fuel Flow Schematic (Credit: Airbus Helicopters via NTSB)

[The] fuel transfer pumps are located within the forward main tank. The transfer pumps deliver fuel from the main fuel tanks to the supply tanks and must be activated during helicopter operation. The capacity of the transfer pumps allows for more fuel to be transferred into the supply tanks than the engines can consume. Any surplus fuel in the supply tanks return to the main fuel tanks via overflow tubes. When the transfer pumps are deactivated, the “F PUMP XFER FWD” and “F PUMP XFER AFT” caution lights illuminate on the annunciator panel. Activation of the respective transfer pumps extinguishes the associated caution lights.

Each supply tank contains a fuel quantity transmitter. Additionally, fuel quantity transmitters are installed in the forward main tank and the 80 kg fuel tank. The fuel quantity indicator (gauge) in the cockpit shows three values in gallons: the main fuel tank quantity, the left supply tank fuel quantity, and right supply tank fuel quantity. Each supply tank contains a low fuel warning transmitter that detects when its respective supply tank contains less than 7.6 gallons of fuel. Activation of the low fuel warning transmitter results in illumination of the “FUEL LOW I” or “FUEL LOW II” caution light on the annunciator panel for the left supply tank and right supply tank, respectively.

AMC require that the transfer pumps are left on throughout the flight, as discussed in their recurrent training.

Previous Accidents and Incidents

On 20 July 2001, BK117C1 N313LS, experienced a dual engine power loss in flight and autorotated into a field near Decatur, Texas. The fuel transfer pump switches had bee set to off for most of the flight.  The pilot did not see any warning lights but the dimming function of the pilot instrument and console instrument lighting was turned on.

On 5 May 2014, a BK117B2 ZK-HJC, experienced a dual engine power loss in flight and performed a successful autorotation near Springston, Canterbury, New Zealand.

  1. Both engines lost power due to fuel starvation because the pilot did not switch on the fuel transfer pumps after starting the engines.
  2. A cockpit lighting modification to the helicopter had adversely affected the readability of the caution lights during daylight when the dimmer switch was on. Brightly illuminated caution lights should have alerted the pilot to the incorrectly configured fuel system and the low fuel levels in the supply tanks.
  3. The helicopter was not designed to generate an aural warning of a critically low fuel level in the supply tanks. An aural warning, as fitted to later designs, would have alerted the pilot to the potential loss of engine power.
  4. A previous event in New Zealand in 2011 involving a BK117A3 that experienced a loss of engine power due to fuel starvation but landed safely. The cockpit lighting dimming function was reported to be on during daylight conditions.

We reported on that here: Dim, Negative Transfer Double Flameout on 23 April 2016 (over 14 months before the Air Methods Perryville accident).

The German accident investigation agency, the BFU, provided the NTSB with correspondence from within the FAA in 1993 and 1994 which discussed fuel starvation accidents involving the earlier MBB Bo105-series.

The correspondence, spanning from 1992 to 1994, discussed accidents in which the fuel transfer pump switches were not turned on in flight. Similar to the BK117-series helicopter, the Bo105-series helicopter contains a main fuel tank and a supply tank, the latter of which feeds the No. 1 and No. 2 engines. Pumps transfer fuel from the main fuel tank to the supply tank. A low fuel warning transmitter in the supply tank triggers a low fuel light in the cockpit annunciator panel but there is no associated aural warning.

NTSB Probable Cause

Fuel starvation due to the pilot’s failure to turn on the fuel transfer pump switches during takeoff, which led to a total loss of engine power.

Contributing to the accident was the pilot’s improper decision to activate the annunciator panel’s dimming function during dusk, which prevented him from seeing the illuminated fuel transfer pump caution light indicating that the pumps were off and the illuminated caution lights for low fuel in the supply tanks.

Other Safety Resources

See: Basics of NVG Lighting

Out past articles include:

On error management:

…and our review of The Field Guide to Understanding Human Error by Sidney Dekker presented to the Royal Aeronautical Society (RAeS): The Field Guide to Understanding Human Error – A Review


Aerossurance has extensive air safety, operations, SAR, HEMS, airworthiness, human factors, aviation regulation and safety analysis experience.  For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com