BFU Investigate S-76B Descending to 20ft at 40 kts En Route in Poor Visibility

BFU Investigate S-76B Descending to 20ft at 40 kts En Route in Poor Visibility

The German safety investigation body, the BFU, is investigating a serious incident that occurred to Sikorsky S-76B D-HHNH on 23 October 2017.

History of the Flight

In an interim report they describe that the flight was on a return to Emden from the DolWin Alpha High Voltade Direct Current (HVDC) platform in the North Sea, which helps transit power from offshore wind farms ashore.

At 0842 hrs the helicopter took off now with four persons on board [2 crew. 2 passengers] for the return flight…[and]…climbed to 2,000 ft AMSL which remained the same up until right abeam of Borkum Island.

At 0854 hrs the flight crew established radio contact with the Flugleiter at [the uncontrolled] Emden Airfield and requested information regarding the prevailing weather conditions.

They were told:

“Right now it is raining, cloud base few one thousand, overcast thousand five hundred and wind south-west 10 to 15 gusts 20 kt, QNH 1,010 and visibility here visual reference point 2.5 to 3 km.”

At 0857 hrs the crew informed Bremen Radar of passing way-point JUIST.

However:

Before reaching way-point JUIST the helicopter began to descend. The descent ended in low altitude with reduced speed near the coast line.

S-76B Flight Profile (Credit: BFU)

S-76B Flight Profile (Credit: BFU)

Then the helicopter climbed again to about 500 ft AMSL and continued the flight to Emden. At 0901 hrs the crew again requested information regarding the prevailing weather at Emden Airfield.

They were told:

“Right now it has decreased a little bit few seven hundred, broken one thousand, overcast one thousand eight hundred, the rain is gradually slowing down”.

The aircraft landed safely at 0911 hrs.

The Aircraft Commander (PIC) had a total flying experience of 11,461 hours, of which 3,025 hours were on type.  The First Officer (co-pilot) had 910 hours flying experience, of which 359 hours were on type.

Investigation

The BFU examined the events leading to the period between 0957 and 0901 when the aircraft lost speed and altitude over the Wadden Sea, the tidal mud flats between the outer island and the mainland.

…during the two legs (outbound and return) the co-pilot in the left-hand seat acted as Pilot Flying (PF) and the Pilot in Command (PIC) in the right-hand seat as Pilot Non Flying (PNF). After take-off from the converter station DOLWIN ALPHA the helicopter had been operated using the Flight Director (FD) coupled in the Upper Modes ALT and NAV.

Between the way-points UTIRA and HW751 the FD had decoupled without reason. The two autopilots had changed from Attitude Retention (ATT) Mode to Stability Augmentation System (SAS) Mode. This resulted in the PF having to actively control the helicopter manually.

Subsequently, the PNF tried to find and eliminate the cause for the FD failure. Both in vain.

After they had enquired about the prevailing weather conditions at Emden Airfield they decided to reduce the flight altitude before reaching the coast line. During descent the helicopter had suddenly encountered heavy rain and significantly reduced visibility. At about 500 ft AMSL the co-pilot had handed over controls to the PIC.

Notably:

He had instructed the co-pilot to extend the landing gear because he wanted to be prepared for a possible off-field landing due to the unexpected bad weather.  According to the Cockpit Voice Recorder (CVR) recording the PIC allowed the helicopter to continue the descent because no visual contact with the coast line had been established.  At that time even in low altitude there was no visual contact with the coast line.

At about 0900 hrs the co-pilot said “100 ft GND”. A few seconds later the computer voice from the radar altimeter [Audio Voice Alerting Device (AVAD)] announcing “One Hundred Feet” and an acoustic signal were recorded. There was no verbal comment about the altitude warning. The helicopter continued to descend.

Significantly (emphasis added):

Both pilots looked outside searching for the coast line or a brighter place in the grey clouds ahead of them.

The co-pilot recognised the low altitude and read the radar altimeter “20 ft GND“. He advised the PF accordingly. At the same time the airspeed had decreased to approximately 40 kt.

The PF stopped the descent and increased the airspeed. A short time later in low altitude the pilots had the coast line in sight. The flight was continued towards Emden under Visual Flight Rules (VFR) with reduced cruise speed and in low altitude. Due to the limited visibility and low clouds it was decided to fly an approach to runway 07.

When the engines were shut off the pilots noticed that the cyclic stick trim switch was not pushed in, i.e. the function was disengaged.

bfu s76b fd

The PIC stated that the heavy shower and the therefore reduced visibility had occurred totally unexpectedly. He had not considered to changing flight rules and conducting the instrument approach RNAV (GPS) Y RWY25 to Emden due to the unclear situation with the FD.

Flight Data Recorder (FDR) data was not available to the investigators as the FDR was faulty.  They do have radar and Flightradar 24 data.  The two passengers, who were facing aft were interviewed:

Then the helicopter had descended in stages until the main rotor generated spray. The passenger estimated the lowest altitude was 5 to 10 m. At the same time the helicopter had had a nose-up attitude. Then the coast line had been reached and they had flown over land slightly above the wind turbines to Emden.

The BFU explain that:

The helicopter was equipped with a Honeywell SPZ -7000 Digital Automatic Flight Control System (DAFCS). This system consists of an autopilot function for attitude stabilisation (SAS or ATT modes), and a flight director function for roll and yaw control (ALT, IAS, VS, HDG, Nav or ILS modes). During automatic flight control one or several flight director modes (upper modes) are coupled with the autopilot in ATT mode. If the cyclic stick trim function is disengaged the AP changes from ATT mode to SAS mode. It is not possible to couple the FD in SAS mode.  Prior to departure the DAFCS has to be self-tested with a two-level test (Level 1 and Level 2) with running engines. The crew stated that at the day of the occurrence the two test levels had been without fault.  [Additionally] the helicopter was equipped with a Flight Management System (FMS) Universal UNS 1K and a fixed installation Garmin GPS 500W with Moving Map indication.

[At Emden] both runway directions (07 and 25) are equipped with RNAV (GPS) and instrument approach and departure procedures. For helicopters approaching from the north way-point JUIST is the Initial Approach Fix (IAF) for RNAV (GPS) Y RWY 07/25 approaches.

[On] the day of the occurrence between 0800 hrs and 1000 hrs three more helicopters and one airplane departed for VFR offshore flights and one helicopter for an IFR offshore flight.

They also note that:

The serious incident occurred at a time where the [operator] was in the midst of reorganisation. It encompassed the postholder positions, the revision of the procedures, the training, and internal reporting.  Deployment of the S-76B [by the operator] was only planned until the end of 2017 / or the beginning of 2018. As a result the number of crews for commercial flight operations was reduced to three (three PICs and three co-pilots).

In relation to the wind farm support sector:

Since 2010 with the construction and operation of offshore wind energy convertors a new field of activity has developed for helicopter operators in Germany. In the beginning there were just a few flights, by 2016 the number had increased to about 14,000 per year. Due to the aeronautical regulations and the airspace and procedure specifications these flights have so far essentially been conducted in accordance with Visual Flight Rules (VFR). In the European neighbouring states the offshore flights in connection with oil and gas production are essentially conducted in accordance with Instrument Flight Rules (IFR) due to the unpredictable weather or often marginal visual meteorological conditions.

…a way-point and route system including stipulated vertical separation and communications procedures were compiled on the initiative of several operators and in agreement with the Deutsche Flugsicherung (German Air Navigation Services, DFS). Since September 2014 a majority of the offshore companies are committed to adhere to this way-point grid and the procedures. In October 2017 the Deutsche Flugsicherung published the AIP AIC VFR 03/17 Reporting point network and special alerting service for helicopters in the North Sea of the Bremen Flight Information as information and recommendation [only].

The helicopter had been equipped with an emergency flotation system for Sea State 4 (up to 2.5 m significant wave height).  The significant wave height at 0830 hrs at DolWin Alpha was 3.1 m.  This would have precluded operations in some countries, such as the UK.  The water conditions in the Wadden Sea would have been significantly calmer.

Although bird strike risk is not discussed by the BFU, the Wadden Sea is

…one of the most important wetlands for migratory waterbirds on earth. It provides a rich food supply for 10-12 million birds (2 million geese and ducks, 7 million waders, 2 million gulls and terns), which use the Wadden Sea for moult, wintering and to build up fat reserves before migration to distant breeding and wintering grounds. The majority, approximately 10 million, are migratory birds, which refuel in the Wadden Sea during spring and autumn migration.

Half monthly totals of birds in the Schleswig-Holstein Wadden Sea area (Credit:  Schleswig-Holstein Wadden Sea National Park)

Half monthly totals of birds in the
Schleswig-Holstein Wadden Sea area (Credit: Schleswig-Holstein Wadden Sea National Park)

We will update this article when the final report is published.

Past Accidents

The BFU choose to highlight three past accidents:

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