HAV Airlander 10 incident in 2016
On the 24th of August 2016, a test flight of the HAV Airlander 10 registered G-PHRG ended badly. Well, the test flight itself was successful. With aircraft, take-off and landing are the critical phases of flight, while most airship issues seem to occur after the flight is finished, when the airship is parking. Or rather, mooring. But I’m getting ahead of myself.
The HAV Airlander 10, designed for air cargo transport, is described as a hybrid mix of helicopter, aircraft and airship. HAV say that it can reach 16,000 feet and travel up to 90 miles per hour with the capability of staying aloft for up to two weeks. It can also carry up to ten metric tones (22,050 pounds) of cargo.
It’s an example of a lifting body, which uses aerodynamic principles to produce lift from the shape of its fuselage, rather than relying on wings to generate enough lift to carry the weight of the body. We looked at the principle before in the context of NASA’s M2-F1 which led to the design of the space shuttle.
I took two pages of notes on the subject which ends with a scribble of “I have no idea how this could possibly work” so I’m going to quote the HAV people on the physics:
The buoyancy of the aircraft is expressed in terms of static heaviness. A static heaviness of zero would equate to neutral buoyancy, such that the aircraft would neither rise nor fall unless acted on by other forces. In normal operation the aircraft has positive static heaviness, being overall heavier than air. Generally, static heaviness reduces as fuel is burned and it may also be affected by precipitation and by environmental warming of the helium gas.
Because it uses both aerostatic and aerodynamic lift, it is neither an airship nor an aircraft. As I understand it (which is to say, “barely”), the Airlander takes off by vectored thrust and once airborne, it transitions to forward velocity which creates lift over the body, keeping it airborne. The buoyancy of helium is used to reduce the amount of lift the Airlander needs. It receives 60% lift from buoyancy, 40% lift from aerodynamics and 25% from vectored thrust, where the vectored thrust seems to be used only during take-off and landing.
Shareholder Bruce Dickinson (who figured hugely in my teenaged years as the lead singer from Iron Maiden but he seems to have moved on since then) explains it like this:
Barnes Wallis R100 airship of the 1920s was a great design but was limited by the technologies of the day. The construction materials were inadequate, the engines were heavy and inefficient, flight controls were cumbersome, radar didn’t exist and navigation and weather forecasting were still at an early stage of development. In later life, Barnes Wallis wrote a note for Roger Munk, saying: ‘Solve these problems and the airship will become an efficient and viable mode of transport.’ The issues to be tackled were stability and flight control, structures, increased payload, more powerful engines, improved capabilities in poor weather and forecasting and easier ground-handling. Now we have the technology to revisit this fundamentally sound design to make it efficient and make it work.
The first test flight of the HAV Airlander took place a week earlier, on the 17th of August. The Guardian waxed poetic:
Above a field in rural Bedfordshire, a shiny, futuristic craft the size of a football pitch ascends majestically into the evening sky, and gawping onlookers crane their necks for a better view. This could be the trailer for the latest Independence Day film, but it is the maiden flight of the Airlander 10, a helium-filled craft aiming to kickstart a new age of the airship.
It has been a while coming – the first flight had been delayed several times and Wednesday’s takeoff was held up for hours – but once in the air, showing off its curves as it banks and soars for its audience, the Airlander is quite a spectacle.
The second test flight took place on the 24th of August. There were two flight crew, a test pilot and a flight test engineer, on the flight deck of the Airlander, a 15-foot long area with four floor-to-ceiling windows. The Airlander was originally designed for unmanned flight so the cockpit on the flight deck was added later. The cockpit has Garmin avionics and a GNS 430 GPS/nav/comms system. There is also a screen fed by cameras installed 15 feet ahead of each engine to allow the pilot a view of the engines, which aren’t visible or even audible from the cockpit. The pilot controls the Airlander with a side stick on the right side.
The flight crew were supported by two teams on the ground, the flight test team and the handling team. The Airlander unmasted at 08:12 for the flight. There was a gentle breeze of around two knots. The crew went through a sequence of test flight tasks and declared the flight a success after about 98 minutes. They returned to Cardington.
Cardington is interesting in its own right. The site of the airfield is a former Royal Air Force station in Bedfordshire, England; this is where the first British airships were built during and after the first world war. Short Brothers, an aerospace company founded in 1908 in London, was the first company in the world to make production aircraft. In 1916, they received a contract to build two large dirigible airships for the Admirality, the British government department responsible for the command of the Royal Navy (its functions are now carried out by the Navy Department of the Ministry of Defence). A 700-foot long (210 metres) airship hangar was built in 1915, which was known as the No. 1 Shed. A housing estate for the employees was built nearby, which is still known as Shortstown.
The airship site was nationalised in 1919, becoming the Royal Airship Works. Between 1924 and 1926, No. 1 Shed was extended for the R101 project, raising the roof by 35 feet and increasing the length to 812 feet. The No. 2 shed started life in Norfolk but was dismantled and re-erected at Cardington.
The R101 crashed during its maiden voyage in October 1930, destroying the aircraft and killing 48 of the 54 people on board. The British government cancelled further dirigible funding and the German Zeppelin Company, purchased the duralumin from the wreck, possibly to re-cast and use in the Hindenburg. Cardington became a storage station.
However, it came back into play in 1936, when the RAF used the site for building barrage balloons: tethered kite balloons meant to defend ground targets from aircraft attacks. The British Balloon Command was established in 1938; barrage balloons were used to protect key targets from dive bombers by forcing them to fly higher. Cardington became the main RAF Balloon Training Unit and by 1940 there were 1,400 barrage balloons in place, a third of them over the London area.
Germany began to use more high-level bombers and the effectiveness of the barrage balloon waned, but Cardington remained busy as it became the home to the RAF meteorological research balloons training unit in 1943 until 1967. The sheds appear not to have been in use, however, as the fence around the site was moved, leaving the sheds outside of RAF Cardington.
The Met Office is still using the site for a Meteorological Research Unit, with a tethered balloon kept in a small, portable hangar. In 2007, English Heritage added No. 1 Shed, by now known as Hangar Number 1, to the At Risk Register. The hangar is a Grade II listed building as the only in situ example of an airship hangar to have survived from the pre-1918 period, however the cladding was failing and the condition deemed to be “very bad”.
In 2015, Hybrid Air Vehicles reopened the completely repaired and refurbished hangar and brought their workforce there to completely development on their hybrid airship, with a press release inviting the public to look out for the Airlander 10 in the skies of Bedfordshire in early 2016.
Where was I? (Actually, I’m thinking about writing a new book in which I collect all these pieces where I fall down a research rabbit hole and highlight them instead of the main story. I would call it What Was I Talking About Again?)
On that day, the 24th of August 2016, the HAV Airlander had completed all of the planned tasks for the test flight and was returning to Cardington. Traditional airships were lighter than air which meant a large ground team was required to lasso them down to the ground. The Airlander is able to make itself heavier than air in order to land more effectively. However, it’s not capable of a true vertical landing: the Airlander can’t just drift over the landing area and then drop down. In order to land, the Airlander approaches the landing area with some forward airspeed, touching down with a short landing roll as the aircraft naturally slows to a stop.
The flight crew brought the Airlander in for landing.
The mooring line is normally stowed within the Mast Mooring Interface, the housing for the TMM, a towable trailer with a telescopic mooring mast which secures the Airlander to the ground, similar to a buoy for a boat. In the stowed position, the free end of the mooring line is attached to the aircraft at a point just below the flight deck windscreens. As the Airlander lands, the ground crew members detach the free end of the mooring line and pull the line out of the Mast Mooring Interface and feed the free end into a winch at the mooring mast. This allows the Airlander to be winched into the mast and secured.
The ground crew extracted the mooring line from the Airlander’s stowage, ready to feed it into the winch at the mooring mast assembly. The mooring mast assembly is driven by hydraulic pumps which are powered by a diesel engine. But the diesel engine wouldn’t start: the starter battery had been drained by an electrical fault.
Now they had a problem: if they couldn’t start the engine, they couldn’t winch the Airlander to the mast.
The test pilot decided to gain altitude in what seems like a very slow version of a touch and go.
There’s no proper storage for the mooring line once it has been extracted from the MMI. The flight crew quickly found a place temporarily stow it through a small access panel in the cabin door.
As the Airlander left the ground, the mooring line fell free under its own weight until the full length was hanging down, about 155 feet (47 metres).
The ground team warned the test pilot that they had about 50 feet of the mooring line dangling from the Airlander, apparently not realising that the full line had pulled free. The pilot remained airborne while the ground team resolved the issues with the diesel engine and the winch. Then, mindful of the mooring line swinging beneath him, he flew a steeper approach in order to avoid the line getting tangled in the trees or the perimeter fence beneath the airship. Effectively, he was trying to get as close as he could to a vertical landing in the very light wind conditions.
The pilot believed that the rope was only dangling about 55 feet and thus would clear the power lines which crossed his approach path, which were about 120 feet above the ground. The mooring line swung into one of the high voltage power cables. The test pilot managed to free the line from the wire but now the Airlander was at around 180 feet, high for the final approach. With no forward airspeed, the pilot had limited control. He trimmed the Airlander nose-down in hopes of bringing the mooring line within reach of the ground crew. This is done by using ballonets: bags in the hull which can be inflated with air to increase the mass of the airship and compress the helium. CTO Mike Durham explains:
As you go up in altitude, the air wants to expand and you can’t cope with trying to contain it with the strength of the hull, so the helium pushes down on the ballonets and pushes air out through valves. When you come back down the helium wants to contract, so the ship would go soggy unless you push air back into the ballonets. So each ballonet has a big valve and fan in it so can vent air in and out and run the ship at a constant delta p. It’s the one system on the vehicle that’s got no parallel to any other aircraft or helicopter.
The key point here is that you can use the ballonets to adjust the centre of gravity; if you only inflate the ballonets at the front, the bow becomes heavier which pitches the nose down.
The pilot had the Airlander pitched to about 10° nose-down when the airship suddenly lurched to 18° nose-down and started to descend. The pilot attempted to reduce the nose-down angle but he couldn’t slow the descent. The Airlander crashed nose-first into the ground, causing structural damage to the flight deck.
The flight crew were evacuated through the gaps in the damaged flight deck and the HAV was attached to the mooring mast and secured.
The gentle crash took place at a somewhat different speed than we are used to in aviation, as you can see in this video (readers on the mailing list may need to click through to the website to view). The video footage begins with the Airlander attempting to correct before eventually coming down with a bump. One person described the attitude of the Airlander as “a very slow motion tumble”, with the nose too low and the back end too high.
Hybrid Air Vehicles said that the repairs should take about three to four months, however some of the specialist tools they needed for the repairs had been scrapped by the US Army and would need to be remade, which would “contribute significantly to the estimated time required”.
HAV ran their own investigation on the test flight and submitted the results to the AAIB who referred to it for their special bulletin for March 2017.
The investigation determined that the higher than planned approach had been necessary in order to allow sufficient clearance for the mooring line. In turn, the mooring line had come to be hanging free below the aircraft because there was no proper stowage facility for the mooring line once it has been extracted from the MMI. The high approach led to a situation that was considered outside of the aircrafts normal operating regime, in that it required a controlled vertical landing in very light wind conditions. The static heaviness of the aircraft had probably reduced as a result of additional environmental heating occurring between the first and second approaches.
Static heaviness is the ratio of buoyancy to gravity. To increase altitude, you need to reduce the craft’s static heaviness, to allow it to rise. To descend, you increase the craft’s static heaviness. It seems like until this point, there wasn’t a specific consideration of naturally occurring changes to the static heaviness over short periods of time.
The internal investigation concluded with a number of recommendations including:
- effective stowage and control arrangements for a deployed mooring line
- review of relevant static heaviness issues
- enhanced maintenance and fault reporting regimes for ground support equipment
The flight crew on board for the test flight walked away with no injuries and the cabin survived the low-speed incident. Hybrid Air Vehicles decided to design a new landing system to allow the Airlander to return to the ground from a greater range of angles, as well as a system for easier recovery of the mooring line.
The repairs were quickly completed, however in November 2017, the Airlander 10 suffered a second accident and this time, HAV decided it was not worth restoring the prototype. This meant that HAV was an airship company without an airship, and at the time it seemed questionable whether they would continue. However, HAV is still around, now with a focus on mass production opportunities and with new funding from a number of sources.