AAIB (H) UK October 2020
 

BK117 HEMS departing off-airfield site manages to lift roof of caravan on a nearby site: here - comments that the trend to use and larger aircraft in this role may lead to an increase in this type of incident.

Well, its still a BK117 on the paper ;-)
But looking through the development of AUM - with not really increased rotor diameter - the downwash is quite more remarkable.
We made trials with high hovering after getting claims about damage done by one go our helicopters - and even experienced pilots were surprised, how much wind is generated in a 200 feet hover.
(Hovering was done above a grass area in front of our hangar (with concrete dispersal) and open hangar doors - and it was windy in the hangar....)

It’s possible the roof of the caravan itself wasn’t in the prime of life; they do rot around the edges. Last winter, on a very windy day I was driving on the M1 motorway and pulled out to overtake a “well used” static caravan being transported by low loader. I was about fifty yards behind it when the entire aluminium roof of the caravan lifted off, blew across three lanes on our side, sailed over the central barrier and wrapped itself around the front end of a white van coming the opposite way.

We tried to look at this at the ICAR Air Commission some years ago. Might have been the 2012 Congress. Someone had a diagram that mapped the downwash for a particular type. The President of the Commission was going to seek out other such mapping so that we could examine this aspect of the rescuers' working environment in an empirical manner. I think is was found that nobody had done the work on this and mapping generally did not exist.

We have often found that the SAR market is not large enough to warrant manufacturers' efforts in certain areas (even though their marketing teams love us). However, the HEMS market these days must surely large enough to warrant work on matter like this?

Jim, it comes back to the discussion on the SAR thread - powerful engines with reduced rotor disc sizes equals more powerful downwash - even on medium twins.

Way back in Borneo with our lowly Whirlwinds 10s when we were visiting a longhouse for intelligence gathering we used to fly around it a couple of times before landing. This was to give them time to organise all the children to line up along the veranda and hang on to the roof when we arrived.
We were going back to the UK so our area was handed over to the FAA.
'The Navy's Here' and they landed in front of the longhouse without warning. It took them a week to rebuild the roof, during the monsoon season.

No doubt the RAF prior to your visits had experienced a similar thing and learned from it...or were you clairvoyant? :oh: