On Sunday we watched a helicopter land at Shoreham Airport, a small airport used mostly by light aircraft. We’d seen the process before, but had kind of assumed we were watching flying lessons; however this appeared to be a regular straightforward landing, but it followed a very particular process.
Here’s a satellite view of Shoreham Airport. The helicopter flew in at height, then on the western perimeter of the airport, hovered and descended to maybe 10 meters above the ground. It then maintained this height as it tracked over the runway, all the way across to the far south east of the airport, and landed in the helicopter pound, just north of the row of buildings (above the word “Cox” in Cox Powertrain Ltd) and just inside the perimeter road. (Incidentally, if you view using this link you can zoom in and see a single helicopter parked in the pound. Just look in the bottom right corner of the field: Google Maps )
I assume there must be a reason for following this routine. I found out on this board about the column-of-descending-air that a helicopter produces, and how this is a danger to light aircraft for a surprisingly long time – were we watching some sort of ground effect that mitigates this? And was the descent simply done as far away from people, buildings, roads etc as possible? On the face of it, tracking across a field at very low altitude doesn’t seem very safe - you would have to have a reason to do it, right?
Not sure what you mean. We’ve seen helicopters do the track across the field on numerous occasions - but we were watching from a different vantage point on Sunday, and realized that it was part of the landing process.
Yeah, it’s a small airport and they certainly do flying lessons there. Hence our earlier assumption that we were watching flying lessons - sort of, get it off the ground and hover a bit, move over there - that sort of thing. And it’s possible that this landing was also a flying lesson with some maneuvering practice at the end.
Possibly - but this still leaves me wondering - why?
A helicopter pilot explained to me that maneuvering a helicopter for landing is best done at a very low altitude. So is taking off, you should see helicopters climb a little than transition to horizontal flight to pick up speed before climbing. If anything goes wrong in takeoff or landing it’s better to be close to the ground. It’s not easy to land right on the circle from altitude and descending to the ground with unpredictable air movement around you is not the time and place to be correcting it’s position.
I know nothing about flying a helicopter - my completely uninformed first thought is that the thing you’re most likely to crash into is the ground, so why do you want to be so close to it? If the answer is “For a soft and easy emergency landing if needed”, then I guess that makes sense.
No, a “hover taxi” to the final parking spot on an airfield is a safe and normal procedure. Aside from traffic management considerations, it’s normal practice at a training airfield to shoot the approach to a wide open area free of obstacles, and then hover taxi as required.
The height/speed safety considerations are:
(1) Power requirements. A high hover requires the most power. Power requirements are reduced by either ground effect (when close to the ground) or translational lift (with forward airspeed). A standard approach profile is therefore at around 10 degrees, gradually bleeding off speed, with the objective of entering ground effect just as speed decreases to a point where translational lift is lost.
(2) Engine failure. There are two ways to obtain enough lift to land safely without power. In a stationary low hover or hover taxi (no more than a few feet off the ground), there is sufficient rotational inertia in the rotor blades alone to increase their angle of attack and obtain a small amount of lift to cushion the immediate inevitable landing. Alternatively, from a greater height in autorotation with sufficient forward airspeed, the much greater inertia of the forward motion of the entire aircraft is used to obtain sufficient lift to reduce the rate of descent to near zero at touchdown, converting forward airspeed into lift in a landing flare. This implies a “dead man’s curve” in a height vs airspeed graph: avoid an unsafe region at low airspeed between between about 20ft and 800ft, where the rotational inertia of the rotor blades is insufficient and where there is not enough height available to convert gravitational potential into the forward airspeed required for a landing flare.
Under these two safety constraints, an expert pilot can smoothly shoot an approach at about 10 degrees to reach a precise aiming point at a few feet off the ground at zero airspeed. But it’s normal to give a student pilot much more obstacle-free room to maintain forward airspeed as necessary, so that if they misjudge the approach they do not end up in a dangerous stationary hover 50 feet off the ground, at high power (and potentially running out of power) and inside the dead man’s curve if the engine fails. Once safely in ground effect and out of the dead man’s curve in a low hover, a hover taxi to the parking spot is quite safe.
I’ve described the reason for shooting the approach to an open area, but if your description of the approach profile is accurate, either it’s a student pilot who fucked up the approach completely, or it’s an expert pilot in a twin engined helicopter where the safety concerns I described above are less critical.
To add: even an expert pilot in a twin engined aircraft with ample power will avoid coming to a high hover out of ground effect and making a vertical descent unless obstacles make it absolutely necessary, because of the danger of vortex ring effect.
In the United States, helicopters are instructed to avoid fixed-wing traffic. This usually means that there is an accepted traffic pattern for helicopters that is separate from the traffic pattern for fixed-wing aircraft. For example, I mainly flew out of Van Nuys (VNY). The helicopters (and other aircraft) are based on the west side of the airport. Approaching from the east, helicopters would cross the runway at 90º at mid-field. We would then make our approaches to the taxiway. Approaching from the west, we’d turn toward the taxiway before reaching the runway. Landing at William J. Fox airport in the Mojave Desert, I contacted the tower and they told me to approach from the south (opposite to their right-hand traffic pattern for the runway in use), and to land in any non-movement area (‘the ramp’). I’ve never flown a helicopter to or from an uncontrolled airport.
Having said all of that, and not knowing about flying in the UK, I would assume that helicopters at that airport follow, or are sometimes directed to follow, the same traffic pattern for fixed-wings. A fixed-wing aircraft will touch down at the approach end of the runway, then ‘roll out’ to the taxiway. Shoreham appears to have taxiways at either end of the runway, rather than having one or more exit midway. Helicopters don’t need to roll out. If they are following an established traffic pattern, they need to get to the other end of the runway.
There are two kinds of taxiing in a helicopter: Hover taxi, and air taxi. Hover taxi is done below about 25 feet (8 metres) above the ground, and at a slow speed. This is useful for short distances. If you need to go farther, an air taxi would be more than 25 feet (so 10 m as in the OP is right), and at a higher speed.
Why not just ‘land long’? It could be simply that if helicopters and fixed-wings are using the same pattern, aircraft are where aircraft are expected to be regardless of type. Why not avoid the pattern and use a separate pattern to get to the heli area? Looking at the map, this might require helicopters to operate too close to the end of an active runway.
I don’t have time to do a lot of research, but here are traffic pattern maps for EGKA:
I think I’ll have to answer this in installments. Thanks for the highly detailed reply.
Backgrounder: here’s a satellite picture of the airport building Google Maps
Just in front of the building, on the left, immediately adjacent the taxiway, you’ll see a couple of rows of square objects. Those are tables. There is a restaurant in the airport, and you can sit at those tables for lunch - it’s ridiculous how close to taxiing aircraft you are. Great fun. When we have seen this hover-taxiing before, that’s what we were doing - you really don’t get a view of helicopters descending from there (or at least, we have never noticed them).
On Sunday, we were standing on the levee alongside the river (far right of the first satellite picture, so around a km from where the descent took place). I didn’t take note of whether the descent was vertical or not - I wasn’t thinking that technically, and we were much more interested to watch the hover taxi. I was estimating the hover height from the blades-to-wheels height of the helicopter, and I placed the flying height at double, or perhaps a little more. Thinking about that now, I guess that would have been less than 10m - but certainly not a couple of feet. It could have been less than 20 feet.
Not exactly - at airports you don’t aircraft of different capabilities mixing. It’s fairly common for fixed wing aircraft to have one set of traffic patterns and rotorwings (helicopters) a different one. Potentially, you could have a couple more than that but I don’t want to bog down the thread.
To some extent this also applies to fixed winged aircraft, and very much to people using parachutes, the concept that you’re safer either very low or up high, but in-between doesn’t give you many options and very little to no time to actually do anything in an emergency but, again, I don’t want to get too bogged down here.
I wasn’t the one flying the rotors, of course, but I have shared uncontrolled fields with rotorcraft. The practice there is, again, for the rotorcraft and fixed wing aircraft to keep apart to avoid confusion and Unfortunate Encounters of the Too Close Kind.
Some of the uncontrolled fields I’ve used had fixed wing, rotor, glider, and ultralights of various configurations all sharing the same field and airspace. It sounds confusing and chaotic, but rather than “uncontrolled” we preferred to call it “pilot controlled”. Having separate areas/traffic patterns actually helped keep things orderly.
Think about a busy intersection of two roads - you might have car/truck traffic, bike lanes, crosswalks for pedestrians… everyone is using the same intersection but different things are separated due to different speeds and other characteristics which actually makes things safer and more orderly than if everyone was in the same lane.
FWIW, most of my landings have been off-airport. Helicopters have their own frequencies, and are required to give the occasional position report. If I was flying into the Santa Clara River wash (west of Magic Mountain), I’d listen for other helicopters operating there, and say my direction and altitude whether anyone’s there or not. Taking off, the practice was to come into a hover, yaw to look downwind, and when clear, radio ‘[4-1-3] on the go from the Wash,’ before taking off.
ETA: I’ve heard uncontrolled airports called ‘out-of-control airports’.
Man, I’ve learned a lot about how helicopters work.
It almost certainly wasn’t. We’ll probably walk the levees again in the next couple of weeks, and I’ll be looking for landing (and indeed, taking off) helicopters. Approach angle, hover height, duration of hover taxi.
j
BTW I should say a big Thanks! to everyone - this has been a real education.
