The choice of helicopter for HEMS (Helicopter Emergency Medical Service) operations is a difficult one. Whilst cost is a huge factor, there are many subtle elements which go into the selection of a helicopter. This article aims to explore those options and present what an ideal HEMS aircraft would be.

In the near future, another option will also be opened up – the eVTOL. We will look at eVTOL for HEMS in another article.

Factors to think about

What needs to be considered when initially working out what the perfect machine is for HEMS? Cost is very important factor – aviation is an expensive business. But what is clear is we want the most capability for the least cost. What are those capabilities that we want? Here are the areas we are going to focus on:

• Speed
• Size
• Range
• Payload and cabin arrangement
• Safety
• Availability
• Skids or wheels
• Cockpit design

Speed

In emergency aviation, the need to do things fast is a given – people’s lives may be at risk and we want to save time to save lives. The need for speed is a lot more nuanced than that. Let’s break it down.

Starting up

The first aspect of speed that is critical for emergency aviation and HEMS in particular, is the speed to get us on our way. We want to minimise where possible the time between a call being received and the crew being airborne en-route to scene. We want a machine that is very quick to start, preferably with sufficient automation that we can just flip switches rather than jockeying throttles. Using an aid to starting such as an APU or GPU may add a delay to the start process which may be unwelcome in HEMS.

We absolutely want to avoid “dead” time during the start where we have to wait for sometime to happen before moving on. We do not want to be staring at a screen watching a “please wait…” message which prevents us for getting airborne. However, it may not be an issue if the boot up time can be used for other tasks – for example I do not need the navigation system while I start the engines, but I do need it before I lift.

We also do not want to have to program lots of “default” data into the system to initialise it. For example, I do not want to have program in the weight and balance in detail before getting airborne on a HEMS mission with a known crew and equipment weight. Often we can mitigate this by having weight and balance/performance functions on an iPad/EFB.

Overall, getting started and airborne quickly is what we want to aim for.

Transit Speed

Going fast is great. The faster we can get to the patient, the more chance we have of positively affecting the outcome. But speed is not free. Travelling at high speed may come with a sharp rise in fuel burn over a more efficient range/cruise speed for little actual change in arrival time. High speed might also come with a drop in ride quality and intrusive vibration or noise.

The need for speed is highly dependent on the circumstances. For a HEMS aircraft with a small operational area, raw speed is going to make less difference to the overall service than the time to start up. A high fuel burn rate and smaller fuel tank might make long range missions impracticable.

Although there are some issues with just flying flat out, a high transit speed is a worthwhile attribute in a HEMS machine.

Turnaround time

Getting started and flying quickly are great, but a mandatory 1 hour stand-down after each flight for maintenance might not be so welcome. Generally things are never this bad, but aircraft will need a period after a mission to reset and refuel before going off again. The length of this turnaround period is generally dictated by 2 aspects – fuel and maintenance.

Refuelling needs to be done quickly but safely. Most HEMS units have their own fuel source or access to fuel from the airfield they use. Maintenance requirements are typically minimal between flights but a very high oil usage rate and a commensurate need to top off the oil tanks would rapidly become an issue for HEMS.

So we want to minimise turnaround time of our aircraft – rapid refuelling and minimal maintenance time.

Sizing to meet the mission

Moving onto size, we need to think about 2 aspects to fully determine our needs: the size of the aircraft and the space inside the machine. There are conflicting demands for this mission and this aspect is hotly debated amongst crews.

External dimensions

The largest overall external dimension of the aircraft is generally from the rotor over the nose to the tip of the tail – this is called the D-value (see my article about D values here: https://rotarywinggeek.com/2d-or-not-2d-how-much-room-do-i-need-to-land-a-helicopter/). The D-value determines the size of sites we are legally allowed to land in and the size of any hospital sites. In general, the closer that you want to get to a patient, the smaller the helicopter needs to be. The minimum dimension for HEMS is 2D. An example of a 2.5D site is below:

So we want a small helicopter for HEMS. Well yes, smaller helicopters fit in small places, we generate less downwash and outwash (see the Vertical Flight Society’s repository on this topic here: VFS – Downwash & Outwash) and we burn less fuel. In general, the smaller the aircraft, the less maintenance and storage cost too. So an R22 for HEMS then?

Internal dimensions

Well no, the inside dimensions directly relate to the aims of the mission. The whole point of HEMS is to carry medical aid to patients at the scene. We need to carry a medical team and sufficient equipment to the patient.

We also may want to take that patient to definitive medical care at a hospital. Normally a patient is fairly stable by the time they are loaded into the machine but this is not always the case – the medical team may need to intervene during the flight so the patient needs to be accessible. An external pod just is not going to work – that’s the Bell 47 out of the running!

Let’s dive a little deeper into what we need. We need to carry the following and have an aircraft sized to match:

• A pilot – automation is great but a HEMS scene is so dynamic that automation simply cannot currently cope with all the variables and judgements that need to made during a landing.
• Two medics – the minimum effective medical team is 2 medics. Many procedures require 2 sets of hands and many others need cross checking of critical points. This is to treat only one patient at a time.
• A patient – A patient is assumed to be prone on a stretcher and be at the top end of height and mass. It is unsatisfactory to size the stretcher for Mr Average.
• Equipment – The equipment load is matched to the tasks. Typically this is in a collection of portable loads/bags.
• Access – We need to be able to access the patient and potentially certain parts of the medical equipment during the flight. A storage and mounting system is likely to be needed.
• Trainee/observer/checking officer – An extra seat is needed over and above the 3 person crew. There will always be a need for training of new personnel and checking of existing ones.

So big enough to carry everything but small enough to fit in small spaces is the answer.

Range

The operating area of the HEMS unit dictates the range requirements for the aircraft. An aircraft covering a small patch of ground with ample hospitals in close range has a very low range requirement. An aircraft covering vast areas of rural land with widely spread hospitals and refuel options have completely different requirements. So the answer to what range is required is “It depends!”.

What we can determine is that more range is good in general. Even in a small operating area, more range gives the opportunity for more missions before refuelling and more options while airborne if things go wrong (weather!). So we should fuel up to the maximum mass every flight then?

Actually no. In HEMS, sometimes mission can happen very close by. If we always operate an maximum weight, for a really short mission we may find ourselves on scene and too heavy to take the patient to hospital. Therefore, on HEMS missions, a fuel load is carried such that a patient can be carried even on a short mission. Alternatively, the fuel that needs to burnt off can be completed in a short time (maybe 5-10 mins of solo orbits by the pilot!).

So for range, we want as much as possible, but keeping size in mind as discussed already.

Payload and Cabin Arrangment

We have already explored some of the internal carriage requirements when discussing the internal dimensions of the aircraft. Size is one thing, but the mass is even more critical. Let’s build a typical payload for a HEMS mission.

• People
  • Pilot – 100 kg with kit
  • Paramedic – 100 kg with personal kit
  • Doctor – 100 kg with personal kit
  • Patient – 100 kg
  • Trainee, observer – 100 kg with personal kit
• Fuel
  • Minimum 1 hr 15 plus 30 min reserve – typical for european/UK HEMS
• Role kit (EC135 in this example)
  • 125 kg for fixed fittings (floor, racks etc)
  • 117 kg for carry on equipment (medical bags, monitors etc)

That’s a total mission load of 740 kg plus fuel. For a example aircraft (eg H145) the fuel load is 350 kg of fuel. So a total load of 1090 kg for the HEMS mission. To allow comparison with eVTOL we need to keep both of these figures in mind as the “fuel” in an eVTOL is part of the basic mass.

So we want to lift 740 kg with our fuelled up machine and more is better (more fuel, more kit, more medics).

Furthermore, we want a big cabin. Some light twins are used frequently for HEMS but have small cabins. In some aircraft, average sized people cannot sit upright in certain cabin seats (EC135 you know I am talking about you!) and some aircraft require the left cockpit seat to be moved or rotated to allow the medic team to work with the patient in flight. Larger aircraft, like the AW169m provide the capability to reach both sides and top and bottom of a patient in flight. This may be unnecessary in HEMS operations with limited flight duration and a medical team capable of stabilising the patient or going by road, but might be critical in longer range operations in adverse terrain where road transfer of the patient is not possible.

It’s safe to say, that the cabin arrangement is very much a “It depends” part of the choice of HEMS aircraft.

Safety

Safety is a priority for HEMS but balanced against acceptable risk in order to achieve the mission. Mitigations must be put in place against hazards to keep the overall risk to a tolerable level.

Two engines please

One specific risk which strongly influences the type of machines used for HEMS in the UK and Europe is an engine failure. When operating over congested areas, substantially used for residential, recreational, commercial or industrial purposes, we need to be able to tolerate a single engine failure. This immediate puts otherwise hugely capable single engine aircraft out of the picture for many HEMS missions. It also provides a minimum size for our aircraft as it is not commercially viable for manufacturers to produce very small twin engine machines.

Automation and technology

HEMS operations can be extremely challenging – extreme operational pressure, poor weather and a complex obstacle environment. A degree of automation is invaluable for improving safety and providing room for holistic thinking and planning ahead. A HEMS landing or take off is not the time for a macho display of manual hovering prowess if a suitable auto-hover mode can provide a low workload way to safely do the same task but allowing the pilot to look around. Appropriate automation also allows operating in degraded visual conditions with the certain knowledge that a full automated go-around and instrument approach is a button push away.

At night, NVIS is a key technology for safe continued operation to ad hoc landing sites as determined by the needs of the patient. For an operating area where night missions are expected, the machine must be capable of integrating with NVIS.

Downwash

The last aspect to be considered on safety is downwash and rotor wakes. Downwash in emergency aviation has killed people and caused lots of damage. It is a key consideration in landing site selection (see my article here – https://rotarywinggeek.com/hems-landing-sites-reliable-places-to-drop-your-medics/). For a HEMS aircraft we want our downwash to be as small as possible, both in terms of the area affected and the speed of the flow. This is intrinsically linked to the rotor system and mass characteristics of the aircraft. In broad terms, the smaller the aircraft, the less downwash is produced. So within the bounds of what we have already looked at, smaller is better.

However, it is more subtle than that. As discussed by Richard Brown, the qualities of the downwash are also important (Sophrodyne_eVTOL_Downwash.pdf). Is the downwash pattern predictable for the pilot? Can the pilot have reasonable certainty about how the downwash will behave in certain conditions and therefore operate the helicopter to minimise its effects? Is the downwash so turbulent and unpredictable that huge margins must be put in place around the aircraft making a 3D or 4D sized site necessary?

However, in simple terms we want to minimise our downwash.

Availability

The last aspect to look at in our perfect HEMS aircraft is the availability of the aircraft to complete it’s mission. You do not want to have the call come in and politely have to decline because your aircraft is grounded by a bit of light rain! There are several aspects to consider here:

Maintainable

We want our HEMS machine to be maintainable in the field. Ideally we want long periods where it can remain online with just some oil top ups and pilot inspections. Short times on components requiring frequent engineering support would be very problematic. Any periodic maintenance needs to be covered in some way to maintain the capability which might involve a spare aircraft, cars or sharing coverage with other operators. Minimising these engineering requirements is highly desirable. The availability and timely delivery of spare parts also needs to be good, along with sufficiency of engineering support.

Crewing

A pilot is essential for the HEMS mission but to maintain coverage for the mission, generally several pilots are required in a rotating roster pattern to cover the designated times. This is compounded for multi-pilot operations or extending operating hours (eg 24 hr ops). These pilots need cover for leave and illness and need to be trained by competent instructors and examiners. To keep availability for the mission up and to improve safety, effective simulators need to be available.

These factors can strongly influence the choice of aircraft. Where a popular machine is flown, availability of pilots and simulators might be abundant but equally the turnover might also be high with pilots taking their skills onto new challenges. Conversely for a type with fewer operators, there may be a more challenging environment in terms of training asset availability. This also applies when introducing a new type until it gains a wider community of people with experience on it.

Weather

An aircraft with the capability to operate in all weathers including icing would help improve availability for the HEMS mission – to a point. For HEMS, the visibility must be such that we can see the ad hoc landing sites we use so operating HEMS in fog is just never going to happen. However, the capability to escape safely in poor weather means that a crew can push just that little bit farther in trying to reach a pilot in the middle of the mountains.

The ability to operate in high temperatures or altitudes may be critical for some regions. People can do all sorts on things half way up a mountain that needs HEMS support and the aircraft needs to be chosen to match the mission requirements.

Generally as twin engine is a minimum requirement, the capability to operate IFR is bundled up in the equipment fitted so there is not a great deal of variability in aircraft capability.

Skids or wheels

The great skids or wheels debate is guaranteed to spark some lively conversations in the crew room. Wheels potentially bring easier ground handling and the ability to ground taxy (important in mixed fixed wing/helicopter airfield environments). There is also the potential to reduce drag in high speed flight if the wheeled gear is retractable. However, wheeled gear brings complexity to the aircraft and the possibility of forgetting to deploy it. In addition, the nature of HEMS operating sites mean that wheeled aircraft can sink into soft ground and require “digging out”. Anecdotally, this is a regular occurrence in some wheeled HEMS fleets with wheeled machines. This is less likely to happen with skidded aircraft, particularly if they are fitted with load spreaders or “Bear paws”.

Wheeled aircraft can also be fitted with load spreaders to prevent the sinking issue but normally these inhibit the retraction capability. The drag is still there leaving only the advantage of slightly easier ground handling. A capable ground handling machine can of course make moving a skidded aircraft really simple so wheels are perhaps not a benefit at all!

It is very interesting to note that both for the AW109 and AW169, Leonardo has developed skidded versions due to customer demand. The Bell 429 wheeled version has seen very limited pick up overall and never (to my knowledge) in HEMS roles.

Cockpit Design

HEMS pilots typically do not spend long periods in the cockpit so design deficiencies of the space in terms of comfort can be overlooked. However, the usability of the cockpit in terms of visibility and real estate for avionics can make or break a HEMS machine. Large door and window posts or poor headroom around the crews heads can make observation of obstacles and other aircraft very challenging. For example on AW109 variants, the cockpit structure around the cockpit door is quite close to the pilot’s head making it difficult to use NVIS without knocking them off the pilots head and potentially obscuring certain arcs of view.

The cockpit also needs to make provision for an EFB tablet computer. This is a vital part of a HEMS crews’ arsenal and adequate stowage for the device in secure mounting is vital for keeping the crew’s heads out. A kneeboard mount is not adequate! Some mounting solutions appear to block safety equipment – see below. Much better in place of a “chart holder”. And please please please can I have a charging point (where the wire does not get in the way of the controls).

So in conclusion, a nice cockpit please and somewhere to put my iPad that is not on my knee or in my way! Also power in flight is really useful.

Conclusion

So we have looked at a wide variety of factors which determine what we want in a HEMS machine. Let’s summarise a bit:

• Speed – we want to start quickly, travel fast but efficiently and turnaround quickly
• Size – large enough to carry our required payload but no bigger
• Range – matched to the operating area
• Payload – large enough for our medical team and to carry a patient with sufficient fuel for the mission
• Safety – tolerant of engine failure with minimized downwash and appropriate automation
• Availability – maintained and crewed, whilst being capable of operations in bad/extreme weather
• Skids or wheels – My vote is skids for HEMS – far simpler.
• Cockpit – Comfortable with safe EFB integration

I know which of the available machines I would choose for HEMS if given the choice. What is your experience?

Next time we will look at how eVTOL match up to these requirements.

Why not take a look at my other articles?

• The Ultimate Medical Helicopter – Selecting the right machine for HEMS
• Aerial teamwork – Practical tips for working together by staying apart in emergency aviation
• HEMS Landing Sites – Reliable places to drop your medics
• Planning to fail – The perils of ignoring your own advice
• 2D or not 2D – How much room do I need to land a helicopter?