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In my answer to this question about fuel dumping I included the following rhetorical question:

Or if you're dumping fuel from a helicopter (does that ever happen?) it could hover over a tank on the ground, pump out the fuel and then land.

Two commenters said that helicopters should never need to dump fuel because if they can take off with a full fuel load then they can land again. Makes sense, but this news story refers to a helicopter dumping fuel in order to land, although it's only one article and the general (non-aviation) press is notorious for misreporting aviation incidents.

So, can/do helicopters ever dump fuel as a safety measure before landing?

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Yes

07-NOV-1971

Sikorsky CH-124A Sea King (S-61B)

Suffered single engine failure on take-off. Pilot attempted to return while also dumping fuel.

From what I've read, fuel-dump systems are more common on military helicopters.

Sometimes, a helicopter may not have enough power to hover in ground effect for a normal landing or take-off because of loss of an engine, high density altitude, or being overweight. The alternative procedure is to perform a running take-off or running landing, which is an advanced, but not an emergency, maneuver.

But if the copter is landing in a confined area, such as a small pad on a frigate in a stormy ocean, there's no room for a running landing, so its better to dump fuel to allow the copter to hover into a landing.

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  • $\begingroup$ Military copters are far more likely to lose power in flight than civilian ones. Nobody's flinging shrapnel or bullets at civilian choppers. $\endgroup$ – Loren Pechtel Oct 30 '14 at 4:16
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    $\begingroup$ can sometimes take off by flying forwards. Several effects come into play. Reduced torque required on the tail rotor as speed increases since the tail assemblage provides lateral stability so more power to the main rotor. "Translational lift" as air flow over the rotor disk increases really kicking in at about 20 kts. If you accelerate slowly forward, without increasing collective pitch, the helicopter will start to climb as translational lift increases. Reduced power as speed increases up to about 65 kts. It's a standard manouveur taught during the PPL. $\endgroup$ – Simon Oct 30 '14 at 8:54
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    $\begingroup$ My 8th grade history teacher was a Huey pilot who had just returned from Vietnam. He said that the choppers leaving on combat missions were very heavily loaded, so on hot days they would have to use the forward momentum method to take off. They would do this by tying them to jeeps and pulling them down the runway. $\endgroup$ – Carey Gregory Oct 30 '14 at 20:34
  • $\begingroup$ Something we didn't know until the end of the cold war... the Soviet MI-24 Hind attack helicopter always did a running takeoff. It didn't have enough power to take off vertically. It could hover only for maybe ten seconds, before the over boosted engines would be damaged. $\endgroup$ – tj1000 Jun 23 '18 at 16:52
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Helicopters can take off when too heavy to hover, either by moving forward while in ground effect or by reducing the pitch of the anti-torque rotor to get a bit more power into the main rotor (there's a book called Chickenhawk by Robert Mason which describes using both of these processes on Hueys in Vietnam - not a bad read IMO). It's risky to use either of these methods in the first place, but there's added risk as landing overweight is so dangerous. So in helicopters which may have to take off overweight being able to dump fuel is a good safety feature.

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    $\begingroup$ "too heavy to hover" and "moving forward while in ground effect" are contradictory. you can't move forward in ground effect if you are not in a hover. what you're talking about (incorrectly) is a running take-off, and it is a normal (albeit advanced) maneuver, not an emergency maneuver, taught to every private pilot. (faa.gov/regulations_policies/handbooks_manuals/aviation/…) $\endgroup$ – rbp Jan 4 '15 at 16:36
  • $\begingroup$ @rbp It reads clearly to me: You may be "too heavy to hover" outside of the ground effect zone. So you hover in ground effect, then before leaving ground effect zone get forward velocity, then pull up. Landing by doing the reverse is harder. $\endgroup$ – Yakk Nov 13 '17 at 19:10
  • $\begingroup$ Define “ground effect zone” $\endgroup$ – rbp Nov 13 '17 at 19:13
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I have never crewed on a helicopter that dumps fuel, nor have I known of any machine in the fleet that even has that capability - though I have been wrong before.

It has been noted already that helos can take off and land at max Gross Take Off Weight (GTOW), or all up weight (AUW) and thus do not require weight shedding to land safely (as we don't have wings to store fuel so we don't have to worry about over-stressing them on landing). However, keep this in mind: different fuel loads alter the emergency course of action in the case of a forced landing.

When considering helicopter performance (and we will talk in terms of torque), we explore 2 main ideas - power required and power available.

Power Required

Power required is the amount of torque that I need on the main rotor to overcome gravity and get into hover flight. Hover flight is always the most power hungry because i'm drawing air down into the rotor in order to create the smooth fast airflow over the blades and then pushing it down to the ground. There are two modes of hover flight - Hover in ground effect (HIGE) and hover out of ground effect (HOGE).

Hover In Ground Effect

Hover in ground effect is where you are close enough to the ground (around 4 feet) that you draw the air down through the rotor disk and it pushes in into the ground. In accordance with Newton's Third Law, as the downwash pushed into the ground, it also pushes back towards the rotor, creating a high pressure "bubble" beneath the helo, and reduces the recirculating air on the rotor tips.

Hover Out of Ground Effect

Hover out of ground effect is where we are roughly over a rotor span in the air - say around 50 feet. We have now lost that cushion of air beneath us because there is no air trapped between us and the ground. It simply recirculates around and around.

Sometimes a picture is worth a 1000 words:

enter image description here

As I hover in ground effect, I get a bit of help because I have that bubble helping keep me in the air, meaning my rotor has to do less work (really its less AOA or pitch pulled from my collective), whereas I don't have that help out of ground effect. The difference (at least for the bell 412) is around 11 percent torque. This means at a given take off weight, I will require 11 percent less torque in ground effect than out of ground effect to maintain hover flight.

Getting back to power required, this is the torque needed to lift the sum of all the weight on board my helo (and the helo itself). This means the more cargo + more fat bodies + more gas in the tanks = more power required. This is going to affect my HIGE and HOGE torques, along with my minimum safe airspeed (which i'll cover later).

Power Available

Power available is the second half - this is what the engines and rotor can produce in terms of power and thrust/lift. The 2 main factors are altitude and temperature. We measure these in terms of torque as well - mast torque for the rotor and engine torque for the motors - but for now we are only going to focus on mast torque. Essentially, this is the amount of final drive power we have in our pocket. Air is power - so the more air we have, the more torque we produce. Cold air at sea level in the Arctic will produce more power than hot air in the mountains of Afghanistan - see density altitude if you want to learn more. For a given altitude and temp, I will be able to produce a certain amount of power measured by rotor torque percent. A cold day I could get 100% mast torque. A hot day I might only get 85% mast torque because I've run out of engine turbine temp to produce power or I've run out of collective pitch because my rotor blades are at max pitch. Most helicopters are also multi-engine, so two engines running will get me 100% torque, where if I lose an engine I might only get 60% mast torque.

Finally there is a safe minimum airspeed for single engine operations (there is the deadman curve for twin engine, but we are talking emergencies here). As a helo hovers, it draws air down from above to create the airflow over the blades - essentially the spinning of the rotor is solely responsible for creating relative wind. As we start to translate into forward flight, airflow is established over the blades by forward movement, creating a relative wind more in line with the blade chord. Our forward flight means the helicopter depends less on pure blade rotation to create air movement for lift because forward flight is providing the clean relative wind over the blades. This lets us stay in the air with less torque required.

If you made it this far, great!

Example

So how does this tie all together? Lets use an example:

All up weight: 11,500 lbs. Fuel on board: 2000 lbs. Mast torque available (twin engine): 100%. Mast torque available (one engine inoperative - 2.5 minute power): 65%. Mast torque available (one engine inoperative - continuous power): 54%. Minimum airspeed (single engine): 25 kts. Hover torque required (in ground effect) 75%. Hover torque required (out of ground effect) 86%.

Helicopters landing at or near maximum all up weight is normally limited to a factor of power required/available rather than a factor of airframe loading limits/overstressing

Scenario #1

You are in ground effect hover and lose an engine. You pull the collective into your armpit and accept settling with power because your power available single engine (65%) cannot meet the power required (HIGE 75%). Fuel dumping is not an issue here.

Scenario #2

You are out of ground effect hover and lose an engine. Again, you pull the collective into your armpit and accept significant settling with power because - you guessed it - power available single engine (65%) cannot meet the power required (HOGE 86%) and you accelerate to the earth. The whole iteration takes about 5-7 seconds, and the result lands you as a flaming fireball on the 6 oclock news - however you don't get to see it. This is why you don't hover heavy OGE, and why you don't violate your deadmans curve! No time for fuel dumping, so not an issue here.

Scenario #3

You are in a high hover (HOGE) - say 150 feet. You lose an engine, but instead of pulling the collective into your armpit and dying, you pull some power and push the cyclic forward - accelerating above safe single engine speed (25 kts). You start to arrest your descent as you trade altitude for airspeed, bringing enough speed in to climb out safely. You can then either choose to stay in the air on single engine for the next hour to burn enough fuel to facilitate a HIGE landing, or you can carry out a run-on landing at or above 25 kts at your current weight. No fuel dumping required.

Scenario #4

You do a no-hover (or rolling) takeoff and climb out above minimum safe single engine speed (25 kts), and then lose an engine. You have two options - continue the climb out and burn some gas until you bring your all up weight down to a torque that will allow you to hover single engine 2.5 minutes (which will take at least an hour - thats why flight engineers bring ipads and magazines), or simply continue your circuit and carry out a run on landing (or rolling landing) at or above 25 kts. Nice, slow, not a lot of runway needed. Fuel dumping not an issue. Keep in mind that a nice slow decent with loaded blades is key - every approach is an approach to overshoot with the option to land.

Summary

Remember, the fuel loads in most helicopters - even the big ones - are measured in thousands of lbs and not tens of thousands of lbs. We don't carry a significant amount of gas to really alter our landing profile - all we need is enough to stay in the air for a couple of hours. After that, our backs, necks and egos are sore from terrain flight and vibrations.

As an extra note, trying to dump fuel in the hover would be a hazardous mistake. As you dump fuel mist, it would recirculate through the rotorwash back onto the helo - into the engines and all over the place in general. Then after it lights off, your land as soon as practical emergency just turned into a one bell alarm. IF it were to happen, you would need an established minimum airspeed to carry it out, but then you certainly wouldn't be transferring it to a ground storage tank.

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  • $\begingroup$ Well, if in #3 and #4 it's hour to burn the fuel to facilitate HIGE landing, fuel dumping would help. Especially for ship-based helicopters that can't do run-on landing because the landing pad is not large enough (note the accident RedGrittyBrick found was a ship-deployed helicopter). $\endgroup$ – Jan Hudec Oct 30 '14 at 6:59
  • $\begingroup$ For sure, absolutely a fuel dump (if able to be done safely) would help. I did not read the accident report, so I cannot comment specifically on what happened. I was not ship ops qualified, however if I were in a situation like that where a run on landing was required to a confined area, I would want that ship into wind and going as fast as it can. The wind speed plus vessel speed should compensate for the 30 kts or less to maintain single engine safe speed - most of the time i'm in the 5-15 kts range. I'm not against fuel dumping, its just a compromise between design and requirements. $\endgroup$ – Tiger963 Oct 30 '14 at 18:21
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As one of the original commenters I suppose I'm as good as any to reply here!

I can't remember my wording off the top of my head, but essentially I pointed out that if a helicopter has the power to lift off at a certain weight, or hover/climb, it is by definition capable of slowly descending and therefore landing.

Unlike an airliner which has to essentially 'fly' onto the runway, straining the landing gear, a helicopter does not. In theory, one can land with virtually no extra force other than the weight of the heliopter. In practice this isn't quite true, but you get the idea.

Now in reality that doesn't mean a helicopter will never dump fuel (or want to) - in an emergency situation it makes sense to reduce weight to improve responsiveness, and if I'm about to crash I'd like as little highly flammable liquid around as possible!

There are also a small number of (mainly military) helicopters which do generate lift when moving, therefore can takeoff at a higher weight than they can hover... In these cases they are more akin to fixed wing aircraft in terms of having to 'fly' onto the ground rather than performing a vertical landing, and therefore may need to reduce weight to reduce stress,

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    $\begingroup$ Each and every helicopter can fly heavier than it can hover. It is a direct consequence of fundamental physical laws—like in fixed-wing aircraft, increasing speed reduces induced drag and therefore power required to maintain altitude. $\endgroup$ – Jan Hudec Oct 30 '14 at 7:14

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