Why don't airports use arresting gears to recover energy from landing passenger planes?

Question

An average plane has 50t of mass and a landing speed of approx. 250 (lets be conservative) 200 km/h. Thus its kinetic energy at landing is about 75 MJ or 21 kWh. The plane catches the wire attached to a pneumatic or hydraulic system, which will decelerate the plane and harvest its kinetic energy. The built up pressure in the system can be released via some kind of turbine to convert the pressure (potential energy) to electricity. Lets say the system has 50% efficiency. Leading to 10kWh harvested energy, per landing.

The airport in Hamburg, Germany is just average. Not large and not small. They had 150 000 flight movements in 2017. Lets say half of them are big planes and 1/2 were landings. That is 37,5k landings per year. Lets say 2/3 could use the arresting gear system: This would provide a cumulative 250 MWh of harvested electricity per year. This is equivalent to the electric energy consumption of 100 two person households in Germany. In Germany 1kWh electricity costs about 30 euro cents (including tax etc). An airport could save 75000 euro per year when harvesting this energy and use it, instead of buying it from the net. I can't imagine, that such a harvesting system would not pay down after a couple of years.

Why there are no systems like this in the aviation industry?

Con:

• You need to refit thousands of planes with a wire catching System

• System has to be safe in case of a go-around

• ???

Answer 1

The energy unit costs for companies in Germany are much lower than for households. Where as a citizen you pay approximately 30 ct / kWh, the airport will pay much less, probably around 15 ct / kWh.

Adjusting your calculation for this price, we have 37.5 k€ per year in terms of energy cost saving to offset the depreciation and maintenance of the installation.

Let's assume our bean-counters want to see their investment money back in 10 years (10% depreciation per year), and the yearly maintenance costs are 10% of the initial system price. That means the cost of such an installation at Hamburg airport can be at most 187500.

But mind you, Hamburg has two runways, and each can be used in two directions. To catch 2/3 of the total traffic, we would need to install at least two systems. Runway 23 and runway 15 combined get approximately 75% of the landing traffic, they would be the preferred choice. (traffic percentages per runway are here)

This brings the maximal cost of a single installation to 95 k€; including project management costs, costs of the equipment, cost of the installation works, cost of closing the runway etc etc.

That is absolutely nothing! You can't develop and install a system that needs to recouperate 21 kWh in thirty seconds (the duration of the deceleration phase) for such a price.

And then there is the airborne side of the story. Who is going to pay for the modifications needed on the aircraft, such as an arrestor hook? And for the fuel that it costs to carry this hook around during flight (a hint, it will cost more energy to carry this extra weight around all the time, than that it would harvest at the airport)? And who is paying for the certification of the system?

There is no business case for such a system, not even close.

Answer 2

You can't simply stick an arrestor hook on a passenger airplane, the fuselage is not designed for the forces that a rear hook would impart. Carrier airplanes are designed from the beginning for carrier landings and have beefed up structures. That extra structure adds weight to them (presuming it's even possible to do so), and it would also add weight to the passenger airplanes as well, making your airplanes less efficient and using more fuel - more than any possible savings you'd get from your proposed system.

Answer 3

Summary: It isn't worth it financially, quite apart from any other aspects:

• The 'savings' would be "lost in the noise" of operating an airline service- they are absolutely minuscle compared to all other costs involved.

• The amortised cost of implementation + operation + maintenance + safety management would greatly exceed the savings.

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Using your own figures, the savings amount to about 3 Euros per flight. That's less than the cost of an airline meal, or a passenger purchased drink or almost any identifiable discretionary customer related service item.

Even before the complexities and costs of development deployment and use are considered, the available income is far far less than could be achieved by action with truly minimal low impact low development and low deployment costs within the airline passenger service structure.

Answer 4

Beyond the other issues mentioned here, let's accept your 75,000 euro/year number for a second and ignore the impossibility of retrofitting aircraft. How many people does it take to operate the arresting gear system? This article says it takes seven people for a US Navy aircraft carrier. But let's say your system is very efficient, someone will work very long hours, and it somehow takes only one person to operate the system, retracting the arresting wire between aircraft, maintaining the equipment and generator, etc... That person will want to be paid. How much does it cost to keep him or her employed? Seems like a pretty skilled position, got to be at least 75,000 euro a year, right? Without even paying for the equipment or any of the other parts of the system, your cost savings have already disappeared.

Also consider that arresting gear cables have a quite limited lifespan and are replaced frequently during use, frequently enough to require replacement every few hours based on the number of operations at a busy commercial airport. You could easily spend more than 75,000 euro/year just on cable replacements. Runway downtime while cables are being replaced may cause aircraft to burn more fuel while they wait; the cost of that fuel could easily exceed the value of the electricity generated by the system.

For another way of looking at it, even if we use your electricity price, consider that 75,000 euro a year is not much compared to the 269 million euro turnover of Hamburg Airport last year. While I'm sure the airport would like to save money where it can, it's clear that retrofitting thousands of planes (if that even was possible, which it's not) to try to save 0.03% of annual revenue would not be practical.

You could also look at the amount of electricity generated. "The electric energy consumption of 100 two person households in Germany" sounds like a lot of electricity, but it's also around the same amount of electricity that you'd get if you could run a single jet engine at takeoff power for a couple hours. Downrate that to a power setting you can maintain continuously, and your whole system is still only producing in a year what a single jet engine outputs in half a day.

Answer 5

One more nail in the proverbial coffin...

Let's say for a second that the airport could install a system like this at zero cost, not entirely unlike the assumption in Zach Lipton's answer.

Let's also say for a second that there are no ongoing costs for the airlines in having their airplanes retrofitted to be able to use such a system. In other words, the arresting hook has zero mass and causes zero net drag increase.

Let's also say for a second that the ground system requires no maintenance and no additional manpower. In other words, the cost for the airport after the system is installed for free is zero.

Let's also say for a second that the electricity can be fed into the ground systems with no losses.

In such a completely unrealistic scenario, now ask yourself who gets the benefit?

The primary benefit here is that airport saves some electricity. Specifically, they need 21 kWh less grid electricity per landing of a reasonably large jetliner. At your €0.30/kWh, that works out to €6.30.

That reasonably large jetliner might have 200 passengers on board.

In other words, a savings of €0.0315 per passenger.

Sure there are people who shop around for low fares, but a €0.03 difference per passenger per flight? And that's assuming that the airport pays full price for the electricity in the first place, and that the airport passes the full electricity cost savings on to the airlines, and that the airline passes the full landing fee savings on to the passengers.

I'll bet you almost anything that there are ways to save €0.03 per passenger per flight that don't require huge, maintenance-demanding, costly ground systems along with adding both mass, drag and certification headaches (and costs) to the airframes. I wouldn't be surprised if something silly like filing a millimeter off one side of each tray table would save more than that from lower fuel consumption because of hauling around less mass throughout the flight.

Answer 6

Setting aside the costs and complexities of such a system, it's also important that the energy you can recover this way is limited to the aircraft's kinetic energy at touchdown. This is a small fraction of the total energy obtained from the energy stored in the aircraft's fuel.

For instance, the Boeing 737NG series, which lands in the 50t ballpark of your example, has about a 20t fuel capacity. Aviation fuel has about a 44 MJ/kg energy density, so a 737NG can get through up to about 800 GJ of energy in one flight, or about 100,000 times the estimated 75 MJ available to an arresting gear.

The vast majority of this consumption is directly related to the aircraft's total mass. The majority of fuel is burned during cruise, when engine thrust predominantly balances drag, about 30% of which is lift-induced drag that is directly proportional to the cruising weight of the aircraft. Other components, like take-off thrust, predominantly increase in more direct proportion to the aircraft mass, so 30% is probably a reasonably conservative estimate of the marginal increase in fuel burn for additional aircraft mass.

It therefore only takes an increase in mass of about 1/30,000 for the energy recoverable to be cancelled out completely by additional fuel burned during a flight. For a 50t plane we can therefore only afford to add a couple of kilograms at most to stand any chance of saving any energy at the end of a flight at the upper end of the aircraft's operating range.

Suffice to say the gear necessary to transfer 75 MJ to systems on the ground in the space of a few seconds will not be this lightweight.

Answer 7

To put things in perspective, 75000€ is the amount an airline will pay in compensations and duties of care on a single flight with ~100 passengers arriving late. So, if a single flight per year is delayed because of the landing system failure in the receiving airport, or the airplane getting damaged after such a landing, you'll instantly lose all the profits. A second incident will make it a net negative.

And I'm not even talking about yearly amortization / service / repair / insurance costs which will be a couple of order of magnitude higher.

Answer 8

No need to modify any airplanes.

You would make the first half of the runway just past the touchdown strips into a conveyor belt. The conveyor belt uses radar to determine the aircraft's ground speed, and expends energy to spin up the conveyor to projected speed of the airplane on touchdown. The airplane touches down, and its wheels never spin up, it just sits.

Now, a braking mechanism on the conveyor belt brakes the the belt to a halt, such that the airplane will be at 0 mph speed (0 kph) by the end of the conveyor belt, recovering all the energy in theory. *Mind you, it is also recovering the energy it expended putting the belt into motion in the first place.

When the airplane is down to about 30 mph/50 kph (or whatever its comfortable fast-exit speed is), the pilot releases the brakes altogether, and is now independent of the conveyor, i.e. Its mass is now disconnected from the conveyor's and its own inertia carries it forward independent of whatever the conveyer may do.

The airplane runs straight off the end of the conveyor and finishes its stop on plain pavement, and/or takes a high speed exit at that point.

The conveyor must failsafe by stopping hard instead of freewheeling. If that happens, the airplane's wheels will suddenly start turning again, and the ordinary braking they are already applying will be effective. Would they be locked up and skid? Probably not. Very likely the ABS will be modulating braking force so the wheels barely turn with the conveyor's normal slowing; now that they're turning again it will reapply.

Handling might be non-intuitive if the airplane tried to exit in the middle of the conveyor belt. Simply don't provide any exits there. If the pilot needs to throw it into the grass at some point, the handling oddity will be the least of his problems.

Of course, this requires essentially frictionless bearings and slide surfaces, and all this for under €187k capitalization.

And when you realize that is futile, visit your local commuter train station and watch the trains pull in. Trains weigh a LOT more than airplanes.

Answer 9

So what you are suggesting is to replace a well-tested process for stopping planes, self contained and redundant (brakes) by a complex mechanism which fundamentally takes away control from the captain (the go-around, as all other maneuvers is the decision of the captain) or has a super complex control handover between plane and ground systems. Not to mention that the time which the system has to recover from stopping a plane may be short or even negative (the time between the plane leaving the runway and the next plane being signaled by the tower to land). You even would have to introduce some additional indicator in the cockpit which requires a last second decision from the pilot in case the hook is not ready.

(and yes, I know they do it on aircraft carriers but for much better reasons that saving 3 euro per landing).

Answer 10

If a plane needs to go around after touchdown (eg: runway incursion), but its already caught on the arrester system, that is one bad scenario.

If the arrester system fails after being caught, then the pilot has an extremely brief window to switch to traditional braking (reverse thrust, control surfaces and brakes). That is another really bad scenario.

Landing is one of the most dangerous and complex phases of flight. This portion of a flight should be about safety, not saving a few dollars. An energy re-capture system increases complexity and decreases safety.