Can a passenger jet be towed to safety if it breaks down in midair?

Question

A car can be towed by a tow truck to the repair shop when it breaks down on the highway. Is it possible for a passenger plane to be towed by an emergency tow plane along the air corridor to the next nearest airport assuming it cannot reach there on its own power (example if it runs out of fuel or both pilots are incapacitated)?

Update: This question actually consist of two parts. Whether it is feasible and whether this is practical. I have seen a small tugboat towing a barge; a man pulling a firetruck. Before writing off this idea completely, can someone address what's stopping a fighter plane from towing a passenger jet if a proper point of harness is installed?

Answer 1

Towing is quite feasible, and even landing while in tow is possible - I've done it myself in gliders. The hard part is catching up with the stricken plane.

To stay with Keegan's Boeing 777: It most likely will travel at Mach 0.8 and somewhere between 30.000 and 39.000 ft when disaster strikes. Let's further assume that power fails instantly, so the aircraft, traveling at 240 m/s, will develop a sink rate of 15 m/s just to continue flying. When it sinks, the increasing density allows it to slow down, so at sea level it will fly at 130 m/s, sinking with 8.125 m/s. For simplicity, assume a linearly decreasing sink rate, and the remaining time aloft will be less than 1000 sec or around a quarter of an hour.

Now the towplane has 15 min to scramble the crew, run up the engines, get take-off clearance and to actually fly an intercept to the stricken airliner. How realistic is that? How many airports are needed to have 777-sized towplanes ready, and how many otherwise unproductive 777s would this scheme need?

A car can stop and wait a couple of hours for the tow to reach it. An aircraft can not.

Answer 2

I am picturing something like a 777 being towed due to a complete engine failure. Please correct me if you had something else in mind.

This is unheard of, but it would be possible, and is a very interesting thought.

The first thing that comes to mind is something like a C-17 being refueled by a KC-135

Of course, a loaded 777 will weigh more carrying passengers than a C-17 loaded with cargo, but these mid-air refueling operations, logistically, are not much different that what would be needed to tow an airborne plane.

Here are the differences I can think of - whether or not they are feasible, I'm not sure.

• Logistically: the air force has their rendezvous planned down to a couple seconds before the plane that needs fueling is even airborne. In an emergency situation everything from planning to execution would have to happen in a VERY small amount of time.
• When refueling, no major tension is put on the refueling hose. If a 777 were being towed the tension would be AT LEAST the amount of air resistance acting on the 777.
• Designing a hook that can bear the huge forces and moments involved would take a lot of work.

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I did a little math to calculate the forward force that would be required to pull a (fully loaded) 777 to keep it above stall speed.

You would need about 43,000lbs - which is not too much in the world of airliners. For example, the 777 has a take off weight of 660,000lbs - and each engine produces over 100,000lbs of thrust. So it would be very easy to have a well equipped aircraft or two to tow a 777 - once connected.

The total force in the rope, assuming the towing airplane is slightly above the airplane being towed would simply be 42,500lb/cos(angle) so if the length of the rope is a generous 300ft and the towing airplane is 50ft above the towed aircraft, then the total force would be 43,100lb.

Just in case people don't trust my numbers, here's a screenshot of the math. I had a 777 model on my computer that I used to get the data from:

Answer 3

If both pilots are incapacitated, towing the plane is moot because it won't be able to land.

In the case of fuel starvation, an airliner that loses its engines at cruise altitude can glide for maybe 20–25 minutes before it descends to sea level. If there was an airport close enough to scramble a rescue plane from, then the broken plane would have a decent chance of just gliding to a landing at that airport.

Answer 4

Its not unheard of - gliders are often carried up by tow planes. However, hitching up a plane on the ground and towing it up is completely different than trying to do the following:

1. Catching up to a plane flying at great speed.
2. Positioning yourself in front of it.
3. Hooking up a tow hook.
4. Pulling it fast enough to maintain flight.

Suppose you were somehow able to do the first 4, you are still left with the small point of having to land two aircraft that are tied to each other.

So its is nearly impossible to do what you are suggesting.

An airplane will maintain flight as long as there is sufficient lift on the wings.

To tow a disabled plane is really not required, because even if all engines fail (a highly unlikely event), there are contingencies in place to power the control surfaces and communications so that the pilot can glide the plane to a safe landing. See this question for examples of incidents where all power was lost.

Lets assume that everyone on board becomes incapacitated. In this scenario (which has happened before) the plane will continue on its programmed flight path until it runs out of fuel, at which point it will drop in altitude eventually until it crashes.

In April 2012 the pilot of a Cessna 421 was incapacitated (possibly due to hypoxia - lack of oxygen) - the aircraft continued on its programmed course until it eventually ran out of fuel (flightaware flight track):

September 10 had another similar incident.

Answer 5

The simple answer is that it's presently not possible, certainly not for large passenger jets.

That said, it's possible to do rather impressive things like towing a glider or setting up a rendez-vous in mid-air for refueling. Towing would therefore not seem completely out of the realms of possibilities and Keegan's calculations suggest it would even theoretically be plausible (I didn't try to verify the calculations and I am probably not knowledgeable enough to do so).

One problem is that if all engines fail, staying put and waiting for a tow is not an option. Fortunately, planes do not “fall off the sky” either, they can glide in a controlled fashion and that's the actual contingency plan. It has happened before, perhaps most famously to the Gimli glider.

As Pieter Kämpf explained, an additional problem in this situation is that, while it's possible to fly the plane to safety, there is not much time available. Imagining a tow plane is one thing, but having one at the ready to reach a specific point anywhere in the world in 10-20 min is something else entirely and would be prohibitively expensive.

Now, if the pilots are incapacitated, things are quite different. Modern planes can maintain speed and altitude and basically continue to fly. This has also happened before, e.g. to Helios Airways Flight 522 and in that particular case there was more than enough time for fighter jets to reach the distressed airliner and escort it for over half an hour. Ultimately, however, they couldn't do anything else than watch it run out of fuel and crash.

The problem is that “towing” in some way or other is neither needed nor helpful. After all the plane is still entirely capable of flying on her own. What you need is taking control and that's presently not possible either. To my (non-expert) eyes, adding some remote control functionalities seems perfectly doable with current technology (cf. drones) but that would mean adding additional complexity, potential for failure, etc. I assume the cost/benefit analysis does not justify it and, as far as I know, there is no commercial passenger plane with anything like that.

Answer 6

Cars and passenger aircraft are designed and maintained in very different ways.

Redundancy

Cars only have one engine, unlike almost the aircraft that carry most passengers. Cars cannot continue their journey if their only engine breaks down or if an axle breaks. Aircraft have multiple independent sets of many of the most critical parts like engines and flight control systems, they are specifically designed to be able to continue when many things break down. An aircraft can continue when one engine breaks down, it can still be flown and make a landing when all engines have broken down. Aircraft have redundant systems to allow for continued operation after multiple failures.

Maintenance and monitoring

Cars might only be looked at by a qualified mechanic once a year or once every 18 months. Even at this time, it is likely that they mechanic just changes the oil in the engine, checks the fluid levels and brake wear and does a few other basic checks specified by the manufacturer. They don't check everything in the car.

An aircraft is more likely to be monitored continuously. It is not unusual for the engines to be sending data to the engine-makers in flight so that the engine makers can measure the health of the engine and can predict when the engine needs maintenance before any problem arises for the aircrew. After every flight, the crew of an aircraft report any unusual behaviour of the aircraft to the maintenance crew so that incipient problems can be dealt with before there is any danger of breakdown.

Car owners are not this diligent

Towing

Cars are expected to break down and are designed to be towed. Aircraft are maintained so that they don't need this and the airframes are not designed to be towed in the air, there is no towing point with enough strength other than that designed into the landing-gear for gentle low-speed towing on the ground.

You cannot attach a towline to an aircraft that is not under good control, if the pilots are incapacitated or if the aircraft is damaged such that steady flight is impossible (e.g. phugoid cycles etc). If the aircraft is under good control it is probably able to fly or glide to an alternate airfield or attempt an emergency landing.

Time is of the essence

Say a 747 lost all four engines, there would probably not be enough time to launch a rescue tow-aircraft from a nearby airport and reach the plane in time to do anything useful in mid air. Particularly if the 747 is in mid-Pacific or flying through a volcanic ash-cloud. It is better to design the aircraft to cope as well as we can make it.

A car can wait an hour or two at the side of the road. An aircraft cannot wait.

Answer 7

There are several reasons that this idea would not be practical.

An gliding aircraft flying at around 30,000ft would have, say, 20 mins before it hits the ground. If there is a suitable airfield within that range it stands a decent chance of landing safely.

The stricken aircraft would be travelling at, say, Mach 0.8 so only a fast fighter jet would be capable of chasing and catching it from behind in the available time. Any other rescue aircraft would have to approach from the front, circle and manoeuvre into position.

The time constraint makes this almost impossible. Assuming the rescue aircraft took 10 mins to prepare and take off, then the manoeuvring and attachment of the tow could add another 5 mins. That would allow only a few minutes to actually catch the stricken aircraft.

Gliders are designed to be very light aircraft and get towed up by relatively powerful tugs. They then release the tow - it is not re-attached! The airframe of a glider is designed to take the forces involved in towing; that of a passenger aircraft is not. Would it be possible to re-design all passenger planes to cope with this very unlikely scenario of total engine failure without adding weight and reducing efficiency? I doubt it.

As mentioned by David Richerby, air-to-air refuelling is currently a very expert skill practised by a handful of the best Air Force pilots. Some cunning new technology would be needed before the principle could be used on commercial aircraft.

Rather than this idea, a more useful and practical suggestion might be to add some sort of remote control function to be used in the case of the pilots becoming incapacitated or terrorists taking over the aircraft. Such technology would need to be robustly fail-safe and many crashes have shown that that isn't easy to achieve.

Answer 8

As others have pointed out, the plane that would benefit from towing would go down before it could be reached by a tow plane.

Even if the tow plane could catch up to the glider it would be difficult to do the hookup quickly. You would need some kind of special apparatus to capture the tow hook and attach it. Doing this would be very difficult because the aircraft would be in different configurations.

For example, lets say it is two 737, one with engines out. The stall speed for a 737 is 150 knots, so it might be going, say 180 knots with a glide ratio of 15:1. At this rate it will be descending at 1200 ft/min. That is a FAST descent rate, almost crashing/out of control. For example, if you descend faster than 1000 ft/min in a flight test you will be automatically failed.

The cruising speed for the tow plane is 450 knots, so he would have to somehow slow down to 180 knots AND be descending at 1200 ft/min AND stay directly in front of the glide plane while the hookup occurred. Doing all three of these things would be very, very difficult.

Answer 9

The OP asks about towing and this has been comprehensively explored in the preceding answers.

However, we could widen the discussion by fitting the stricken aircraft with a docking mechanism on the underside. Then the rescue aircraft (with a matching docking unit on the top side) could maneuver beneath it and lock on. We would then have a composite structure like the 747-space shuttle transporter configuration. A sufficiently powerful rescue ship could then fly the pair down to a safe landing.

Further enhancements occur: (a) The docking port could have a manhole through which the PAX could be evacuated into the rescue ship. (b) A command-interface could enable the rescue pilot to control the flight surfaces on the stricken plane (assuming they were still functional), thus making it easier to fly.

Answer 10

This has happened under wartime conditions.

Pardo's Push Captain Bob Pardo pushed another F4 Phantom that had lost its engines into Laotian airspace after a raid on N Vietnam, to where the pilots could eject into less than hostile ground. All while Pardo's plane lost one engine due to battle damage and a fire.

Robbie Reisner pushed an F86 Sabre that had run out of fuel, out of N Korean airspace and over the ocean. Unfortunately, the pilot drowned when he ejected, landed in the ocean, and became entangled in his chute lines.

However... If you undertake a study of high profile commercial aviation incidents, you will find that almost none of them could have been mitigated with this scheme. Here's a quick spot check...

AF447 High altitude stall in the middle of the Atlantic. Went down in less than ten minutes.

JAL 123 loss of horizontal stabilizer... aircraft uncontrollable. Would have just taken the tow plane down.

USAir 427 malfunctioning rudder control unit on landing approach - no time, aircraft uncontrollable.

ValuJet 592 Fire in the cargo hold. Burned through control lines and probably incapacitated the crew.

BA 9 Lost engines due to volcanic ash. Three engines restarted. "I trust you are not in too much distress"

USAir 1549 Lost engines due to bird collision shortly after takeoff. Not enough time to do anything other than ditch.

Alaska Air 261 Vertical stabilizer jackscrew malfunctioned due to poor maintenance. Aircraft uncontrollable.

TACA Air 110 Lost engines due to storm. Landed on levee without damage by unbelievably talented pilot.

Lauda Air 004 Thrust reverser deployed inflight. Aircraft out of control. We learned that the certification that the aircraft could fly with a thrust reverser deployed was inadequate from this incident.

In all of these, only TACA 110 or BA 9 might have benefited from a tow plane, and even so, there probably wasn't enough time, as both planes could remain airborne for maybe 20-30 minutes.

So it's an interesting idea, but as of today, an airliner losing all engines is extremely rare, as contemporary gas turbines are very reliable - enough to certify crossing major oceans with only two engines. The funds for modifications for a tow hookup, the training in how to do it, and the maintenance of dedicated tow aircraft, would be better spent addressing the issues that are causing most accidents.

Answer 11

There has to be consideration for whether the towing process would remain at a stable altitude, or just provide assist to the nearest airport, calling into question the amount of power that would be required to assist. Given the amount of turbulence that would be generated from the towing craft (and the bigger it is, the worse it would be) it seems unlikely the disabled aircraft would be able sustain stable flight and structural integrity.

You also need to consider the relatively small number of aircraft that become completely "disabled" without being impacted by some related catastrophic event that otherwise compromises it's flight worthiness. There are very few instances of US Air 1549 and most would be too close to the ground to allow any time to react without the famed Star Trek Tractor Beam

Perhaps with a highly specialized design process some of the challenges could be overcome, but the practicality of being able to actually have the proper assistance available would negate the feasibility.

Answer 12

With enough engines and all the right hardware...sure, it's possible. See this report on tow launching an F-106: http://www.nasa.gov/centers/dryden/history/pastprojects/Eclipse/index.html

The problem is that it's just not practical. Nevermind the actual execution of a pickup maneuver. As mentioned in previous answers the costs of a standby fleet required to execute reliable pickups are very high. There would be zero utilization of such a fleet.

Your edit asks about a fighter jet serving as the tow plane. Yeah, sure, it's technically possible to build a trap of some kind to be a tow rig on an airliner. If afterburner is required to keep both planes aloft tow time would be extremely limited though.