How heavy is too heavy to land on a grass strip?

Question

This is actually a tricky question, because it's not just about gross weight, but about how much weight each individual tire imparts into the ground. This, of course, depends on the design of the suspension and the number of wheels. Thus a really big craft, with a well designed undercarriage, can land on grass just fine (like a C-130 for example).

The point being, "how heavy is too heavy" has more to do with specific weight supported by each wheel during landing, and less to do with total landing weight.

So, kind of a two parter. Firstly, how much weight can a grass field generally support when an aircraft is landing? Secondly, how do aircraft designers make landing gear that can land on a grass field without plowing into the ground and ripping off?

Answer 1

Your second question is really easy to answer:
Q: How do aircraft designers make landing gear that can land on a grass field without plowing into the ground and ripping off?
A: Big Fat Low-Pressure Tires.

A wide, low-pressure tire has a larger contact patch with the ground, spreading the aircraft's weight over that larger area helps prevent the tire from digging into the ground, and lets the aircraft roll more easily on rough or soft surfaces.

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Your first question - How much weight can a grass strip support? is a much tougher one. The bearing strength of an unimproved surface depends on a number of factors (some big ones: The composition of the runway bed, its drainage, and what kind of surface cover - turf, rocks, etc. - it has).

If you look at aerial photos of Edwards Air Force Base you'll notice part of the runway complex includes a dry lake bed:

Some pretty heavy stuff has taken off from and landed on those runways - space shuttles have landed there.
Similarly TACA Airlines Flight 110, a Boeing 737, landed on a levee at NASA's Michoud Assembly Facility (and was subsequently repaired and took off again, though they used an access road for the takeoff rather than the grass levee).

Conversely if you tried to land a Cub on a grass strip that's saturated with water after a rainy month there's a chance your tires might sink into the mud.

CBR Technologies has a nice presentation on unpaved runways, which includes some discussion of how their bearing strength (and thus maximum landing weights, tire pressure limits, etc.) can be calculated, but each unimproved runway will be different, and the values will likely change over time.

Answer 2

If you have 24 wheels to spread the load out, even 400 tons might not be too heavy. Especially when the tire pressure can be adjusted from the cockpit (however, reducing tire pressure quickly lowers the MTOW of the An-124). See here for some hearsay on the issue.

The aircraft designers keep tire pressure low to enable this. Everything below 80 psi should have no problem on a dry grass strip. Gliders have around 50 psi and use grass strips (and the next available field, if needed) routinely. When tire pressure exceeds 100 psi, I would look more closely at the grass strip - if the soil is hard enough, why not land there? The C-130 has a tire pressure of 125 psi (8.6 bar) and routinely operates from unpaved strips.

Polish Air Force C-130E with serial 1502 during grass strip take-off and landings on 4 April 2014 (Image © Capt. Włodzimierz Baran / Dowódca Generalny Rodzajów Sił Zbrojnych)

Here is a video of an Illyushin 62 landing in a field. Note that it was not anywhere near its maximum flight mass at this event. It was the last flight of this bird; now it sits idle as a tourist attraction in the German countryside.

However, I once landed an ASW-20 C on a muddy field soaked after days of rainfall. The wheel immediately dug in and the aircraft slid for 20 meters along the bottom of the forward fuselage. It took me a week to repair the cracks - the ASW-20 has a great landing gear, but is poorly suited to plough like this through a muddy field.

Answer 3

The operative term to look up here is ground pressure. This is simply the force required to support the object divided by the area that force is spread over. You can reduce the ground pressure of an aircraft by increasing the size of the landing gear - most obviously, using more wheels and/or larger wheels. On low-friction terrain (ice, snow), try skis - they're lighter.

The ground pressure of a large Main Battle Tank at rest is about 15psi. This is very low, even though the tank is very heavy, because its weight is spread over a pair of very long and broad caterpillar tracks. This allows the tank to move and fight effectively on almost any terrain short of an actual river. The ground pressure of a normal car is more than twice that, and is closely related to the tyre pressure. Some off-road vehicles have very large tyres that can be partially deflated for this reason.

The actual force to be supported by the ground is not just the weight of the aircraft, but the dynamic forces caused by descending onto the ground during landing. These are obviously reduced if you make a very smooth landing. Dedicated STOL aircraft have many features to control their trajectory at low speed to assist with this, such as blown flaps and even tilted engines. Their slow forward speed at touchdown also minimises the effects of the wheels digging into soft ground.

Answer 4

Any aircraft can land on a grass field. It is actually easier and safer to land on a grass field than a hard surface. The reason that hard surface runways are used is because a dirt runway will develop ruts and require frequent resurfacing, but a hard surface can be landed on repeatedly with no change. Also, if you land on dirt it kicks up a lot of dust and possibly rocks which are bad.

Landing gear will not "rip off" an aircraft unless it hits an immovable object, like a brick wall. The landing gear is the toughest part of a plane, so if you hit a brick wall what happens is that the plane rips off the landing gear, not the other way around. I know of small planes that have hit steel highway dividers and come to a dead stop. You have to have a very severe sort of accident to pull apart an aircraft from the landing gear. Normally wings and stuff will come off way before the landing gear does. The landing gear is heavily integrated with the fuselage, so they basically act like a unit.

Dirt is very good for landing because the aircraft comes to a stop more quickly. With concrete you just keep rolling and rolling, which is much more dangerous. If I had to land off field, I would look for the muddiest place I could find. Shallow water is excellent too for the same reason: it slows you down quickly.

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To respond to some of the asphalt-loving comments... In any off field landing you should ALWAYS land on dirt, grass or mud (even better). Landing on streets and highways is incredibly dangerous because there are cars and people on roads and other dangerous obstacles such as railings, signposts, fire hydrants, traffic lights and highway lights. Also, in any settled areas there will be wires and steel cables strung across the roads which are invisible from the air and can cut a plane in half.

People who advise landing off field on asphalt are just power pilots who don't have off field experience, so they don't know what they are talking about. As any glider pilot or bush pilot will tell you grass is way way safer than trying to land on a road. Even at an airport, it is much safer and easier to land on the grass strip at the airport than the runway.