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A question I've always wondered is: can jet aircraft - not turboprop in this case - operate from 'rough' airstrips. My definition of 'rough' is grass or maybe a smooth mud airfield. I know that there's the dry lake at Edwards AFB that is used by aircraft for testing. Are aircraft such as the Boeing/ Airbus passengers series - 737, 777, 747 etc - designed for or have the capability to regularly handle these airfields? Are there currently any operators using these kinds of fields? I can under stand that tarmac/ concrete makes for a much smoother and more pleasant surface along with the reduce risk of FOD, hidden cracks that wheels could get caught in etc.

Any other examples such as executive jets, fighter aircraft that regulary handle rough fields are more than welcome.

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See Antonov stuck in mud –  RedGrittyBrick Jun 25 at 19:19
    
Or TACA Flight 110: not the landing, the take-off... –  User58220 Jun 26 at 0:33
    
@User58220 Good example, I didn't think of that. I suppose you're referring to the take off after replacing the blown engine right? –  shortstheory Jun 26 at 10:03

5 Answers 5

up vote 18 down vote accepted

The main problem with most jet airliners is that the engines are placed close to the ground. If the field is not paved, this increases the risk of damage to the engines from ingesting FOD (foreign object debris).

There are, however, jets that can operate from un-paved fields.

The 737-200 has an option called a "gravel kit." Despite having engines close to the ground, this kit helps to prevent the engine from sucking in FOD, which allows the aircraft to operate off of gravel runways. It also includes an addition to the nose gear to help prevent gravel from getting kicked up. For this reason, airlines like Canadian North still operate multiple 737-200 aircraft in order to provide service to airports with gravel runways.

High-wing aircraft are better suited for this type of mission (see Pros and cons of high-wing vs. low-wing design). The C-17 is a military airlift designed to operate from remote airfields. Its wings are placed above the fuselage rather than below, which allows the engines to be mounted further above the ground. The IL-76 is a Russian aircraft of a similar configuration that also operates out of unpaved fields.

Aircraft with engines mounted on the rear fuselage also protect the engines from ingesting FOD. The new Pilatus PC-24 jet advertises the ability to operate from unpaved strips. Older aircraft like the 727 or the Tu-154 have this engine placement as well, and both can operate from unpaved fields.

737 Gravel kit
737-200 landing on a gravel runway. The gravel kit is visible as the small extension just below the front of the engine, and the addition to the rear of the nose gear.

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2  
Most aircrafts designed in former Soviet Union are also capable of operating from unpaved runways because they needed to service remote locations in Siberia that didn't and don't have paved runways. –  Jan Hudec Jun 25 at 18:06
    
Better to use a T-tail aircraft like a DC-9, B727, MD83, F-100, etc since their wings screen FOD debris. –  user2357 Jul 19 at 23:51

The mechanical strength of an airfield can be measured in CBR, meaning California Bearing Ratio. Another way is Westergaard's theory, which uses a more analytical approach. ICAO has used both as the basis of ACN, the aircraft classification number which can be used to express the effect of a particular aircraft on the runway. The pavement in turn is classified by PCN, the pavement classification number. Here is a lengthy Boeing document on the calculation of PCN. If you know both values, you can determine how damaging the operation of a particular aircraft will be on a particular airfield.

Generally, civil aviation wants to use clean, hard surfaces. Tire pressure is a good indicator how high the tire load is, and how soft the surface may be. Commercial jets have 100 - 220 psi (7 - 15 bar) tire pressure and need a hard surface (concrete or asphalt; gravel operation is really unusual and needs lower tire pressure). Carrier-based aircraft have up to 350 psi (24 bar) tire pressure when operated from the steel deck of a carrier. On the other extreme, some military transports are still jet aircraft but need to land on unprepared surfaces, so their tire pressures go down below 100 psi (the C-17 uses 144 psi normally, but this can be lowered to 120 psi).

As Jan mentioned, many Russian aircraft have traditionally been designed for operation from unpaved airstrips. Even fighters like the MiG-25 or the MiG-29 have (relatively) low-pressure tires (10 bar / 150 psi) which allows their operation from unprepared airstrips. The MiG-29 can even close its air intakes to prevent foreign object damage, sucking air through louvres on the upper wing. Here is a picture of the intake of a Polish MiG-29 in take-off configuration, taken from this website:

MiG-29 intake

Left air intake of a MiG-29 with main doors closed and louvres on the wing root opened.

With the MiG-25, you could take off from a grass strip, climb to FL800 and fly at Mach 3, and land on that grass strip again. Fascinating!

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A MiG-29 can hit 80,000 feet? –  ptgflyer Dec 15 at 15:08
    
@ptgflyer: The picture is of a MiG-29, but the text mentions the aircraft which can zoom up to FL 800: The MiG-25. It's not a caption, maybe I should add more space. –  Peter Kämpf Dec 15 at 15:17
    
Yeah, MiG-25 makes a lot more sense. –  ptgflyer Dec 16 at 16:09

I can't vouch for other airliners, but the 727 was used on dirt airfields in Africa quite often.

Video of 727 taking off from a dirt strip

Photo of a 727 taking off from a dirt strip

Photo of a 727 taking off from a dirt strip

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and then there's the VFW-614 which was designed with overwing engines specifically for rough field operations in developing countries.

It sadly failed to sell, and the aircraft is now largely forgotten.

enter image description here

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There are several problems: 1) Whether the landing strip can handle the weight of the aircraft and landing impact(s), 2) Whether the aircraft landing gear itself survives repeated abuse and 3) How much debris does get sucked into the engines.

With regard to fighter planes, I know for a fact that MIG-29 fighters have several slits on top of their intake (and top of their wing, resembling a shark's gills) and can actually close the main intake when landing or taking off; This way you get much less dust operating from less-than-ideally maintained runways.

With regards to landing gear, there are special designs for that; Swedish SAAB fighters have had longer landing gear to be able to land on the roads, refuel and rearm and fly again against the Soviets, or you could do the C-130 (I know, a turboprob, bear with me) and have multiple wheels to distribute the landing load so that the plane does not sink into the runway.

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none of which problems are specific to jets... –  jwenting Jun 28 at 20:09

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