After being immersed in complex airspace with rapid fire clearances and sometimes holding number fifteen for takeoff, I really look forward to flying into the mom and pop grass airstrips scattered across the countryside. Theres just something about being able to slip on final and glide into a nice landing without a controller in my ears telling me to take the next high speed exit because theres another airliner on my tail.
Ah, such fun! Back to simple stick-and-rudder once again.
Fun? Absolutely. Low risk? Not necessarily. If you rank the biggest dangers in aviation, icing and thunderstorms might appear near the top of the list. Flying into grass airstrips isnt something that causes most pilots to lose sleep at night, but Ive had more than a few takeoffs and landings on unimproved airstrips that left my heart racing and me saying to myself, Geez, I wont try that again!
Its almost always an unknown or hidden risk that has the potential to cause the most trouble. The FAAs practical test standards regarding soft field operations concentrate on the soft field technique but do little to really prepare you for operating on unpaved strips. What you dont know can hurt you!
When you fly from a concrete airstrip, you can consult the Airport Facility Directory for the exact dimensions of the field. Widths, lengths, slope, runway edge markings, runway markings – theyre all known and published. The edges of the concrete are (usually) clear, and on most of the county and municipal concrete airstrips, the surface is smooth.
None of these conditions exist on many unimproved airstrips. The lengths and widths may have been measured, however its not always clear where those edges are. Some grass airstrips have runway edge markers, but most dont. The edges are very indistinct on many grass airstrips, sometimes leaving you wondering if you are on the runway, a taxiway or just somewhere out in left field! Yes, Ive taxied on a grass strip and looked at my wife wondering if I was still on the runway or driving through a farmers field.
Dark, With Winds Unknown
Grass airstrips seldom have lighting. If you do fly into a grass airstrip with night lighting, you still cant check out the condition of the surface before landing. You are taking a risk trying to land at night on grass. Has it been done without incident? Yes, many times, but eventually Murphys Law will catch up and an airplane will try landing at night when someone has dragged a hose across the runway or maybe even parked their vehicle for a view of the stars.
Many grass airstrips lack even a wind sock. In this case, you are left to determine the winds on your own. Sometimes you can see drifting smoke nearby as a clue, or maybe flags or banners on nearby farmhouses. Sometimes the taller vegetation will render a clue about the wind. Sometimes your aircrafts drift angle over the ground is an obvious aid.
Most grass airstrips also lack a VASI or PAPI, so gauging the proper glide path depends on your visual perceptive skills. Since many grass airstrips are surrounded by trees, perhaps hills, and might have some slope involved, a huge variety of visual illusions await your arrival. Usually you judge your glide path angle by the relationship of the runways dimensions to the horizon. However, since grass airstrips have all sorts of dimensions, and can be located in varying terrain, its very easy to misjudge your glide path.
When trees surround a grass airstrip, not only do you get the squirrely spill-over of the winds when they flow over the trees, they can also hide obstructions such as powerlines. When you are in the air looking down at the earth, its very difficult to distinguish power or utility lines. Usually the most revealing characteristic of power lines is the presence of tall utility poles, but when thick trees surround the airstrip, those can be hard to see.
A friend of mine landed his glider on a grass airstrip one time. From the air it looked quite nice, but during the roll-out, he suddenly noticed the vertical posts of a barbed wire fence. He applied maximum braking and managed to bring the glider to a stop just inches from the barbed wire. The barbed wire would have sliced through the canopy at eyeball height if he hadnt been able to stop in time. Unbeknownst to him, a fence had been installed on the end of the airstrip.
Livestock and Wildlife
Grass airstrips usually are found in rural areas, and with that comes livestock and wildlife. Im currently doing an extensive study on emergency medical service accidents and noted that a handful of EMS accidents at the same airport involved collisions with horses. Even when the EMS pilot had contacted the FBO to assure that the runway was clear, livestock were still in the runway environment when these aircraft landed.
Some grass airstrips lack good fencing, allowing animals (including humans) to simply wander out onto the runway, completely unaware of your takeoff or landing in progress. At night, pilots should pay particular attention to wildlife that may graze on the grass in the dark hours. Whitetail deer have adapted very well to mans presence and are not easily deterred from airport environments.
At some of my favorite airports, antelope like to run alongside landing aircraft. Since antelope are incredible runners, they will easily keep up with a landing or taking off aircraft. Some local wildlife can be more hazardous than others. There are some grass airports in Florida where alligators can be found sunning themselves. Running into an alligator would definitely ruin your day.
Unknown Runway Condition
Most surfaced runways have fairly similar characteristics. Concrete is hard and, for most general aviation aircraft, the concrete is sufficiently thick to handle the weight. Some runways – particularly those paved with asphalt – may be bumpy from patches and cracks, but seldom do you have to worry about the aircraft sinking into the runway.
However, an unimproved strip is another matter. The first time you land on a water-logged grass strip, youll really understand why the FAA uses the term soft field. Its a helpless feeling to land on a soft surface and begin sinking.
If you have no other way to check the hardness of a surface ahead of time, you can try to do an immaculate soft-field landing, just barely touching down and trying to get a feel for the hardness of the terrain. In practice, your pilot skills have got to rival Bob Hoovers to do so. Its really, really hard to do, plus you are chewing up runway and diverting your attention while trying to do this.
Experienced Alaskan bush pilots land on beaches at low tide, but if the sand is too soft the airplane can flip over. When Ive flown out to friends hay fields, I usually try to visit the site on foot first and drive a truck down the field, just to see how deeply the tires dig in. If the ground is soft, or has wet, mushy spots, you can be in for a nasty surprise when the airplane lands.
Other surface hazards are not so apparent. Grass thats three inches tall or more and soil that is very sandy leads to a sticky situation when the airplane touches down. Moderately long grass is tremendously powerful at adding drag to an aircrafts wheels. If you land into long grass, add power to keep some speed and hopefully get enough lift to get out of there. Even moderate length grass can really impede your ability to takeoff, and the faster you try to go, the higher the drag of the grass on the wheels. Moderately tall grass is definitely a retarding force with immense drag potential.
Airports used for commercial purposes have design features in the runway to help drain the rain. Porous-filled concrete (PFC) is specially designed to help drain the rain. Also, many runways have grooves across the width of the runway (unless you happen to remember the Air Force engineer who put the grooves lengthwise in the runway at Patrick AFB, and then had to re-do the entire runway). The grooves are quite effective at draining rain and preventing puddles.
An unpaved strip, however, is extremely unpredictable. The soil underneath the unimproved airstrip can vary along the runways length. One part of the runway may have a soil type that readily absorbs and drains away water, while another part of the runway may have a soil type that is easily saturated and hold standing puddles. Certain parts of an unimproved airstrip may remain slippery even when the rest is dry.
The substrate underneath the surface layer will also help or hinder drainage. Some substrates resist compaction better than others. The net effect is that differential settling of the runway is inevitable, creating high zones, which tend to be drier, and low zones, which tend to retain water longer.
Some grass runways have a drainage system, either through crowning or sloping the runway surface or through a series of drains underneath. I once belonged to a soaring club that installed a drainage system underneath its runway because of the rainy climate. The airstrip was also well graded and smooth, and the water seemed to wick right away.
There were no soft spots in that runway, but it is an exception. Usually every grass airstrip has a soft spot or two or three. Those spots are very difficult to detect unless you know where to look for them. Moisture gathers there, which helps the grass grow more abundantly. The taller grass hides the spot and its presence is unknown to you until you taxi through it.
If you taxi through a soft spot like this with just one main wheel during takeoff or landing, control of the aircraft becomes an instant panic situation because of the immense drag, which suddenly veers the aircraft to one side. Because grass has a much lower coefficient of friction with your tires than a concrete runway, your movements on the pedals will be less effective than on concrete (assuming the nosewheel or tailwheel is on the ground). Secondly, these soft spots usually decelerate the aircraft fairly rapidly. If you are trying to takeoff, this added deterrent is no help.
Many grass airstrips have a well-worn and well-compacted center lane from high use, but the outside edges are a bit iffy. Sometimes the centerline is easy to see from the air because the grass is well worn. However, just off to the side, the grass can be much higher. If you happen to stray from the well-worn areas, youll suddenly encounter control problems because one main wheel will be dragging in the softer ground or taller grass.
Maintaining control of the aircraft under these circumstances is guaranteed to get your heart going, particularly if youre in a taildragger, where the aft center of gravity will encourage the airplane to swap ends.
There arent many grass airstrips that dont have some form of surface irregularity. When possible, walk an airstrip before operating from it. Sometimes locals will warn you about soft spots, sometimes not. Guess what? If you bend some metal, the feds arent going to be more lenient on you just because someone else gave you bad information. You need to check it out for yourself, if possible.
When walking a grass airstrip, look for hidden features. Big rocks can lie under the grass, just waiting to take a bite out of someones prop. Take an extra look at the darker, greener grass. Usually that is a soft spot or a small local depression. Look for animal holes, such as those made by ground squirrels or gophers. They can build new holes overnight, much to the owners dismay.
In the backcountry airstrips of Montana and Idaho, elk will create wallows on the airstrips, some of which can be dangerous if you happen to try landing over one. On any unimproved airstrip, its very easy to encounter these hidden gotchas and really ding your prop well or lose aircraft control long enough to hit an obstruction.
When you land on grass, be prepared for the possibility of wet grass. Its very hard to tell from the air if the grass is wet. If it rained recently, or there was a heavy dew during the night, you should definitely suspect wet grass. Braking potential is significantly compromised on wet grass.
In addition, recent rains may have prevented mowing, in which case the grass will be long and lush. Long grass can of course help slow an airplane down, but wet long grass isnt something Im willing to bet my pilot license on.
Although most unimproved airstrips are turf, in the arid West and Alaska they are usually gravel, rock, or sometimes plain old hard-packed ground. In some areas unimproved airstrips have instrument approaches, so proper radio work is crucial to safely making it to the runway. Many times in Alaska I broke out at 300 feet on an approach and was greeted by someone flying beneath the overcast.
In certain parts of the West, roads are used as temporary runways. My favorite resort does this. EMS operators frequently use roads in the rural west. Roads should be approached with caution unless youve checked them out ahead of time.
Hard-to-see utility lines usually run alongside roads, as well as mile markers, road signs and roadside markers. Roads arent very wide and its easy to divert from the centerline just a few feet and hit a roadside marker almost before you can see it. Road surfaces are usually firm, but thats about the only thing they have going for them.
Gravel has its own special risks. Because gravel is composed of small rocks of varying size, its easy for the low pressure area behind the prop to pick up rocks and really damage a propeller. Thats why taildraggers are far superior on unimproved strips. The prop on a taildragger has far more clearance from the ground and is less likely to be damaged by loose gravel.
Gravel is also picked up by your main gear and kicked back on the tail surfaces. Not only will this severely erode the leading edges of your tail surfaces and potentially cause some damage to these susceptible surfaces, but small rocks can become lodged in the control surfaces, inhibiting control movement. Its especially dangerous during takeoff or landing.
The depth of gravel can be difficult to determine. Ive had similar scary takeoff and landing events on gravel as in moderate length grass. Ive landed and quickly slowed down because the gravel was deeper than I thought. Gravel runways dont have to be very deep before they can slow your aircraft so much that its impossible to take off.
Its also scary during landing because the drag of the gravel can pitch the aircraft quickly toward its nose. Gravel of moderate depth can retard your aircrafts speed very effectively. Before operating on gravel, you need to accurately predict its effect on your aircraft – but the catch is that its something only experience can tell you.
Gravel can also have soft spots, or spots where the gravel is deeper. The effect is the same as rolling through long grass. Ive rolled through these soft spots and been surprised when the aircraft quickly jerked and yawed. It takes very quick and aggressive control inputs to keep control of the aircraft. Its also more difficult to keep control of the aircraft on gravel because it can be slippery.
Mud can also cause control problems, and this is why I try to avoid taxiing through mud holes. The mud is sprayed on the underside of the flaps and can coat the control hinges so that it becomes difficult to move the flight controls in flight.
Some unimproved airstrips in the arid Southwest are composed of rock. Theres a reason that we call it Slickrock! When it is wet, or there is a layer of dew or frost on the rock, its very slippery.
When parking at a grass airstrip, seldom will you find covered hangars or tie-downs. This means your aircraft will be left exposed to the weather. If you dont anticipate any harsh weather such as hail then its not much of an issue. However, tying down an aircraft is always a proper precaution and thats very difficult to do when there are no tie-downs. I recommend carrying those screw into the ground tie-downs as a good back up. Their holding strength is remarkable and sufficient for most wind conditions.
If you are going to park your aircraft frequently on strips where livestock may graze, I would recommend carrying a portable electric fence to keep the animals away from your aircraft. Its easy for a 1,200-pound horse or steer to brush up against your aircraft and do some damage. In addition, some animals, particularly cattle, have been known to snack on fabric-covered airframes.
Not only will this make you mad, but chances that appropriate repairs can be made at the airstrip to ferry your aircraft to an appropriate repair station are lean to none.
-by Pat Veillette
Pat Veillette is happiest when he trades in his B-727 gear for a J-3 Cub and a grass strip.