Risk Factors Of Using Unpaved Runways, Part 1

One of the major advantages of business aviation is the ability to save time by getting directly to the desired location. Whether it be mining operator that needs to transport engineers to a remote mine, a service technician needing to service a large irrigation device, or a humanitarian aid organization seeking to deliver emergency supplies to a disaster-stricken area, getting to the site quickly and efficiently is necessary in modern-day aviation.  

In these circumstances often the nearest airstrip is “unpaved.” Flight operations at these airstrips carry considerable potential risk and require extensive preparation.

For example, the methodical analysis on the suitability for operating to/from an unpaved airstrip must include the synergistic effects of the nearby topography and winds. A pilot can’t simply look at one or two performance charts and decide that his aircraft can fly into a destination such as Smiley Creek, an unpaved airstrip not far from Sun Valley, Idaho, that is popular with recreational flyers. 

Smiley Creek is surrounded by high terrain that limits the airstrip to “one way in, one way out” operations. It is normal to land on Runway 14 (to the southeast) and depart on Runway 32 (to the northwest). An attempt to take off on Runway 14 would be ill-fated due to the rapidly rising terrain.

Those who aren’t deeply knowledgeable about localized mountain meteorology can be caught unaware of the predominant diurnal wind pattern and how this has a significant effect on takeoffs and landings at Smiley Creek and similar locations.

The diurnal wind pattern is southerly in the morning through afternoon. Why? Air along the north-facing slopes of the tall mountains to the south of the airstrip cools overnight. That cooler air flows down the surfaces and funnels through the ravines. Since Smiley Creek airstrip sits near the bottom of that topography, the terrain funnels this fresh cooler inflow of southerly wind over the airstrip. The morning diurnal wind flow creates a favorable headwind for landing on Runway 14. However, this simultaneously creates an unacceptable tailwind hazard for taking off on the normal departure runway of 32.

The official advisory from the Idaho Division of Aeronautics advises pilots to remain on the ground until more favorable conditions exist. This means that a morning departure is unlikely with those adverse winds. Unfortunately, this translates into an afternoon departure, when the density altitudes reach excessive levels.

The Smiley Creek airstrip is located at 7,160 ft. MSL. On a summer afternoon, with the temperature at 82 deg. F, the density altitude would be 10,362 ft. The takeoff performance of an airplane at that density altitude is anemic at best. Every performance chart in an aircraft’s operating manual is based on specific conditions and contains notes on how to adapt the information to flight conditions. A pilot must carefully understand the assumptions and limitations of the takeoff performance charts to determine how well those charts apply to this particular condition.

The density altitude under these conditions will also significantly impact a jet’s second-segment climb performance. Proper performance planning for a multi-engine aircraft would need to assure a sufficient second-second climb gradient to out-climb terrain in the event of an engine failure at/after V1.  

Go-Around Analysis
A thorough analysis of an unpaved airstrip must also consider the go-around. An attempt to do a late go-around from Smiley Creek’s Runway 14 (which is the “normal” landing runway) will face rapidly diminishing separation from the rapidly rising terrain to the south and east. Having an aircraft with suitable climb angle and turn radius performance is a necessity, and this especially applies during the performance-robbing high-density altitudes during the normal recreation season.

Not only does the terrain confine a go-around attempt to the southeast, but a turbojet has an additional limitation because of the spool-up time. I was exposed to this very scenario in the simulator when attempting to fly a go-around while in the flare at Telluride. The 6-9 sec. required for engine spool-up time from idle to go-around thrust resulted in our aircraft going off the end of the cliff each time we attempted this experiment. The spool-up time at high altitude is a risk that isn’t well known. This needs to be factored into a pilot’s “go/no-go” point for a go-around.

Visual Inspection Before Landing

Unpaved runways serve as key logistical ports in sparsely populated regions. These runways are the norm in many parts of Canada and Alaska. Thirty-five miles north of the Arctic Circle is the village of Bettles, Alaska, which has a gravel primary runway. The notes for the runway include, “Runway condition unmaintained. Recommend visual inspection prior to landing.”

A safe flight begins with good planning and getting accurate information from reputable sources on the weather and up-to-the-minute assessments of the runway’s surface and operating environment. This is yet another challenge with flights into these locations.

A visual inspection is considered standard practice at unpaved airstrips.  This begins with a “high recon” to conduct an initial evaluation of the airfield and environment, then a “low recon.” The high recon is an evaluation of the hazards for an approach such as power lines and the spacing for a traffic pattern. The low recon focuses on the landing surface, looking for sprinklers, animals, cones, persons and moving aircraft as well as trying to ascertain the condition of the landing surface.

Visual inspections are flown at “canyon maneuvering speed.” This requires the pilot to compromise between a speed that minimizes the aircraft’s turn radius in the terrain while maintaining a sufficient margin above the stall. An aircraft operating in this environment must have the capability to fly with a sufficient margin above the stall while also having a slow enough speed to make the relatively tight turns in the confined environment.  

There is a limitation to the “low recon” method. The angle of viewing is from above, and depending on the sun’s angle and shadows, small undulations in a runway’s surface will not be visible. Some of these imperfections are best viewed from a flatter angle with the sun angle at a lower angle over the horizon.

The varying conditions of unpaved runways can present handling control challenges, we explain in Part 2 of this article.