The Deer Fence A Risk Benefit Analysis

Carol Simpson


In the September 06 PMLAA newsletter, Jim Thomas (JT) asked for feedback on his efforts to keep deer off our runway with the electric deer fence. We have a range of opinions among us on the effectiveness of the fence and the safety or dangers associated with it. These opinions are emotionally charged, no doubt because this is something we care about - safe flight operations at PML Airport. I'd like to share my observations and reasoning regarding the deer fence. I hope everyone has responded to the deer fence survey that Jim sent out at the beginning of November.

Unfortunately, we have no baseline data on the numbers of deer on the runway before the fence was installed. And, to my knowledge, no one has collected any systematic data on the numbers of deer on the runway following the fence installation. We have observations from individuals, taken from their particular vantage point, and several differing opinions.


Rather than continue the discussion based on opinions regarding the numbers of deer on the runway before and after the fence went up, I take a different approach, a comparison of risk versus benefit. I make a few assumptions, assumptions that I think most will agree with. (1) With or without the fence, there are still deer on the runway. (2) It only takes 1 deer to damage an aircraft and possibly injure pilot and passengers during take-off or landing. (3) The larger the group of deer, the greater are the chances that you will hit one. (4) The deer are not the only dangerous obstacle in the runway environment; the fence is also an obstacle, albeit in a fixed location on either side of the runway whereas the deer may be on the runway or on either side of it in unpredictable locations.


The following are my own observations and those reported to me by others who made them first-hand.


Observation 1: The fence may discourage deer from going onto the runway. This observation is based on reports from individuals who live along the runway at various locations. If the total number of deer on the runway is reduced, there may also be more or longer intervals of time during which there are no deer on the runway.

Observation 2: The fence does not prevent deer from going onto the runway. They go around it, they jump over it (the fence is 7 feet tall) and they actually go through it. I am told that JT and Supervisor Mark Thornton saw deer go through the fence at a recent Boy's Club meeting. Conclusion: even if we put a fence around the entire runway including the runway ends (which would be unthinkable), we would still have deer on the runway.

Observation 3: The fence also is a hazardous obstacle for aircraft and their passengers in the event of an accident or incident that results in a departing or landing aircraft going off the runway and into what is supposed to be, by Federal Aviation Regulation Part 77, the clear zone on either side of the runway. In addition to any damage and injury that result from the accident itself, an aircraft that impacts this electrified fence will likely suffer additional damage. For aircraft with propellers in the front, the wire will get tangled up in a still-spinning propeller, may cause sudden stoppage, may then slice into airframe, windscreen, pilot, and passengers. We have already had one incident which was, in my opinion, made far more serious by the fence. I refer to the Stitts Flybaby that lost brakes on landing, headed for the fence, got its prop tangled up in the fence, and nosed over. Repair estimate is $5000. Had the plane just rolled to a stop, I think there would have been less damage. It could have been worse had there been a fuel leak. I don't know if the fence was powered at the time. Obviously fuel and electricity are a bad mix.

Observation 4: Since the advent of the fence, deer tend to accumulate on the runway between the two parallel fences with the result that the peak numbers of deer are higher than pre-fence times. Even if the total number of deer is a smaller number with the fence, still the concentrations of deer are of larger size with the fence.

Observation 5: When a large concentration of deer is on the runway between the fences and an aircraft or vehicle traverses the runway, the deer tend to panic and run back and forth across the runway, not being able to exit the runway.

Observation 6: The fence is an obstruction to rescuers in the event of an aircraft accident on the runway. A rescuer on foot who is abeam the accident location will have to run all the way around the fence, a distance of up to 2000 feet, instead of 150 to 300 feet to get to the aircraft, turn off fuel, help pilot and passengers get clear of the aircraft, etc. Observation 7: The fence violates FAR part 77 SubPart C Obstruction Standards regarding requirements for the Runway Object Free Area (ROFA), also called the primary surface, which for our airport with a Utility Runway and Visual Approaches extends 125 feet from the runway centerline on each side of the runway and within with all objects regardless of height are prohibited unless they are navigational aids, runway markings, or runway lighting. CALTRANS is responsible for licensing public use airports in California and applies Part 77 as part of this process. See their website for a lot of good information on this, CALTRANS also has the authority to close our airport if it violates Part 77.


Table 1 is my list of the benefits and risks associated with having or not having the fence. I think the items in each of the four cells logically follow from the 3 assumptions 7 observations I have listed.












* May discourage deer from getting on runway.

* May result in fewer total numbers of deer on runway

* May result in more or longer intervals with no deer on the runway

* No additional obstacles for aircraft that run off the runway

* No additional fire risk for aircraft that run off the runway

* Unobstructed access to aircraft accidents by rescuers

* Smaller groups of deer on runway at any given time

* Airport in compliance with FAR Part 77








* Obstacle hazard to aircraft that run off the runway edges

* Obstacle to rescuers in event of an accident

* Fire danger to aircraft that run off the runway edges

* Conducive to accumulation of large herds of deer on the runway, increasing risk of hitting one of them

* Violates FARs for Runway Object Free Areas.

* Not having the fence might result in more or longer intervals with small numbers of deer on the runway.


You may not agree with my assignments of risks and benefits, but I hope I have given you something to think about as we weigh the risks and benefits of the deer fence at our airport. My own conclusion is that we will be at risk of hitting deer with or without the fence but that the overall risk to safe operations at our airport will be lower if we remove the fence. I also applaud JT for starting to work with an alternative method of deterring the deer, small high frequency sound emitting devices which have worked well for Bob Appleby and Karen Bound for 2 years now to keep their yard free of deer without any fences. We will have to see them work at ground level, however, and not mounted on poles, in order to not have obstacle issues similar to those we have with the fence. Hopefully JTs testing will show that we can put these devices at ground level and still deter the deer.