Counting Ops at GA Airports
Current methods used by FAA, states need to be refined
By Dr. Maria Muia, Manager of Aeronautics Section, Indiana DOT
In an industry with exploding growth and limited funding, it’s appropriate that measures be taken to accurately record growth and to ensure funds are allocated properly to accommodate this growth. That’s the opinion of Maria Muia, aviation manager for Indiana, who recently researched methods used to count aircraft operations. Here are some of her findings.
U.S. accounts for 50 percent of all general aviation traffic in the world,
and GA aircraft make up over 90 percent of all aircraft in this country.
Almost 84 percent of the 5,357 airports open to the public in the U.S.
are general aviation airports, and the vast majority of these airports
do not have air traffic control towers. General aviation airports provide
service to more than 5,300 communities while the airlines serve just a
handful; yet, we don’t have a uniform way of counting the traffic
that uses our general aviation airports.
Air traffic controllers count the aircraft that use the commercial airports, but who is counting the aircraft at the general aviation airports? In some cases the state aviation agency is, but in most cases the number of operations recorded at a general aviation airport is simply a guess. Albeit an educated guess in certain cases, but usually just a guess all the same.
We care about this information because it’s being used for a whole host of purposes: helping to justify airport development projects, air traffic control towers, and navigational aids. It’s being used in airport environmental documentation, forecasting, economic impact statements, airport planning, and the FAA Airport Master Records. If inaccurate methods are being used to estimate operations, then anything the resulting information is used for is questionable.
Surveying the States
I recently surveyed each state aviation agency in the country to determine exactly what method, if any, was being used to count and estimate aircraft operations at non-towered airports in their states. I further analyzed these methods for accuracy, efficiency, and cost-effectiveness.
The most popular method to estimate aircraft operations at non-towered airports is to simply ask someone at the airport what they think the operations are. Thirty states just ask the manager, the FBO, or other commercial operator. The accuracy of this method is highly questionable.
Ten states report that they multiply the airport’s based aircraft by a predetermined number of operations per based aircraft. For example, if an airport has 30 based aircraft and each aircraft is believed to perform 500 operations per year, then the airport records having 15,000 operations. The accuracy of this method is also questionable.
Three states base annual operations on information contained in guest logs at the airport. Many airports keep registers for pilots to sign when they come into the terminal. These are tallied to determine an annual estimate. But how many people actually sign these logs? (If I am typical of most pilots, I only sign them about 50% of the time.)
Eighteen states take a sample of actual operations and estimate annual operations from the sample. This method has some degree of statistical accuracy, but there are still concerns about how and when the samples are taken. Furthermore, there is concern about how the sample is expanded into an annual estimate.
Two states use information received in planning studies as their source for operations estimates. These studies include individual airport master plans, regional plans, and state system plans. One state contacts commercial carriers located at the airport and asks what the annual operations are. These are tallied into an annual count for commercial operations — but what about the non-commercial?
It’s interesting to note that of the states that responded to the questionnaire, 15 indicate that they use more than one method, making the results incomparable with other airports within the same state because of the different methods used.
Testing for Accuracy
Also, I researched the accuracy, efficiency, and cost-effectiveness of each of the methods indicated. Elev-en states report that the accuracy of their method is unknown. Thirty states indicate that they had not performed any type of accuracy test and estimate the accuracy of their method to be anywhere from zero to 80 percent. The most common method used by states that don’t perform accuracy tests is simply asking the airport manager, FBO, or others what they think the operations are.
Fourteen states performed some type of accuracy tests on the method used. Taking samples of actual operations is the most common method used by states that had actually performed accuracy tests (11 of the 14 took samples). States that perform accuracy tests generally believe their methods to be more accurate.
The least time-consuming and least expensive method to estimate operations is by simply asking the airport manager, FBO, or other personnel what they think the operations are — the cost being just the time it takes to ask the question. Normally, the state airport inspector is already at the airport doing an inspection and asks the question. This is the least accurate method. Since 37 states actually use operations information to justify airport development projects, there is considerable incentive for an airport to exaggerate its operations.
The most accurate method to count and estimate aircraft operations, on the other hand, is to actually sample operations and then expand the sample into an annual estimate. The sample, however, must be long enough and done throughout all four seasons to be accurate. Generally eight weeks is believed to be the most accurate with two weeks sampled in each of the four seasons. Although this is the most accurate, it is also the most expensive and time-consuming.
States that take samples use three different methods: visual counts, pneumatic counters, and acoustical counters (RENS counters and Larson Davis counters). Taking visual sample counts is the most accurate, expensive, and time-consuming of the sampling techniques, and most states find it too cost-prohibitive to do for eight weeks over the entire year.
Pneumatic-type counters are connected to a tube placed across taxiways and ramps. When an aircraft passes over the tube, a pulse of air is created and travels through the hose, triggering the counter. Pneumatic counters can be very inaccurate depending on the layout of the airport and the amount of non-aircraft vehicular traffic using the taxiways. Also, these counters fail to record touch-and-go operations.
The most accurate and least expensive approach is to use acoustical counters, which actually record the noise the aircraft makes and tallies the operations. However, there is not consensus on the accuracy of the acoustical counters currently in use. While the Larson Davis counter is less labor intensive than the RENS counter, the accuracy is still under question. Eight states that use Larson Davis report varying degrees of accuracy — from 60 to 99 percent. (The Indiana DOT conducted extensive tests of the RENS counter and found them to be approximately 90 percent accurate.)
States estimate aircraft operations at non-towered airports for a variety of reasons, the most common being for the FAA Airport Master Records. FAA has a statutory obligation to "collect, maintain, and disseminate accurate, complete, and timely airport data for the safe and efficient movement of people and goods through air transportation."
An Airport Master Record (Form 5010) for each airport is updated for the Airport Safety Data Program. Forty-two states use airport operations data to fill in item numbers 100-105 on this form. Information on the Airport Master Record is made available to the general public. At least three states have intricate traffic counting programs, but still use the airport manager’s estimate on the Airport Master Record.
FAA Airport Safety Data Program Order 5010.4 directs the airport inspector to record the total number of operations on FAA Form 5010. It further stipulates that the inspector is to use "FAA tower counts where available. If not available from FAA sources, use estimates based on discussion with airport management and/or the FBO."
It makes little sense for FAA to fund certain state’s aircraft traffic counting programs with federal grant money, which they do in many cases, and not use the information on the official Airport Master Record. It’s recommended here that FAA revise Order 5010.4 to include the state’s aircraft traffic counting programs as the second source for operations information when tower counts aren’t available. If a state doesn’t have a traffic counting program and there’s no tower, it’s recommended that this data element on the Airport Master Record be recorded as "unknown".
While states use annual aircraft operations data at non-towered airports for a variety of purposes, it’s not recommended that they use it at all if it’s simply a guess. If the information is going to be used for project or tower justification, environmental documentation, forecasts, economic impact statements, or the FAA Airport Master Record, then statistical sampling should be used to determine the airport’s annual operations.
Federal and state governments should not base capital funding decisions, safety decisions, or environmental requirements on information received from aircraft traffic counting programs that simply ask the airport manager or other airport personnel what the annual operations are. FAA shouldn’t provide funding for aircraft counting programs and then not require the resulting information be used in the official Master Record.
By 2010, hours flown by GA aircraft are expected to increase to 34.1 million from the 1998 level of 28.2 million, while general aviation aircraft are expected to increase to 220,804 from the 1998 level of 194,800. Yet, Congress continues to fight over levels of funding actually needed to maintain this country’s infrastructure. In order to invest appropriately in our aviation system we must understand the system as it exists today.