Every Two Years

It comes with the same surefootedness as the tax collector, but with a somewhat reduced frequency, it sneaks up with all the stealth characteristics of the most advanced tactical fighter and the consequence of oversight can be every bit as deadly. In most cases, its presence is anticipated and dealt with in stride. Such are the dealings with Air Data Indicating and Reporting systems. Regulations for reporting systems

Most Airworthiness Agencies throughout the world realize the importance of proper operation of the devices that inform the flight crew as well as advise ground based air traffic controllers or other aircraft of altitude and position. This realization comes in the form of Airworthiness Regulations. The United States Federal Aviation Regulation (FAR) 91-411 addresses the certification of barometric altimeter systems and FAR 91-413 is concerned with guaranteeing that the Transponder system is operating within specification. Most Airworthiness Agencies throughout the world require similar tests. In addition, anytime an aircraft is accepted for registration in a specific country, the authorities must be assured of continuing airworthiness and will require a plan outlining normal maintenance actions. Most often, these maintenance plans are derived by the airframe manufacturer and then customized for each unit as needed and finally approved by local airworthiness authorities. It is then up to the aircraft operator to make sure the maintenance and inspection program is followed and documented. Some aircraft manufacturers have already included the testing of the air data and reporting systems in their recommended maintenance programs. Customized tests

Certain customizations may also be included such as, testing Mode S Transponder and verification of Reduced Vertical Separation Minimum (RVSM) qualification. The tests developed by the manufacturer often includes testing Altitude Alerts, Over-speed Detection, and possibly the testing of airframe systems that require air data input. The ways and means of testing also will be a variable. In some cases, Altitude indicators or Air Data Computers are removed from the aircraft and shipped to an appropriately rated facility where testing and certification is accomplished. In other cases, all components are tested and qualified while still installed in the aircraft by using calibrated test equipment. Many times, the aircraft manufacturer will stipulate how the equipment should be checked.

Guidelines for operation
Airworthiness regulations provide a guideline for insuring a minimum standard for equipment operation. Frequently, equipment manufacturer’s tolerances are more restrictive than federal regulations and for this equipment to remain in service, the most restrictive calibration limits are observed.
Equipment that requires certification can include everything from the most basic pneumatic altimeter to the most advanced air data computer.
United States Federal Aviation Regulation 91-411 states "No person may operate an airplane or helicopter in controlled airspace under IFR conditions unless within the preceding 24 months, each static pressure system, each altimeter instrument and each automatic pressure altitude reporting system has been tested and inspected and found to comply with FAR Part 43 Appendix E." The regulation goes on to read that with the exception of moisture drains, any opening of the system will also require inspection and test in accordance with both Appendix E and F of Part 43.
Another stipulation of 91-411 addresses the qualification of those performing the tests. The aircraft manufacturer is of course granted this authority, but so are Certificated Repair Stations possessing a Class 1 Instrument rating and the equipment needed to carry out the assessment. A limited instrument rating appropriate to the make and model of the equipment to be tested or a limited rating pertaining to the test to be carried out. A repair station with a specific model aircraft rating can also be considered qualified. A U.S. government certificated Airframe Mechanic is authorized to perform static system checks and inspections only.
Altimeters and Altitude reporting equipment approved under Technical Standard Orders (TSO) are considered to be tested and in compliance as of their date of manufacture. This can often be a factor when calculating future testing.
It is also stated that no person may operate an aircraft in controlled airspace under Instrument Flight Rules (IFR) above the maximum altitude where the altimeters and altitude reporting systems have been tested.
The certification of Altitude Reporting Systems has similar requirements to Altimeters. United States Federal Aviation Regulation 91-413 states, "No one may use a Transponder unless within the preceding 24 months, the system was tested and inspected in accordance with Appendix F of FAR Part 43." Testing is also required following any installation or maintenance where data correspondence error could be introduced. Those authorized to conduct the evaluation of the equipment include a certificated repair station properly equipped to perform the functions necessary and holding a Class III Radio Rating.

Limited radio rating
A Limited Radio Rating is also considered adequate qualification as long as the Radio Rating is appropriate to the make and model of the equipment to be tested. The aircraft manufacturer is also authorized to perform this check if the manufacturer originally installed the transponder. The holder of a Continuous Airworthiness Program as specified by regulations covering commercial aircraft operations may also have the authority to certify the altitude reporting equipment in the aircraft covered by the continuous inspection program.
Compliance with FAR 43 Appendix E requires each person conducting the check to investigate the aircraft static systems for the presence of moisture as well as verify that the system is free of obstruction or restriction and that any leakage is within the tolerances outlined in the regulation governing the specific aircraft certification basis.
Federal Aviation Regulation 25 Part 1325 defines acceptable tolerances. This definition includes rules for both pressurized and non-pressurized aircraft. A non-pressurized application requires the static system to be evacuated to about one inch of mercury or to an altitude about 1,000 feet above aircraft position. Without additional suction the static system should not leak in excess of 100 feet in a one-minute time frame. For a pressurized aircraft, the static system should be evacuated until an indicated altitude is produced that would give a differential pressure between the static system and cabin pressure that is equivalent to the aircraft maximum differential pressure while in flight. For example, an aircraft that has a pressure differential of around nine pounds per square inch would be able to maintain a sea-level cabin up to around 25,000 feet. Therefore, the static system would need to be pumped down to 25,000 feet indicated altitude. Once this is achieved, no further suction should be applied and within a one-minute period, the leakage should not exceed two percent of the altitude or 100 feet, whichever is greater.
In the previous example, this would be 500 feet. As mentioned earlier, aircraft manufacturers publish maintenance procedures as part of their aircraft maintenance program. Often these procedures publish specific static system tests and may have a leakage criteria that is more restrictive than what is published in the Federal Aviation Regulations. The manufacturer’s tolerance should, in that case, be observed. It is considered almost always acceptable to use the most restrictive limit to certify the aircraft. Many airframe manufacturers will require a leak test of the entire air-data system and will test the total pressure system (Pitot) in conjunction with the static system. During the one-minute leak test period, it is usually a good idea to check the airspeed indication as a leak in the static system may result in static pressure damaging certain equipment. In fact, FAR 25 Part 1323 dictates that air speed systems be calibrated to indicate True Airspeed at sea level with a standard atmosphere. The minimum error is three percent or five knots, whichever is greater, throughout the aircraft speed range. Even though testing the Pitot system is not contained within FAR91-411, this bi-annual test is usually a suitable time to test the Total Pressure Indicating system as well.

Altimeter testing
Testing and certification of the Altimeter includes a check of scale error. This involves setting the barometric knob to 29.92 Inches of Mercury and then adjusting the static reference pressure up to the maximum operating altitude of the aircraft. The pressure change being applied to the instrument should not exceed 20,000 feet per minute. A table included in Appendix E of Part 43 lists all of the required test points and allowable error. The altimeter is stabilized for between one to ten minutes at each test point before a reading is taken. The tolerances go from 20 feet at 1,000 feet below sea level to 280 feet at a scale reading of 50,000 feet. A Hysteresis test is accomplished next and involves operating the altimeter at an altitude that corresponds to aircraft maximum altitude, after a stabilization period the test begins. Static pressure is increased, simulating a rate of aircraft decent between 5,000 and 20,000 feet per minute down to within 3,000 feet of the first test point and then within 3,000 feet per minute. The first test point is 50 percent of the maximum altitude and the test reading is taken five to 15 minutes of reaching the test point. After the reading is taken, static pressure is again increased in the same manner as before until the second test point (40 percent of maximum altitude) is reached. Here the Altimeter is stabilized for one to ten minutes. Once again, after the reading is taken, static pressure is again increased as before until atmospheric pressure is reached. Then, not more than five minutes after reaching an ambient state, another reading should be taken and should be within prescribed tolerance of the original altitude. In addition, a friction test as well as a case leakage test round out the certification process.
Part 43 Appendix E has several other requirements including verification that the static port heater (if installed) is operating properly. In addition, a visual inspection of the static port and surrounding area to verify that no alteration or deformation of the airframe surface has occurred that would affect the airflow over the static sensor for any flight condition. Many aircraft have multiple static systems as in Pilot, Co-Pilot and Stand-by. All static systems that are used for flight deck altitude display or supply altitude data to an altitude reporting system are subject to the rules of Part 43 Appendix E.

RVSM certification
In the event the aircraft is certified for Reduced Vertical Separation Minimums (RVSM), the displayed flight deck altitude must meet an even tighter tolerance than those not RVSM compliant. As this program is designed to allow aircraft operating between 29,000 and 41,000 to fly with only 1,000 feet vertical division, the altitude indicating system must be very accurate. In fact, the tolerance at 29,000 is plus or minus 48 feet and at 41,000, a 72-foot deviation is all that is allowed.

Transponder tests
Transponder tests called out in FAR 91-413 may be conducted using a test bench or a portable test set. When a test is conducted on sight at the aircraft the interrogation rate has to be much higher than usual to prevent interference from the Air Traffic Control Radar Beacon Service (ATCRBS). Also, a 3-dB signal loss is allowed to compensate for antenna coupling during receiver sensitivity measurements. As transponder systems all transmit on a frequency of 1,090 MHz, a consistency check is made to verify that the frequency is within an acceptable limit, which varies between 1 and 3 MHz, depending on the type and mode of transponder used. The system must reply to at least 90 percent of the interrogations. In addition, transmitter power output is also checked and verified to be within prescribed values. Checks will also verify the transmitted altitude data is within specification. Mode S transponders used in conjunction with Traffic Collision Avoidance Systems (TCAS), require some additional testing.

Every two years
As mentioned earlier, FAR 91-413 specifies that not only is testing required every two years, but also when there is a chance of correspondence error. This could be a result of component replacement such as an encoding altimeter or air data computer (the device that supplies altitude data to the transponder). In addition, a radio tuning unit can supply the "ident" code to the transponder so anytime it is disconnected, proper system operation needs to be verified.
In today’s digital world, with computerized components incorporating Built-In Test Equipment (BITE), many assume that initiation of internal tests and absence of faults will meet the testing requirements. Every FAA Maintenance Inspector that I have talked to says, "There is nothing like seeing a transmitted signal on a test set to verify proper transponder operation." Although with new technology, testing is still somewhat of a gray area. It would be beneficial to communicate with a local airworthiness agent to verify that any test procedure not directly in compliance with the FAR or manufacturers data, is acceptable.

Jim Sparks is Manager of Technical Information Support Services for Dassault Falcon Jet. He is an A&P and an Electrician, who began his aviation career as a technician in General and Business Aviation. Later, he became a Technical Instructor on Falcon aircraft and then a Field Representative.