IFR Inspections Testing Breakdown

This page breaks down the testing required for IFR inspections to comply with 14 CFR 91.411 Altimeter System and Altitude Reporting Equipment Tests and Inspections and 14 CFR91.413 Transponder Tests and Inspections using references listed at the bottom of the page. There are different ways and sequences to perform the test’s, this summary describes the testing as it is performed at Fuse Aviation (Certified Repair Station #FUSR966D). It is important to note that the following inspections do not certify your aircraft for IFR flight, only the systems being tested.  Refer to the items listed in 14 CFR 91.205 for additional IFR flight requirements.

An IFR inspection consists of testing each altimeter, each pitot/static system, each altitude reporting system and each transponder in an aircraft. Testing should be conducted at a minimum every 2 years. If any discrepancies arise during that 2-year period, a test of the component or system should be performed.

What is an IFR inspection?

How is the altimeter tested?

Testing of the altimeter is performed by a certified repair station in accordance with Appendix E of Part 43. The testing must be done with the test equipment connected directly to the case of the altimeter. In most instances the altimeter is removed from the aircraft for testing but if access permits the altimeter could be tested in the aircraft. The altimeter must pass all tests listed below to be re-certified, if any of the tests fail the altimeter must be sent for repair to a certified repair station. Repairs or adjustments are not allowed in the field. Digital or ADC (Air Data Computers) must be tested in accordance with the manufacturer’s Instructions for Continued Airworthiness.

1. Scale of Error Test – The barometric scale of the altimeter is set to 29.92 inHg and the altitude is increased to multiple test points provided in Table 1 of appendix E to part 43. Each test point is held for a minimum of one minute and then the altitude difference is recorded. As the test points increase in altitude the tolerance for each test point also increases. The altimeter is tested to the maximum operating altitude of the altimeter, usually 20 or 35 thousand feet.

2. Hysteresis Test – The hysteresis test is conducted after the maximum altitude test point reading is taken on the Scale of Error Test. From the maximum altitude, the vertical speed rate is set to descend at a rate of 5K to 20K feet per minute. There are two test points 50% and 40% of the maximum altitude. Once those test points are reached the altitude is held for a minimum of 5 minutes. The readings are then compared to the initial readings taken during the Scale of Error Test and must be within the tolerances provided in table II.

3. After Effect Test– The after-effect test is conducted no more than 5 minutes after the hysteresis test, once the two test points of the hysteresis test have been accomplished the altitude is reduced to ground altitude. The altitude is then compared to the altitude recorded at the beginning of the test. The altitude difference must not be more than the tolerance provided in Table II.

4. Friction Test – The friction test is usually conducted after the after-effect test. The friction test consists of increasing the altitude at a rate of 750 feet per minute to the maximum operating altitude of the altimeter. At each test point in Table III the altitude is held, once the altitude is stabilized the altimeter is tapped (usually with a pen). The needle deflection during the tapping must not be more than the tolerances provided in Table III.

5. Case Leak Test – The case leak test is usually accomplished at the same time as the Scale of Error Test. The case leak test is performed at 18K feet. When the altimeter is at 18K feet the test set is turned off (Leak Test Mode) and the altitude should not drop more than 100 feet in one minute.

6. Barometric Scale Error Test – The barometric scale error test is conducted after all other tests of the altimeter have been accomplished. With the altimeters barometric scale set to 29.92 inHg the test equipment is set to 0 feet. The barometric scale is then moved to each test point provided in Table IV. Each test point must not exceed the tolerances provided in Table IV.

How is the static pressure system tested?

Testing of the pitot/static systems can be performed by either an airframe certified technician or repair station and is performed in accordance with CFR 25.1325, or the airframe manufactures instructions. This task is usually performed after completing the altimeter test. Pitot/Static systems are broken up into two categories, unpressurized and pressurized aircraft. Connecting the test equipment to the aircraft is the same for both systems but the testing requirements are different. The testing starts by performing visual inspections of the static ports for any damage or deformation and the drains are inspected for moisture. Once the visual inspections are complete the test equipment is connected to the pitot tube(s) and static port(s) and testing of the systems can be performed.

1. Unpressurized aircraft - The system is brought to an altitude of 1000’ feet above field elevation. Once the required altitude is reached, the test equipment is turned off (Leak Test Mode) and the altitude is monitored for a minimum of one minute. The leak is not to exceed 100 feet during that minute.

2. Pressurized aircraft - The system is brought to the maximum cabin pressure differential altitude that is rated for the aircraft. This altitude is determined by the aircraft manufacturer and can be found in your POH. The POH usually will give a differential PSI reading, that reading must then be converted to a test altitude. Instructions for converting the given PSI to altitude are provided in AC 43-6D Section 9.1.7.3. Once the maximum differential altitude is calculated the test equipment is taken up to that altitude and the test equipment is turned off (Leak Test Mode). Altitude is then monitored for one minute, the loss of altitude in that minute must not be more than 100 feet or 2 percent of the maximum differential altitude, whichever is greater. If the system drops in altitude more than 100 feet or 2 percent, troubleshooting must be accomplished to find the source of the leak.

While performing the following test it is very important that the transponder is in the OFF position or other precautions are taken to eliminate the risk of transmitting a false target to ATC

How is the altitude reporting system tested?

There is one test to be accomplished for the altitude reporting system to comply with 91.411. The test is done in accordance with Appendix E for Part 43, Automatic Pressure Altitude Reporting Equipment and ATC Transponder System Interrogation Test. The test is to confirm that the transponder system is reporting the appropriate altitude compared to the aircraft altimeter. The altitude reporting system consists of the transponder, altitude encoder, wiring and static pressure system. The altitude reporting system is usually tested to the operating altitude of the aircraft.

To perform the test, two tools are required; a transponder ramp test set and a pitot static ramp test set. The pitot static ramp test set is connected to the aircraft in the same way as the static system leak test. For that reason, this test is usually performed after a successful pitot/static leak test. The testing is performed using a transponder ramp test set (e.g. TR-220) that can be connected to the aircraft in multiple different ways; direct connection, over the air, or using antenna couplers. At Fuse Aviation we prefer using the direct connect method to avoid transmitting false targets to ATC.  

The altitude is increased to multiple altitude points provided in Table I. At each altitude point the transponder output altitude reading will be compared to the reading of the aircraft’s altimeter. The difference between the two altitudes must not exceed 125 feet at each test point. If the altitude readings are more than the 125-foot limit the encoder should be calibrated per the manufacturer’s instructions.