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newSchoolWorks.com › VoTechWorks › Transportation › Aviation Maintenance › Avionics: Aircraft Electrical, Instrument, and Communication Systems › Lesson 5
TRN-AVIA22-04 · Lesson 5 · 47 clock hours · lab

Pitot-Static Systems and Air Data Instruments

Objective. Students will inspect a pitot-static system for leaks and blockages, connect a pitot-static tester, and verify altimeter, airspeed indicator, and vertical speed indicator readings against reference values within the tolerances specified in FAR Part 91 and the instrument manufacturer's data.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Pitot-Static Systems and Air Data Instruments

The pitot-static system feeds three of the most flight-critical instruments in any aircraft: the altimeter, the airspeed indicator (ASI), and the vertical speed indicator (VSI). A fault in a tube, fitting, or drain valve can give a crew false altitude or speed information with potentially fatal results. As an aviation maintenance technician, you are responsible for keeping this system accurate, sealed, and legal.

How the System Works

The pitot tube faces forward into the airstream and senses ram (impact) air pressure. The static port — usually a flush opening on the fuselage side — senses ambient atmospheric pressure. The altimeter and VSI connect to static pressure only. The ASI compares pitot (ram) pressure against static pressure; the difference is dynamic pressure, which converts to indicated airspeed.

Most aircraft also have an alternate static port inside the cockpit, selected by a valve when the primary static port is suspected of blockage or icing. Cabin pressure is slightly lower than outside pressure in many airframes, so switching to alternate static introduces a small error; the AFM/POH quantifies this error for the specific aircraft.

Drain points at low spots in the static lines remove moisture that would otherwise affect readings or freeze. Inspect every drain valve for security and freedom of movement during your visual check.

Regulatory Requirements

14 CFR §91.411 requires that the altimeter system and altitude reporting equipment be tested and inspected within the preceding 24 calendar months before the aircraft is operated in controlled airspace under IFR. The test must be performed by a certificated repair station or the aircraft manufacturer. FAA Form 337 is required whenever the altimeter is replaced as a major alteration or repair.

14 CFR §91.413 sets the same 24-calendar-month interval for ATC transponder tests.

Both requirements apply to IFR operations. Memorize the citation numbers — they appear on FAA knowledge tests.

Safety Before You Begin

Pre-Test Visual Inspection

Before connecting any tester, walk the system with a flashlight and mirror:

1. Check the pitot tube opening for insect nests, moisture, or physical damage.

2. Trace accessible static lines for kinks, chafing, security of clamps, and corrosion at fittings.

3. Inspect the alternate static port valve for freedom of movement and proper seating in each position.

4. Verify all drain valves are closed and secure.

5. Record every finding on the standardized inspection checklist. If anything is out of tolerance or damaged, tag it before proceeding.

Connecting and Using the Pitot-Static Tester

A calibrated ramp tester (pitot-static test set) applies controlled pitot and static pressures to simulate altitude and airspeed conditions on the ground. Connection typically involves capping or plugging all ports not under test, then attaching the tester hoses to the pitot tube and static port with the appropriate adapters.

Static system leak test: Evacuate the static system to a specified altitude (commonly 1,000 ft above field elevation) and monitor for one minute. The system must not leak more than the limit stated in the tester manufacturer's instructions and the aircraft maintenance manual — a common reference limit is 100 ft per minute or less, but always confirm against the specific aircraft data.

Altimeter accuracy check: Apply static pressure equivalent to at least two reference altitudes. Compare the altimeter reading to the applied pressure setting. Tolerances are found in TSO-C10b or the instrument manufacturer's calibration sheet. For most general aviation altimeters, the allowable error at sea level and mid-range altitudes is ±75 ft, but verify the exact figure in the instrument data.

ASI functional check: Apply a known combination of pitot and static pressures and verify the ASI reading falls within the manufacturer's stated tolerance.

VSI: Observe that the VSI reads zero when static pressure is stable and shows an appropriate trend during pressure changes.

Documenting Discrepancies and FAA Form 337

If you discover a leak or an out-of-tolerance instrument reading, document it on the inspection checklist and red-tag the aircraft. An altimeter replacement that qualifies as a major repair or alteration requires an FAA Form 337. The form must identify the aircraft, the work performed, the replacement part, references used, and must be signed by an appropriately certificated individual. One copy goes to the FAA within 48 hours of completion; one copy is retained in the aircraft records.

Understanding why the 24-month rule exists reinforces your documentation habit: altimeter diaphragms drift, static systems accumulate moisture, and transponder encoders can develop errors — all invisible to the crew but detectable with proper test equipment.

Key terms9

Ram (Impact) PressureThe increased air pressure created by the forward motion of the aircraft forcing air into the pitot tube opening; the raw input for airspeed measurement.
Static PressureAmbient atmospheric pressure sensed through flush static ports; used by the altimeter, VSI, and as the reference pressure for the ASI.
Pitot-Static Ramp TesterA calibrated ground test unit that applies and measures controlled pitot and static pressures to simulate flight conditions and verify instrument accuracy without leaving the ground.
Alternate Static PortA secondary static source, usually inside the cockpit, selectable by the crew when the primary static port is blocked; introduces a small pressure error documented in the AFM/POH.
14 CFR §91.411FAA regulation requiring the altimeter system and altitude reporting equipment to be tested and inspected within the preceding 24 calendar months before IFR flight in controlled airspace.
FAA Form 337The official FAA document required to record major repairs and major alterations on type-certificated aircraft; one copy is sent to the FAA and one is retained in the aircraft records.
Static System Leak TestA procedure in which the static system is pressurized to a reference altitude and monitored for pressure loss over a set time period to confirm the system is airtight within allowable limits.
Vertical Speed Indicator (VSI)An instrument that displays rate of climb or descent in feet per minute by measuring the rate of change of static pressure through a calibrated leak in a sealed case.
TSO (Technical Standard Order)An FAA minimum performance standard that avionics and instruments must meet; altimeters are typically manufactured to TSO-C10b, which defines accuracy tolerances.

Worked examples4

1. During a static leak test, the tester is set to simulate 6,500 ft MSL. After one minute the tester indicates 6,620 ft. Does the system pass if the maintenance manual limit is 100 ft per minute?
  1. Step 1 — Find the pressure loss in altitude equivalent: 6,620 ft − 6,500 ft = 120 ft of altitude change in one minute.
  2. Step 2 — Compare to the allowable limit: 120 ft/min exceeds the 100 ft/min limit stated in the maintenance manual.
  3. Step 3 — Determine pass/fail: The system FAILS the leak test. The leak must be located, repaired, and the test repeated before the aircraft can be returned to IFR service.
Answer: The system fails; the leak rate of 120 ft/min exceeds the 100 ft/min limit.
2. The tester applies a pressure equivalent to 3,000 ft MSL with the altimeter baro set correctly. The altimeter reads 2,910 ft. The instrument manufacturer's calibration sheet allows ±75 ft at this altitude. Is the altimeter within tolerance?
  1. Step 1 — Calculate the error: 3,000 ft (reference) − 2,910 ft (indicated) = 90 ft low.
  2. Step 2 — Compare the absolute error to the tolerance: |90 ft| > 75 ft.
  3. Step 3 — Determine pass/fail: The altimeter is OUT of tolerance. It reads 90 ft low, which exceeds the ±75 ft allowable error.
Answer: The altimeter fails; it is 90 ft out of tolerance, exceeding the ±75 ft limit.
3. A technician checks the altimeter at two test points: 1,000 ft (reads 1,060 ft) and 5,000 ft (reads 4,960 ft). The tolerance is ±75 ft at each point. Which point passes and which fails?
  1. Step 1 — Error at 1,000 ft: 1,060 − 1,000 = 60 ft high. |60| < 75, so this point PASSES.
  2. Step 2 — Error at 5,000 ft: 5,000 − 4,960 = 40 ft low. |40| < 75, so this point also PASSES.
  3. Step 3 — Overall result: Both test points are within the ±75 ft tolerance; the altimeter passes at both altitudes.
Answer: Both points pass: errors of 60 ft and 40 ft are each within the ±75 ft tolerance.
4. An altimeter replacement was completed on a Cessna 172. The Form 337 must reach the FAA within how many hours of the work being approved for return to service, and how many copies are involved?
  1. Step 1 — Recall the regulatory requirement: FAA Form 337 for a major repair or major alteration must be submitted to the FAA within 48 hours of the work being approved for return to service.
  2. Step 2 — Recall the copy requirement: Two copies are made — one is forwarded to the FAA Aircraft Registration Branch, and one is kept in the aircraft permanent maintenance records.
  3. Step 3 — State the answer clearly.
Answer: The form must reach the FAA within 48 hours; two copies are required — one to FAA, one retained in the aircraft records.
In class: Perform a pre-test visual inspection of pitot-static plumbing, alternate static port, and drain points on a training airframe, recording findings on a standardized inspection checklist and tagging any discrepancies. · Connect a calibrated pitot-static tester (ramp tester) to the aircraft system and perform a static system leak test, an airspeed indicator functional check, and an altimeter accuracy check at two altitude settings, recording all readings and comparing to manufacturer limits. · Identify and document two instructor-inserted discrepancies (one leak, one out-of-tolerance altimeter reading), complete a sample FAA Form 337 for a hypothetical altimeter swap, and discuss the regulatory basis for the 24-month altimeter and transponder check requirement.
Materials: Training aircraft or airframe section with complete pitot-static system, calibrated pitot-static ramp tester, inspection mirror, flashlight, airspeed indicator, altimeter, VSI, manufacturer's instrument calibration data sheets, FAA Form 337 blanks, 14 CFR Part 91 excerpt (§91.411 and §91.413), safety glasses
Check: Leak test results recorded correctly with pass/fail determination matching instructor reference; all instrument readings compared to tolerance and discrepancies correctly identified; Form 337 accepted by instructor as complete — all three required for pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Regulations
1. Which FAA regulation requires the altimeter system and altitude reporting equipment to be tested within the preceding 24 calendar months for IFR operations in controlled airspace?
Inspection and Testing
2. During a static system leak test, the system is pressurized to simulate 5,000 ft. After one minute, the tester reads 5,115 ft. The maintenance manual limit is 100 ft/min. What is the correct determination?
Aircraft Instruments
3. What instrument uses only static pressure as its operating input?
Safety and Hazard Control
4. Why is it important NOT to blow through pitot or static ports with your mouth during an inspection?
Inspection and Testing
5. An altimeter is tested at 4,000 ft MSL reference pressure and reads 4,082 ft. The manufacturer's tolerance is ±75 ft. What action is required?
Regulations
6. After replacing an altimeter as a major alteration, how many copies of FAA Form 337 are required, and where does each copy go?
Aircraft Instruments
7. A pilot selects the alternate static port in flight after suspecting the primary port is blocked. What effect does this typically have on instrument readings in most aircraft?
Aircraft Instruments
8. Which of the following correctly describes what the airspeed indicator measures?
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