Altitude switch test rig. 7600 m.

Background: the altitude switch in IJT-LSP

The Intermediate Jet Trainer — Limited Series Production (IJT-LSP) is an advanced trainer aircraft programme. The altitude switch is a safety-critical avionic component that monitors cabin altitude and provides an electrical signal when the aircraft descends below a critical altitude threshold (7600 m ±150 m). This signal drives warning systems or activates protective circuits depending on the aircraft configuration.

Because the switch operates on absolute atmospheric pressure rather than gauge pressure, it must be tested under controlled low-pressure conditions that replicate the atmospheric pressure at 7600 m. The Absolute Pressure Switch Test Rig provides this environment with precision and traceability.

Test methodology

Step 1 — Electrical setup

The UUT altitude switch is connected to the rig via the MIL D 38999/26 FA 35 SN mating plug. The DC power supply is set to the required test voltage and the ammeter monitors the switch circuit. The insulation tester verifies dielectric integrity of the connector and wiring before pressurisation.

Step 2 — Pneumatic pressurisation

Pressure is applied to the altitude switch pipe union. The bellow assembly expands in response. The operator monitors the pressure stimulus until the micro switch closes — confirmed by the green indicator lamp illuminating and a current reading on the ammeter.

Step 3 — Threshold verification

The pressure at which switch-over occurs is recorded and compared against the 7600 m ±150 m specification. Pass/fail is determined by whether the switch activates within the tolerance band. Out-of-tolerance switches are rejected; in-tolerance switches are accepted and logged against their part and serial number.

Instrument calibration and traceability

All three primary instruments — DC power supply, insulation tester, and ammeter — are supplied with individual test and calibration certificates traceable to national standards. This satisfies the traceability requirements of defence procurement quality plans and allows the test data to be submitted as part of the component acceptance record.

MIL-grade construction

The rig uses shielded PTFE wiring throughout — the standard for defence avionics test equipment — with ferrule and thimble terminations at all connection points. The mating plug (MIL D 38999/26 FA 35 SN) is the correct series for the IJT-LSP altitude switch connector, ensuring proper mating without adapter risk.