Aircraft fluid systems — hydraulic, fuel, oil, and pneumatic — depend on tubing, hose, and fittings that can reliably contain working pressures without leaking, deforming, or failing. Every line in the system is assigned a working pressure, a proof test pressure, and a burst pressure, each representing a different threshold of stress. For an Aviation Maintenance Technician (AMT), knowing how to interpret these ratings, when proof testing is required, and what the results mean is fundamental to airworthiness. A line that passes visual inspection may still harbor hidden weakness that only a pressure test will reveal.
This article explains the engineering logic behind pressure ratings, the specific proof test requirements FAA guidance establishes, and the practical steps technicians follow on the shop floor and in the field. Because these requirements appear frequently on the AMT General written test, the key numbers and decision rules are highlighted throughout.
The Three Pressure Ratings Explained
Every fluid line specification includes three pressure values that describe its capability at different stress levels. Understanding the difference between them is the starting point for all pressure-related decisions.
Working Pressure
Working pressure (also called operating pressure or normal operating pressure) is the maximum pressure the line is designed to carry continuously during normal system operation. For a typical light-aircraft hydraulic system, working pressure might be around 1,500 psi, while high-performance aircraft can operate at 3,000 psi or more. The working pressure is the value published in the aircraft's maintenance manual and the tubing manufacturer's specification. The technician must never install a line or fitting with a working pressure rating below the system's maximum operating pressure.
Proof Test Pressure
Proof test pressure is the pressure applied to a line or assembly to verify its integrity after manufacture, repair, or installation. Per FAA guidance, proof test pressure is typically 1.5 times the working pressure (also expressed as 150 percent of working pressure). The line is pressurized to this value, held for a specified time, and then inspected for leaks, permanent deformation, or any other sign of distress. If the line holds proof pressure without failure or measurable deformation, it is considered airworthy for service. A line that leaks, bulges, or permanently deforms during proof testing must be rejected and replaced.
Burst Pressure
Burst pressure is the pressure at which the line is expected to rupture. FAA guidance specifies that lines must have a minimum burst pressure of at least 4 times the working pressure (400 percent). This 4:1 safety factor accounts for pressure spikes, fatigue, material variation, and the consequences of failure. Flexible hose assemblies certified to industry standards such as MIL-H specifications carry burst ratings verified by the manufacturer; technicians confirm these ratings by referencing the hose's identification markings and the applicable specification. The burst test is a destructive test performed by manufacturers during product qualification — it is not performed in the field on installed lines.
Rigid Tubing vs. Flexible Hose: Different Standards
Aircraft plumbing uses both rigid metal tubing (aluminum alloy, stainless steel, or titanium) and flexible hose. Each category has its own construction standards and pressure rating approach, though the proof-test logic is the same.
Rigid aluminum tubing used in fuel and low-pressure oil systems is typically made from 1100 or 3003 alloy for low-pressure applications and 2024-T3 or 5052-O alloy for higher-pressure hydraulic lines. The tubing's rated working pressure depends on its outside diameter, wall thickness, and alloy. Technicians use manufacturer tables (or the AC 43.13-1B data) to verify that a replacement tube's dimensions and alloy match or exceed the original specification.
Flexible hose is used where vibration isolation, relative motion between components, or ease of routing makes rigid tubing impractical. Flexible aircraft hose assemblies bear identification markings that include the hose specification number, size, working pressure, and manufacture date. FAA guidance emphasizes that flexible hose must never be used to replace rigid tubing in locations where the original design calls for rigid lines, unless specifically authorized by the aircraft manufacturer or an approved data source. Hose assemblies also have defined service lives based on the manufacture date coded in the hose markings.
Proof Testing Procedures
Proof testing is performed after fabricating a new line assembly, after any repair involving a fitting or flare, or when contamination or damage is suspected. The general procedure follows these steps:
- Prepare the assembly. Install end fittings, ensure all connections are clean, and plug or cap openings not under test.
- Connect to a test rig. A hydraulic hand pump or pneumatic test stand is used. Pneumatic proof testing of hydraulic lines is generally avoided whenever possible, because compressed gas stores enormous energy and a burst would be explosive. Hydraulic fluid or water is preferred for safety.
- Apply proof pressure gradually. Pressure is raised slowly to the proof test value — 1.5 times working pressure — and held for the specified duration, commonly one to five minutes depending on the applicable specification or maintenance manual requirement.
- Inspect during and after the test. The technician examines every fitting, flare, and section of the line for seeping fluid, bulging, or twisting. Any deformation of the line or fittings constitutes failure.
- Document the results. A successful proof test is recorded in the maintenance record, noting the test pressure, hold duration, fluid used, and the technician's certification.
After a successful proof test, the line is drained, dried if water was used, and either installed or preserved for storage. If any portion of the line was rejected, the entire assembly is replaced — partial repair of a failed section is not acceptable unless specifically authorized.
Why Pressure Ratings Matter
The consequences of installing an underrated or damaged line in a pressurized aircraft system can be catastrophic. A hydraulic line failure can cause loss of brakes, landing gear, or flight controls simultaneously. A fuel line failure can lead to fire. Pressure testing exists precisely because visual inspection alone cannot detect internal corrosion, micro-cracks in flares, or manufacturing defects in hose inner liners.
From a regulatory perspective, installing a component that does not meet the pressure rating of the original is an alteration that may not comply with the aircraft's type certificate. The AMT is responsible for ensuring that replacement lines are at minimum equivalent to the original in material, rating, and form. Substituting lower-grade tubing — even temporarily — is a violation of airworthiness standards.
Key Numbers and Rules
- Proof test pressure = 1.5 × working pressure (150 percent of working pressure).
- Minimum burst pressure = 4 × working pressure (400 percent of working pressure).
- The proof test is a non-destructive check performed in the field; the burst test is destructive and done only by the manufacturer.
- Flexible hose must be replaced if it shows signs of leakage, blistering, chafing through the outer cover, or if it has exceeded the manufacturer's specified service life.
- Pneumatic proof testing of hydraulic lines is hazardous because of stored energy in compressed gas; hydraulic fluid is the preferred test medium.
- Replacement tubing must match the original alloy specification and wall thickness — a visually identical tube of lower alloy may have a significantly lower pressure rating.
- All proof test results must be documented in the aircraft or component maintenance record per 14 CFR Part 43 recordkeeping requirements.
Common Test Traps
- Confusing proof pressure with burst pressure. Proof pressure is 1.5× working; burst is 4× working. Mixing these numbers up is a common knowledge-test error.
- Assuming a visual pass means a line is airworthy. A line can look perfect but fail proof testing due to internal flare cracks or internal hose deterioration. Proof testing is required after any repair, not just when damage is visible.
- Using flexible hose to replace rigid tubing without authorization. This is never acceptable as a routine substitution, even if the hose has an adequate pressure rating.
- Ignoring hose date codes. Flexible hose has a finite service life regardless of appearance. The manufacture date is coded into the hose markings, and an expired hose must be replaced even if it shows no external damage.
- Choosing the wrong test medium. The test asks about the safest proof test medium — hydraulic fluid or water is correct; air or nitrogen is hazardous due to stored energy potential in a burst scenario.