Hot-Day Roughness on a Fuel-Injected Trainer: Vapor Lock or Nozzle Fault?
A flight school's fuel-injected, four-cylinder horizontally opposed engine (angle-valve, 200 hp) shuts down for a 50-minute lunch stop at a sea-level airport on a 36 °C afternoon. The pilot reports a normal start but roughness during the run-up at 1800 RPM: the fuel-flow gauge fluctuates between 7 and 11 gph, and EGTs on cylinders 3 and 4 sit about 80 °F higher than 1 and 2. Selecting the electric boost pump to HIGH smooths the engine almost completely; returning it to LOW brings the roughness back within about 40 seconds. Fuel pressure at the servo reads 12 psi against a 15–30 psi band. The boost pump and the fuel servo were replaced or overhauled four months ago, and the fuel selector was on the fullest tank. The wing surface is reading 38 °C after the aircraft sat in the sun. The shop is fully booked and the owner wants the airplane back for an afternoon training block.
1. State the most probable cause of the roughness and describe the specific checks that separate it from a clogged injector nozzle or an induction leak.
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Most probable cause: vapour formation (vapour lock) in the engine-driven pump and injector lines after heat soak.
Reasoning chain:
- The fault appeared only after a hot shutdown and disappeared the moment the boost pump raised line pressure. That is a pressure problem, not a distribution problem.
- 12 psi is below the 15–30 psi band, so the engine-driven pump is not holding rated pressure, and downstream fuel is at the point where light ends flash.
- Two cylinders were affected and the flow needle fluctuated. A blocked nozzle affects one cylinder, gives a steady lean EGT on that cylinder only, and does not respond to the boost pump. An induction leak also disturbs one or two cylinders but is present cold and at idle, not only after heat soak.
Discriminating checks:
- Fuel pressure at 1800 RPM with the boost pump OFF, then ON, compared with the AFM limits. A large delta points to the pump, its check valve, or vapour, not the servo.
- Flow test at the servo outlet into a calibrated container per the overhaul manual, to confirm delivery volume.
- Gascolator and selector for water or debris; tank vent clear; line routing and heat shielding near the exhaust.
- Only if all of the above are normal, flow-check and clean all four nozzles.
Release criteria: pressure inside the band with the pump OFF and smooth operation across the mixture range.
2. The owner asks you to 'just lean it out more and let it go'. Explain why that is unacceptable and what you would tell him.
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Leaning further does not fix a delivery fault, it disguises it while making the engine more dangerous.
- The engine is already indicating low fuel pressure. Leaning reduces fuel flow further, shrinks the detonation margin and raises CHT on the two cylinders that are already running hot. The roughness is evidence of uneven or intermittent delivery; leaning an engine with marginal fuel pressure can turn roughness into a misfire or a power loss at exactly the worst moment.
- The boost pump is a backup, not a normal source. If the engine-driven pump is not producing rated pressure, the aircraft is not airworthy regardless of how it runs with the boost pump on HIGH. A pilot should not be trained to fly behind a system that only works with a backup pump energised.
- The published pressure band is an operating limitation. A reading outside it is a discrepancy that must be corrected before return to service, not managed by technique.
What I would tell him: the aircraft is not being released today for training. I would explain the failure mode in plain terms, offer the option of operating the afternoon block on another aircraft, and give a realistic diagnosis window. If the servo is still under warranty, I would contact the overhaul facility with the pressure and flow data so the fault history is documented.
3. What records would you make, and what would have to be true before you sign the aircraft back into service?
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Records
- A discrepancy entry in the work order and aircraft log: date, tachometer and total time, the reported symptom, the measured fuel pressure (12 psi) against the limit, and the result of the boost-pump test.
- A description of the troubleshooting actually performed and what it eliminated - pressure checks, flow test, nozzle check, vent and line inspection.
- The corrective action, the approved data used, and the signature and authorisation number of the person approving return to service. If the defect is traced to the servo or pump, record the part and serial number of the replaced or reworked unit.
- If no defect is found and the condition is judged operational, that judgement must be written down and supported by data (AFM limits, manufacturer's hot-weather starting and operating guidance), never left as an undocumented opinion.
Return-to-service conditions
- Fuel pressure within 15–30 psi at the specified RPM with the boost pump OFF.
- Smooth operation through the mixture range at idle, 1800 RPM and full power, with EGT spread acceptable for the engine type.
- No fuel leaks, correct line support and heat shielding, and the boost pump checked for output pressure and check-valve function.
- The pilot briefed on the manufacturer's hot-start and hot-day operating procedure.
Signing before these are met would be a false record and would put a training aircraft into service with a known fuel-delivery fault.