The compression check, and what a number does not tell you

Twenty-five elements, six station elements. The synthesis unit of Powerplant, taught last, and the engine equivalent of AF-D.

1. The differential compression check is where the section turns

S1, R1. It is the primary condition assessment of a piston engine, required at every 100-hour and annual inspection, and it is the task you will perform most often.

How it works: the cylinder is brought to top dead center on the compression stroke, regulated air is applied, and two gauges compare the pressure applied with the pressure held. The difference is what is leaking past.

The number alone is nearly useless. The diagnosis is in where the air goes, and you find that by listening:

That is the whole skill, and a mechanic who records "62/80" without listening has taken a measurement and not made an inspection.

Two refinements that matter:

• A low reading may be a stuck ring, not a worn one. The standard response is to run the engine, warm it thoroughly, and re-check before condemning a cylinder.
• The engine must be warm. A cold engine reads low because clearances have not closed up, and a cold check condemns serviceable cylinders.

2. R1 — the propeller will kick, hard

Applying regulated air to a cylinder near top dead center pushes the piston down, and the propeller turns with it — suddenly and with force.

This is the task in which a candidate is most likely to be hurt by a propeller, and not because the engine starts: because the compressed air turns it.

The controls:

• Magnetos grounded (PP-H Procedure 0) — you are turning the engine repeatedly.
• Hold the propeller against rotation as pressure is applied, by the manual's method, with a second person where the procedure calls for one.
• Nobody in the arc.
• Bring the cylinder past top dead center carefully, and expect the kick.

3. It is GEN-I applied to an engine — deliberately

K1, K4, K6 restate the regulation. S4 restates AD applicability. S3 and S13 are record work.

Learn the regulation once, in GEN-I, and apply it here. What this unit adds is the engine — and one thing candidates do not expect.

S5 and S6: an engine has its own type certificate. Whether this engine may be installed on that airframe is a documented question with a documented answer, and the engine TCDS carries its own limits, its own approved fuels and its own accessory eligibility — independent of the airframe's TCDS.

K2 and S11 add the clock. Life-limited parts on engines are frequently counted in cycles rather than hours, which is a different accounting from anything in AF-D Concepts §5, and it must be tracked from the records rather than estimated.

4. S14 — a small element under a large obstacle

Inspect an engine mount to determine serviceability.

Engine mounts on most light aircraft are welded steel tube, and they crack at the welds — which is where the heat-affected zone is, and where the stress concentrates.

And the crack is underneath an engine that nobody wants to remove, which is precisely why it goes unfound.

K8 and S14 are the only place in the certificate where engine mounting structure is examined as structure rather than as fasteners, and AF-A Concepts §1's welding knowledge — which you will never perform — becomes an inspection you will.

Vibration is the usual cause, which connects it to PP-M §4: a cracked mount is frequently a propeller balance story.

Check the rubber isolators too. Perished or collapsed mounts change the engine's position and its vibration path, and they are easier to see than the cracks.

5. R2 and R3 — maintenance on a running engine

Adjusting anything on a running engine puts your hands near a turning propeller and a hot exhaust, and PP-I §1 lists the four adjustments that require it.

S12 — perform an engine start and inspect operational parameters — is the whole of GEN-F Procedures Procedure 7, with the addition that you are looking at numbers rather than merely completing a start.

6. What consumes this unit

This is the last unit of Powerplant because it consumes all of them.

License

CC BY-SA 4.0 (Licensing).