8
$\begingroup$

I have some concerns about the level of maintenance and service on some rental airplanes on an out of town trip.

The first airplane I rented had some misgivings, so I went on a different one today. It was a Cessna 172 and had a Cylinder Head Temperature (CHT) monitor, which I hadn't seen before. So while on an extended leg I fiddled with it. Three of the cylinders where 340-350. However, cylinder 2 was only 280. That seems like a significant outlier.

Since most rental airplanes don't have a CHT monitor, I don't have experience with what is the normal temperature. Should I be concerned that one cylinder isn't close to the other three?

Below is some information about the airplane and that flight:

  • Power: 2400 RPM
  • Altitude: 7500 feet
  • Oil Quantity: 7 quarts
  • Oil Pressure: Top of green arc
  • Oil Temperature: Normal

Airplane was fresh out of a 100-hr maintenance

Anomalous Cylinder: #2

There was some issue with the engine during the 100-hr maintenance which held it up, but was signed off by a mechanic with an IA.

I am unsure about the quality of maintenance service there. We had two flights cancelled in a row because the carburetor was fine when it was warm, but this cold snap (38°F) is making it hard impossible to start. In addition, there are all sorts of small squawks such as trim that was out of whack etc.

The bottom line is that I am concerned about my safety as probably I will be renting one more time. Is the above mentioned scenario something which could indicate a problem, or more likely something harmless? I have doubts that the maintenance is top quality, but I'm unsure if it's bad enough that I should just opt out.

$\endgroup$
  • $\begingroup$ What power setting? Altitude? OAT? EGT? Cowl flaps, if you have them? $\endgroup$ – rbp Dec 29 '14 at 1:12
  • $\begingroup$ Also, oil pressure and temp? All of these factors can help diagnose engine problems or anomalies in flight. $\endgroup$ – rbp Dec 29 '14 at 1:22
  • $\begingroup$ Added more information. Oil temp was great about 1/3 into the green, maybe 150-175f. The oil pressure was slightly disconcerting it was just about the top of the green arc (~85psi). Power was 2400 RPM (60% base on the POH). OAT was -9C, at 7500'. EGT I don't recall the numbers, but I had leaned slightly rich of peak. No cowl flaps. $\endgroup$ – John Dec 29 '14 at 3:56
  • $\begingroup$ @John If I've missed anything, please add. Rollback if you don't like the new form. $\endgroup$ – Farhan Dec 29 '14 at 21:05
  • $\begingroup$ I have an 0-470R in a 1969 C182 that has one cylinder (#6 which is the left front of engine looking forward) that has always run cooler at altitude & leaned rich of peak than the rest, especially in cold winter. Compression is good, oil consumption nill, engine runs fine on mag tests. #6 EGT consistently runs 50F in summer to 150F in winter lower than the highest cylinder which is #1. CHTs run 20F in summer to 100F in winter lower than the highest cylinder. $\endgroup$ – Mike Tessman Mar 13 '17 at 16:11
7
$\begingroup$

A 60 degree difference in CHT is something worth investigating: It may not be a safety-related issue, but you want to know if it's "normal" for the engine or an indication of a problem.

In order from roughly "Most Benign" to "Worst Case", the CHT difference could be the result of:

  • A faulty CHT probe
    Either the probe is not seated in the cylinder head well properly, or the probe/wiring is defective and causing an inaccurate reading.
  • Different CHT Probe Types
    Spark plug gasket probes can read higher than "normal" cylinder head probes, so if gasket probes were used on cylinders 1, 3, and 4 and a standard bayonet probe used on #2 that would account for the difference.
    (I can't see why this would be the case though: usually you have one gasket probe - on the cylinder where the manufacturer's CHT gauge was connected because it's using the normal bayonet probe hole - and bayonet probes in the rest, not the other way around.)
  • A mixture imbalance
    Cylinder #2 is running richer or leaner than the other cylinders. Since you say the engine is not running rough Cylinder #2 would probably be running rich. (This is normally something you'd notice with Exhaust Gas Temperature rather than Cylinder Head Temperature.)
    You can check for this by watching the engine monitor as you lean the engine: Cylinder #2's EGT will peak at a different time than the rest of the cylinders (early = leaner, late = richer). On fuel-injected engines this can be adjusted for by using tuned injector nozzles. On carbureted engines you just have to live with it - but at least you'll know the cause and be able to determine what "normal" is for this engine.
  • Fouled/Failed Spark Plug
    Easy to check in-flight: Cycle each magneto; If the cylinder dies on one you've found the culprit.
    (Something you should probably catch on your preflight mag check too.)
  • A cooling problem
    If the cooling baffles for your engine are worn/damaged/incorrectly positioned you might have anomalous airflow that's directing more cooling air over the #2 cylinder head than the rest of the cylinders.
    (Normally you find baffle problems because one or more cylinders are running hot, but cold is possible too.)
  • A cylinder problem
    An exhaust valve that's not fully closing can cause low CHTs - your mechanic can confirm this with a borescope (or a compression test -- they'll hear air escaping through the exhaust if the valve isn't closing).
    If this is the case it's something to attend to, because if it's ignored it can lead to an exhaust valve failure, which can ruin your day.
$\endgroup$
2
$\begingroup$

Seems low, but there could be some non-maintenance reasons that a pilot can either see or adjust that have an impact on CHTs. The three most important factors relating to CHT are oil, airflow, and power setting.

Did you check the oil quantity before you flew? It should be in the normal range and of the proper type. You can tell what oil is likely to be in the engine because that'll be the extra quart in the baggage compartment.

Often the front-most cylinders will be the coolest because they are directly exposed to the airflow. For other cylinders, there are rubber air gaskets that direct the airflow over the cylinder heads. These gaskets can get dislodged and cause assymetrical CHTs. Pitch attitude and cowl flap position also affect airflow.

Power setting and leaning can be adjusted in flight to control CHTs. Lower settings and richer mixtures will show lower temps.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.