Lean for Taxi

                                                                                                                                             First Edition, Vol. 2


This month’s topic is one that really gets in my craw. I recently saw this hand written addition on an aircraft’s pre-flight checklist. It’s a topic that just makes me shake my head in utter amazement at the obvious misunderstanding of a concept one is willing to comply with without really knowing why he/she is being submissive to it. The topic is of course as the heading eludes to, “Lean for taxi”. This is just another sack of whowee that seems to be a common misconception that everyone accepts. Except me.

Who in their right mind should have to remember to lean the aircraft engine when taxiing? Why should one have to do this in the first place? Is it one of the items on the approved Pilot Operating Handbook, (POH) or pre-flight checklist? What if one forgets to enrichen the mixture for take-off and the engine is starved for fuel because of this? I can see why some pilots are accustomed to this task and are arbitrarily conditioned to do so. It is directly proportional to the maintenance, or lack thereof, regarding their aircraft and the frequent check and adjustment of the engines fuel metering device. This device, regardless whether its mode of fuel delivery is by Bernoulli’s principle or by impact tubes, differential pressure diaphragms, rotating spools and drip type injectors. Proper fuel mixture is paramount to efficient engine operation regardless of the engine RPM or fuel flow numbers. This task of leaning the engine for taxi is not only dangerous, it’s totally unnecessary. And, I’ll tell you why.

Checklists for most aircraft maintenance manuals have a line item on them to check the idle speed and the idle mixture every 100 hours or annually and adjust accordingly. Let’s use the Cessna 185 for example. Now the Aircraft Maintenance Manual, (AMM), D2000-9-13 for 180 & 185 series, doesn’t state specifically at what intervals the idle speed or idle mixture is to be checked, but TCM, (Teledyne Continental Motors)  SID97-3E does.  The Cessna AMM does explain how to check and adjust both the carbureted and fuel injected systems on either model so equipped this manual covers. TCMSID97-3E explains it in quite a bit of detail, on how to adjust the fuel injection system for proper operation at all operating parameters for all TCM Continuous Flow Fuel Injected engines. The FAR’s state that you “shall run the engine, [FAR 43.15 (c) (2)], for the manufacturers recommendation of power output, (full static and idle), magneto operation, fuel and oil pressure and CHT’s and oil temps”. But, nothing is really definitive in the AMM. Now, if nothing is definitive in the AMM, then what instructs the mechanic or technician when to check for, and adjust the idle speeds and idle mixtures on the aircraft he is working on, other than TCM SID97-3E? Well, that would be our long lost friend, common sense. Remember him? Yeah…….I thought so. It’s been a while right? Yeah……I know what you mean.

What I find puzzling, is the wide range of various procedures regarding inspection methods and the processes that is neither methodical nor conducive to the flow of the task at hand. For example, I run each and every aircraft before I start any inspection. I need to know how the engine is running. I perform this check to make sure everything is working as intended and the engine is operating correctly. I check all of the instrumentation for function and accuracy.  I slowly advance the throttle to 1,500 RPM and once the engine has warmed up so the oil temp is at 120° or “in the green”, I advance the throttle and run the engine up to 2000 RPM. I run the checklist and write down what I observe for each line item, including performing a magneto performance check. After that, I perform a full power run to check for full static RPM. I check manifold pressure, (MAP), and fuel flow, (FF) and that the published full power static RPM is obtained and observed.  I do this as briefly as possible, and with the cowling installed. (If you do any run up over about 1800 RPM’s for most engines, for any extended time without it being properly cowled, you will overheat the bottom of the cylinders, guaranteed). I then bring the engine RPM slowly back down to an idle and check the recommended idle speed RPM and adjust it if necessary. If it’s a carbureted engine, I check the idle mixture here at idle. I’m looking for about a 50 RPM rise as I slowly pull out the mixture control to idle cut-off (IC).  I push it back in before the engines dies and perform a “hot mag” check by turning the ignition completely off. If the engine starts to die, I then quickly return the switch to “both” or “on” depending on the set up. If it starts to die, then all is good and I have properly working “P” leads. If not, I need to check out why. Caution: If you’re going to do this check, only do it at an idle. If you do it at too high of an RPM, you’ll run the risk of blowing off your exhaust system. If I have a fuel injected engine I’m running up, I check the idle mixture at 1000 RPM. I’m looking for a 25-50 RPM rise. I use 1000 RPM because it’s high enough to overcome any mis-firing from the ignition or fuel atomization issues that may occur, but low enough to still be on the idle circuit of the system. Plus, this is the recommended RPM out of the book. If I can’t get these RPM rise numbers, then I need to perform some adjustments accordingly.


If I have less than the desired rise in RPM, then I have a lean situation and will need to enrichen the idle mixture. If I observe a rise of more than desired, then the mixture is too rich. I set the idle mixture at the field where the aircraft is based. (KSZT is 2,126 ft. MSL). This setting will be leaner at a sea level airport and conversely richer at a higher elevation field. But it will be optimal at the home field. (I then perform a compression test as soon as I can while the engine is as close to operating temperatures as possible, so I can try to get as accurate compression numbers as possible).

Now, some fuel metering devices have automatic altitude compensating circuits within the device. For example, some Bendix PS series pressure carburetors have an automatic mixture control assembly integrated within the carburetor. The Automatic Mixture Control, (AMC) assembly in these carburetors is a device that works independently of, and in parallel with, the manual mixture control to automatically correct for natural enrichment at altitude. The idle mixture must still be set at the home field for the intended operation of this system to be precise and effective.

Some TCM Continuous Flow Fuel Injection systems have an Altitude Compensating feature in the engine driven fuel pump. This system uses an aneroid that manipulates a needle to adjust for mixture derich and enrich with altitude, (barometric) changes. As with the PS system above, a “calibration” must be set at the home base field elevation to be effective as intended. TCM SID97-3E explains how to set the TCM AMC fuel pump’s unmetered fuel pressures. Which, brings up another aspect of the TCM fuel injection system. Periodic service and adjustments.

In the last twenty-five years I’ve been doing this aircraft maintenance thing, I have only seen about a half a dozen times where a shop before me actually stated in the logbooks that they checked and/or adjusted the TCM fuel injection system. This begs the question; What are these shops doing during the annual inspection? This task is to be performed every 100 hours or annually per TCM SID97-3E. It’s part of the service and “tune-up” portion of the annual inspection. Which, is more than just an “inspection”, it’s a service opportunity also. And, another issue I always see, is the lack of any lubrication whatsoever. Just about every maintenance manual for every aircraft, has a lubrication chart that states with what and when. What medium to use where, and at what interval to lubricate the particular component or system. Yet, I continually find components that are drier than a popcorn fart due to the lack of lubrication.

Sorry, I wandered off topic again. The Aircraft Maintenance Manual, (AMM) has all of this information in it. Yet, very few if any maintenance personnel or shops, are doing any of this. Why, you ask? I don’t know. Maybe so as to meet that two to three day turn around demanded by the owner. Or, just to get it out the door in the spirit of quick turn-a-rounds. In any event, “shit ain’t gettin’ done properly”. The aircraft suffers from it now, and the owner’s wallet suffers for it later. It’s a perfect case of a “false economy”. The aircraft is turned around quickly. The owner gets his aircraft back quickly. It doesn’t cost him as much as it should. Everyone’s happy……………for a while. Until something goes awry, like an overheated cylinder or some catastrophic event that the root cause can be traced back to the lack of attention given to some component. Or, if he’s lucky, another shop that is really on the ball, finds all of these discrepancies at the next annual inspection, and corrects them. And then, the cost savings of all of those “cheap annuals” catches up to him.

Many years ago, I got in a Piper 235 from one of the resident pilots at Redlands. He had been taking his aircraft to a shop over at Banning airport for the last eight or so years. When the annual became due, he contacted this shop to make the usual appointment, but was told they couldn’t get him in, and that he would have to find someone else to perform the Annual Inspection. So, he contacted me. I’m sure that he regretted this after I was done with his aircraft, because the invoice came to over $6,000.00. The reason for this high number, for him anyway, was the fact that for the last eight or so years that he had been taking his aircraft to those guys over at Banning, they apparently hadn’t been doing much, if anything, towards service and maintenance for the $500.00 to $800.00 he was paying them each year. For example. The magentos had over 700 hours on them since they were last inspected, so they needed to be rebuilt. The engine vibration isolator mounts were dated 7/86, the year the engine was installed. And, of course, the idle speed was too high and the mixture was too lean. The flight controls were so dry and absent of lubrication that the elevator controls could actually hold the aircraft in any attitude chosen without the use of the trim. The list goes on. After I was finished, I put the aircraft back in the hangar, and sent him the invoice. Before he came by to discuss, (that’s a nice way to say “complain”) the invoice, he went to his hangar to check out the aircraft. He apparently was alarmed at the ease of which the controls moved, that he actually called a friend to come out to make sure the controls were hooked up. They were. He and his friend then decided to take the aircraft for a test flight to check out the other issues that I charged him to rectify. When they came back, the owner said he couldn’t believe how different the airplane was, and noticed that even the fuel flows were lower at the same RPM and MAP settings than before. “I’ll have to learn how to fly it all over again” he exclaimed. He wrote the check and thanked me for fixing his airplane. He did come back the following year, and I finished some of the items on the original discrepancy list.

A few years ago, I had a client that had an RV-10. In the logbook, the shop in Carson City, Nevada claimed they had inspected the aircraft per the checklist in the Van’s RV-10 Maintenance Manual. I’m not sure how they could have done this, as no Maintenance Manual exists for the RV-10 that would even contain a checklist for this model of aircraft. It’s an “amateur built experimental” (E-LSA) aircraft. The only aircraft that Van’s has an AMM for, is the RV-12, which is a Factory Built S-LSA. It was obvious that the technicians at this shop were basically just “pencil whipping” the inspection, because two of the Van’s issued Service Bulletins for this model were not complied with. I guess the idea that because it’s an experimental aircraft, no product service difficulty communiqué as part of the continued product safety programs from the manufacturer, exists. Their lack of familiarity of this brand of aircraft was evident in their maintenance and servicing, as well. Even routine tasks that are common on all components of certain systems were not addressed. Including the idle speed and idle mixture settings.

Forty years ago when I was taking flight instruction, I was taught to always observe the RPM rise when shutting down the engine with the mixture control. Any deviation from the normal shut down response, was reason for investigation. A periodical check of the magneto switch as described above, was also taught. These are important tasks to perform. They allow you to monitor the fuel and ignition systems operation for safety reasons. If your engine doesn’t shut down during that quick “mags off” test at idle, then you’ll know you have a “hot” mag. This piece of information is very beneficial and will lessen the chance of you getting bit when you align the prop to attach the tow bar. Any time I have to turn the prop on an aircraft that I’m not familiar with, I always turn it in the opposite direction of normal rotation, to prevent any bloody mishaps. Now, I know what some of you are saying. “You’re not supposed to turn the prop backwards”. I say, Blödsinn! Again, the obvious misunderstanding of a concept some are willing to comply with without really knowing why he/she is being submissive to it, is evident once more. They’ll say it’s common knowledge. But when asked what the science behind this opinion is, or even a reason as to why this is, these “believers” can’t give you a straight answer or even an answer at all.

Before I return the aircraft to service, I run the engine through the same procedure again and compare the two engine test runs against the published numbers. The numbers should be the same, or better,  and meet or exceed published book numbers.

So, there you go. The next time you fly your airplane, before you shut it down, perform the “hot mag” check and while you’re at it, check for the appropriate rise in RPM when you bring the mixture control back to IC.

The moral of this story is simple. The pilot should not have to be adjusted to adapt to the aircraft. The aircraft should be adjusted to adapt to the pilot.