Pelican’s Perch #76: Those Dreadful POHs (Part 2)

Last month, AVweb's John Deakin described some strange events from his airline days that were caused by too-strict reliance on 'The Book.' This month, the pelican gets on his perch to tackle a few GA POHs and finds (gasp!) inconsistencies, errors, and just plain dangerous recommendations.


In reviewing Part 1 of this column last month, I remembered another story that illustrates “cockpit culture.” Remember the story of the Flight Engineer (FE) and “backing up” the thrust levers, as a carry-over from the props days?

Levers And Fingers

Well, one late night in the DC-8 simulator at Japan Airlines (JAL), I was taking a six-month check with an arrogant Brit as my partner. Not many Brits are arrogant, but when you find a Brit who once was — or fancied himself as — a BOAC captain, you have the epitome of arrogance … double last name and all. I’d flown with him on the line; I’d been at parties where he’d held forth; and now I had to put up with his utter arrogance and disdain for all things American or Japanese, as well as his crappy performance, which somehow always seemed to be someone else’s fault.As usual, one of us would play captain for two hours while the other played copilot; then we’d break and switch seats to get both pilots done. He went first in the left seat, struggled through the program with difficulty, and then we started my session.The cockpit layout of the DC-8 — and the tasks required of the FE — meant that things were done a bit differently. For some reason the JAL culture was for the pilot-not-flying to fine tune the thrust setting for takeoff, relinquish the throttles to the pilot flying, then drop his hand to the base of the throttles to “back them up.” The problem I saw with this was that the “run” levers were short, stubby levers right at the back of the throttle quadrant, and there was zero space left with the throttles closed. It was a real finger trap.I didn’t like this, but it was the “book procedure,” so I always warned my cohort to either keep his hand out of that space, or he might lose a finger on a sudden aborted takeoff.So I warned this clown during the cockpit briefing. He looked at me down a long nose, and frostily informed me, “It is SOP, old chap, and I, for one, follow the SOP. We’ll do it by the book, if you please.” I’m sure that made some points with the JAL check pilot.We made a normal takeoff, and he dutifully backed up the throttles, “by the book.” On the roll, I pointed to his fingers, and said, “You could lose a finger there, if we have an abort.” Again, I got a snotty response, and a “Pay attention to the takeoff, old chap, I’ll handle my own fingers, thank you very much.”JAL check rides are totally “canned,” so we all knew that the V1 failure was coming on the next takeoff. (JAL policy was to abort at V1, another poor idea.) JAL had the weight and other parameters set in the simulator so that absolutely all the runway was needed for an abort, and I prided myself on always making it without going off the end. I knew the trick that JAL never learned. First, slam on the brakes, hard to the mechanical stops (let the anti-skid do the work) at the same time as you close the throttles and snatch on full reverse. It’s hard on the airplane, but it’s very effective, and it’s the way they do it in certification.I gave Mr. Snotty-Arrogance one more warning, about 20 knots before V1: “This is the abort coming, you’d better get your @#$%! fingers outta there!”He didn’t, and when the engine failed, I laid into it full force, and broke his finger in the process. Whooie, I’ll bet that hurt!Off he went to the hospital with that hand under the other armpit, and the check pilot filled in for him in the right seat. His only comment was, “That was a great abort, Deakin-san.” We both got a chuckle out of that.Anyway, that’s another case in point where the POH is not always the best, and sometimes needs revision. It never did get revised, though.But, enough of war stories … let’s get specific.

The Magnificent IO-550

In the TCM “Operation and Installation Manual” for the magnificent IO-550-A, B, C, G, & N (the finest “flat six” ever made), March 1998 edition, on page 6-17 (Operating Limits), bottom of the page, it says:

Cylinder Head Temperature

Recommended Max. operational temperature: 420F
Limit: 460F

Compare this with the TCM “Operator’s and Installation Manual” for the IO-520-B, BA, BB, C, CB, M & MB, March 1994 edition, page 15-15-01.

Cylinder Head Temperature**

Max. Cruise: 380F
Max. Allowable: 460F

**Bayonet thermocouple AS234 with AS 236 adapter

Why the difference in terminology, please? The very titles of the manuals don’t match. Yeah, I know, that’s really being picky, but these are technical manuals: There’s no good reason not to get them right, and this is symptomatic of all the modern manuals. This may be the first clue that different people wrote them, four years apart. No problem — people burn out, move on, up, or out, and new blood comes in. My impression is that the new hires get stuck with revising old manuals for new engines, then the lawyers take over, and no one much gives a hoot how accurate the final product is. It’s not just engines, either. When is the last time you saw a really good, clear manual for any consumer device?This is amazing, in this day and age of computers, when a mistake can be corrected instantly. The old manuals came as close to perfection as I’ve ever seen, and they were hand-typed, on manual typewriters! If there was an error, the whole page had to be re-typed! In cases where text was inserted, many pages might have to be re-typed. There were people then who accepted nothing less than excellence. Few individuals or corporations today measure up to those standards.But, I digress.Much more important: Why are there differences in recommended temperatures? Well, some will huff, “One’s a 520, and the other is a 550, stupid.”Not so fast, partner. Both engines use the same cylinders and parts, right down to the part numbers. The only thing that gives the 550 more “cubes” is a different crankshaft, with a slightly greater “throw,” and pistons with a slightly different location of the wrist pin to keep the compression ratio the same at 8.5:1.

Why Those Obscene CHTs?

We need to understand more about how these obscenely high CHT limits come to be. The engine manufacturer runs the engine on a test stand, with elaborate ducting and very powerful fans to cool the cylinders properly and evenly. Under these conditions, a max limit of 460, and an operating limit of 420 is “stressful,” but probably OK. On the test stand.Keep in mind, the engine-certification run may theoretically only last 150 hours on one engine, and maybe only 300 hours in practice — total! Worse, the FAAs concept of a durability test is to do a lot of full-rich, full-power operation. The full-rich part of that test is almost meaningless, because the rich mixture holds down the peak cylinder pressures, compared to real-world operation of the engine.With the new-engine type certificate in hand, the engine manufacturer ships the engine, and the FAA-approved TCDS (Type Certificate Data Sheet) has those limits. Some Lycomings even allow up to 500F!The aircraft manufacturer then stuffs the engine into a cowling that may be designed more for looks than cooling, and slaps some baffling on it the same old way they’ve always done it. They do a single climb-cooling test to prove that no cylinder goes over the 460F or 500F redline in a hot-day climb — and then they sell the airplane to the unwary public. This is a very different bundle of baffles — and a hugely different cooling airflow — than the engine saw during certification.What limit does the aircraft manufacturer slap on the CHT? Why, the same as the factory, of course. If the airframe manufacturer used some lower temperature, then some competitor (or magazine editor) would be shouting, “Ah, they must have installation problems!”The key here is uneven cooling. In the airplane, the less-than-good baffling, coupled with the asymmetric geometry of the cylinder, cause the temperatures measured around each cylinder head to vary, sometimes as much as 150F difference from one side to the other of the same cylinder. As the CHT rises past about 420F, the hottest part of the cylinder expands enough in relation to the cooler part to cause the cylinder to change very slightly out-of-round. The piston remains round, leaving a round peg running up and down at high speed inside an oval hole. At some point this creates enough friction to cause extreme local heating, and a scuffed cylinder/piston (or worse) is the usual result. This can happen fairly quickly, once that “thermal runaway” begins.

The TSIO-520BE (Malibu) Engine

Let’s look at the TCM “Maintenance and Operator’s Manual” for the TSIO-520-BE engine, from February 1990.This was the engine introduced in the Piper PA-46 Malibu, with some interesting results. (“Interesting” in the Chinese sense.)On page 13-2 we find:

3. The maximum recommended cruise setting is 235 HP at 2400 RPM and 31.0″ Hg. MAP with the mixture set at 25 F to 50 F lean of peak T.I.T. At cruise settings below 65% engine may be operated at peak T.I.T. or below if obtainable.

They actually got this one reasonably right, if you read it carefully. The engine is rated at 310 HP, so 235 HP is about 75%, and this is clearly lean of peak (LOP). By dropping the HP to 65%, it’s OK to operate it slightly richer, at peak TIT.Some of the blowhards who shout, “I wouldn’t tell my worst enemy to run LOP!” and “Running LOP will burn up your valves!” just hate this engine, because they cannot explain it. There’s nothing special about the basic parts, or the valves, or the metallurgy.TCM probably used up the talents of the last true engineers in the development of this engine during heavy competition with Lycoming for Piper’s powerplant selection for this new airplane (according to Carl Goulet, the long-time VP of engineering at TCM). Unfortunately, they missed one little detail. They thought they were getting uniform fuel flows to all intake ports (by the famous “coke bottle test” — see sidebar above at right), but ultimately they did not realize that their stock nozzle configuration wasnt quite good enough to get balanced fuel to the combustion chambers. In missing this key point, they went at the problem backwards, and spent the better part of two years tweaking the induction system, adjusting the airflows to match the fuel flows and trying to bring the “fuel/air balance” into line. This worked fairly well, but it did leave some “unbalanced” power between the cylinders, which meant the engine wasn’t as smooth as it might have been.Having properly set the engine up to operate LOP, TCM decided they could squeeze one more bit of performance out of the engine by moving the spark timing away from their normal 20 or 22 degrees before top-dead-center (TDC) for turbocharged engines. They reset the timing out to 24 degrees TDC. No other TCM big-bore engine has timing set this far before TDC. But it was the right thing to do, because they had the engine set up for LOP operations, and they required LOP operation, where the fuel/air charge burn-times are relatively slow, which placed the peak combustion pressure far enough after TDC to be effective and safe.This advanced configuration — together with some of the old wives tales in GA — led to a thought process in the pilot population that killed some people, and — in some uninformed circles — gave the engine a bad reputation. The thinking went something like this:

“Well, gee, the (holy) book says I can run 75% at 25F to 50F LOP. But I don’t like the slight roughness I find there, and this LOP stuff scares the daylights out of me. I don’t need to save every last gallon, and the wife wants to make a pit stop, anyway.”

Sounds reasonable, right?Our new Malibu owner muses on …

“Everyone knows leaner is hotter, so I’m going to enrich just a bit, and cool things down.”

When they did that, they got more power, the engine smoothed out nicely, and they were happy. What they didn’t know — due to lack of instrumentation — was that by enriching, the burn-time of the combustion event speeded up and moved closer to TDC, the CHTs went much higher, the internal combustion pressures went much higher, and parts began failing. That early timing, so clever for LOP operations, now became devastating.In this case, had they followed the TCM advice on page 13-2 of the manual quoted above, the engines probably would have made TBO pretty routinely. In fact, if you go to a Malibu Mirage Owners meeting and ask around among the pilots who fly with the TSIO-520 BE engines — and religiously operate them LOP per the book — they usually do make it to TBO or close to it.Years ago, the editor of a major flying magazine flew one. Apparently the engine was not set up properly and it did not run very smooth LOP. The editor correctly noted the problems, and by experimentation, found that by running 25-75F ROP, the engine ran nice and smooth, and produced a lot more power and airspeed. He then related his experience to a few hundred thousand pilot/readers and effectively put his reputation behind the ROP operation of the engine, regardless of the fact that the engine manual does not authorize ROP operation of the engine at high-power cruise settings. Word spread, with many “gurus” making comments like “fuel is cheap, engines are expensive.” Many — perhaps most — Malibu pilots followed that terrible advice, and ended up operating their engines at much higher actual power outputs than 75%, at the hottest possible mixture setting! Its no wonder that the pilots who followed that advice ended up with poor results.The very next line in the engine manual, right after it states pretty clearly that 25 to 50F LOP is required, says:

Caution: Do not operate with E.G.T. settings of 25F on the lean side of peak at any power setting.

How on Earth did this line sneak into this manual? The Malibu, as delivered, didn’t even have an EGT gauge (it did have an analog TIT gauge), and all-cylinder monitors were not then in common use.Why do they speak of TIT in one paragraph, and EGT in another? TIT and EGT both peak at the same point on the mixture curve, so 25F LOP TIT is the same mixture setting as 25F LOP EGT. In any event, this line is a complete and clear contradiction with the preceding paragraph.A little poking around reveals one possible answer. That line is probably nothing more than “boilerplate” left over from some previous manual! That same line is found in every TCM manual I could get my hands on. Whoever wrote the manual simply typed in the “new stuff,” and left the old alone. Or, the lawyers proofed it, and said, “Hey, they forgot this standard line,” and stuffed it in. Certainly no real engineer read it at the factory, because a real engineer would have spotted the inconsistency. Inconsistency my foot — that’s a direct contradiction!But what about that line, on its own? What is the evil magic with exactly 25F? Does that mean that 24 and 26 LOP are OK, and only exactly 25 is “bad,” and prohibited? If you get that needle right on exactly 25F LOP, the engine will just explode? Wanna buy a bridge?


Here’s another interesting manual, the one for the TSIO-360LB, October 1983, page 13-2. This engine is found in the Mooney 231 and the Turbo Arrow. Under “Cruise Control by T.I.T.,” we find:

3. The maximum recommended cruise setting is 160 BHP at 2450 RPM and MAP (per Chapter 13 Performance Charts) with the mixture set at 25F rich of peak T.I.T. At cruise settings below 65% engine may be operated at peak T.I.T.

No, dear readers, that “25F rich of peak” is not a typo, at least not in this column. As I sit here, typing this, and double-checking to be sure I am re-typing it correctly, I am aghast at this terrible, terrible advice. That’s 76% power, and the “sacred book” is advising pilots to run it at the worst possible mixture setting, with the absolute maximum abuse they can inflict. We teach the concept of the “danger zone,” or “the red box,” and this power setting is squarely in the middle of the worst possible place to run any of these engines at high power.The advice in this paragraph is exactly backwards. At 75% power, the engine needs to be run at peak TIT/EGT, or leaner, and at 65% or less, it can be operated anywhere on the mixture curve. Those are still very aggressive power settings — still a bit above what I’d like to see.To add insult to injury, the very next sentence is our old familiar boilerplate again:

Caution: Do not operate with T.I.T. settings of 25F on the lean side of peak at any power setting.

There’s that “deadly” 25F LOP, again. I know of no possible rational reason for this line.What is truly amazing about this particular manual is the presence — in the same manual — of a specific recommendation to lean the mixture so that the engine operates at a BSFC(min) of 0.42. As it turns out, 0.42 BSFC on this engine is LOP. Once again, the original manufacturer’s engine manuals are internally inconsistent, with specific conflicting instructions.

The Airplanes

The Beechraft Baron 58 manual dated October 1984 carries on the tradition of misinformation, or lack of information, mixing truth with non-logic. On page 4-15 we find the section:

LEANING USING THE EXHAUST GAS TEMPERATURE INDICATOR (EGT)A Thermocouple-type exhaust gas temperature (EGT) probe is mounted in the exhaust system. The probe is connected to an indicator in the engine instrument array. The indicator is calibrated in degrees Celsius. Using the EGT system to lean the fuel/air mixture when cruising at 2500 rpm and 25 in. hg manifold pressure power or less in the following manner:1. Slowly lean the mixture and note the point on the indicator where the EGT temperature peaks. Further lean or enrichen the mixture to the desired cruise mixture. Further leaning is referred to as operation on the lean side of peak EGT. Enrichening the mixture is referred to as operation on the rich side of peak EGT.2. At lower power settings, the engine may be continuously operated at any mixture settings from full rich to 27C on the lean side of peak EGT. At higher power settings, as indicated on the manifold pressure vs RPM graph, (in the Performance Section) the engine should not be operated closer to peak EGT than 20C (rich side or lean side).

Note that 27C is 48.6 F, and 20C is 36 F. These are not absolute temperatures — they are differential temperatures — and are converted with the simple 1.8 factor.Someone was trying really hard here, and they almost got it right. It’s obvious that whoever wrote this thought peak EGT was the “worst” place to be, at high power settings. Had he shifted that point about 40F towards the rich side, he’d have been correct.

1985 Turbo Centurion

On page 4-26 of the POH for the 1985 Turbo Centurion we find:

The recommended cruise mixture is at peak TIT or 1750F, whichever occurs first. Leaner mixtures may be used but are not recommended considering that only a very small gain in economy is possible, accompanied by an added loss in speed and possibly cabin comfort. However, richer mixtures may be used as desired for smoothness or power at the expense of increased fuel consumption.

Uh, excuse me? What’s “cabin comfort” got to do with all this mixture stuff? Can you spell “vibration”? How about “roughness” — would that qualify as “cabin comfort?” Are we dealing in euphemisms here, or what?

The Old Data

In the past, both TCM and Lycoming had real engineering talent at the factory. Some of the old data charts found in the back of the older engine manuals are wonderful, and reflect accurately what’s going on in the engine. They also agree with the old data from Pratt & Whitney and Curtiss Wright from before and after the war. It’s a real pity that the current-day authors of the textual material in the front of “modern” POHs don’t understand that old data. Sadly, the trend is now to not even include the old charts, and the old data, leaving only the pap written by the young new-hires, who seem to make it up as they go.A perfect example of this can be found in Lycoming’s now-famous Experts are Everywhere to Help You op-ed piece.Frankly, I don’t think this should have made it out the door, and I’ve previously done a rebuttal.

In Summary …

I don’t recommend you throw away your POH. There is a lot of good info in it, and it’s usually just about all you’ve got. But don’t take it as holy writ, and read it with a jaundiced eye, remaining alert for errors, typos, and poor logic. Read it for entertainment value, for there is often some really funny stuff. Share some of the better ones with me, and I may even do another column.Be careful, up there!