POH and AFM Limitations Explained

[@JustPlaneSilly]

Just bought a new plane and I am going through the book as I am watching this. Very interesting stuff Martin. Edit: I had 2 instructors fly with me in my new plane. Both taught the 25 squared on climb out. Now I am curious about why.

[@martinpauly]

It's a myth that won't die, Bryan. I'll address it in a future video sometime, but I doubt I will be able to put the topic to rest. In the meantime, grab a few POH/AFM of popular airplanes and see if you can find a limitation (or even a recommendation) to pull throttle and RPM back right after the wheels leave the runway. - Martin

[@Stepclimb]

It’s such a stupid and counterproductive OWT. Rumor has it that it originated with CFIs of the late 1940s when GA was in a postwar growth period. Most CFIs in the 1946-1960s period were former WWII Army Air Corps pilots that flew the T-6 Texan and later, large radial powered bombers or fighters. Many of these aircraft had a “METO” power setting in the flight manual. Maximum Except for TakeOff. Takeoff power was usually redline MAP and redline RPM. After a period of 1, 2 or 5 minutes (depending on the aircraft), the MAP and RPM HAD to be reduced to the METO setting. On a lot of normally aspirated large radials, this was 25”, 2500 RPM. When these postwar pilots became civilian CFIs, they continued to apply these procedures (unnecessarily) to the horizontally opposed engines of the Bonanzas and Cessnas of the day. Unfortunately, the law of primacy is very hard to overcome. When someone is exposed to “25 squared for climb” from the start, it is pretty well stuck.

[@sw538]

I learned complex in a Cessna 172RG. The POH lists for normal climb 25 inches and 2500 RPM. My instructor further clarified to reduce to this setting at 500’ AGL. I’ve been doing this ever since then.

[@Stepclimb]

@@sw538 Take a look at the Limitations section for the O-360-F1AB. You will find that 2700RPM is the maximum continuous allowable RPM. There are no manifold pressure limitations. Also, in the normal procedures section of your POH, below your cruise climb setting of 25”, 2500 RPM, you will find a Maximum Performance Climb power setting of FULL THROTTLE and 2700 RPM. If you look at your time, fuel and distance charts, you will find that climbing at WOT and 2700RPM will get you to cruise altitude more quickly and save fuel over the length of the trip compared to slowly climbing at 25, 2500. This effect is magnified if you choose to cruise LOP. In addition, if you are equipped with an EGT/CHT monitor, your engine temps will be cooler in the climb at WOT, 2700RPM. Two reasons: at WOT, there is a fuel enrichment circuit that richens the mixture slightly to increase detonation margins. Also, reducing rpm acts in the same way as advancing the timing. The peak pressure pulse occurs closer to TDC and increases maximum ICP and therefore CHT. While some may think that 25”, 2500RPM is “going easier” on the engine, it is actually making the engine run hotter (CHT) and stressing the components more. All of this data based evidence is available at advancedpilot.com. Also, google Avweb’s “pelicans perch” articles about engine management and POH recommended climbs. There’s a lot of bad data and old wives tales contained in these legacy POHs. Again, most CFIs were taught the “25 squared” years ago, just like the CFIs that taught them. The law of primacy is very hard to overcome.

[@BruceAirFlying]

Excellent overview Martin. And thanks for the shoutout.

[@martinpauly]

Thank you again for your help, Bruce! - Martin

[@da40fan27]

Thanks for this review. Watching lead me to go back to my AFM Supplements section and review the operating limitations of things I've added to my DA40. Interestingly, my two-blade prop (vice original three-blade) increased both my landing distance (bad) and gliding distance (good) by 5 percent.

[@martinpauly]

You replaced a 3-blade prop with a 2-bladed one? Interesting; most people do it the other way around. Did you get a cruise speed improvement, or what was the reason for the change? Thanks, Martin

[@da40fan27]

@@martinpauly My DA40 originally came with a three-blade composite MT prop (1st generation). The original owner replaced it with a Hartzell two-blade aluminum prop. (The MT was prone to chips, etc.). The two-blade aluminum increased cruise by about 2kt and is much more durable. The downside is that it is 15lb heavier and causes more vibration, especially on shutdown. I'm considering returning to the three-blade composite MT prop, but second generation, which supposedly is more durable. There is a Hartzell two-blade Kelvar composite prop option that is the best of all worlds, but currently lists for $27K. Bernie

[@LimeyTX]

Another great thought provoking video, Martin - thanks!

[@martinpauly]

Thank you, Martin! - Martin

[@stevenovi7093]

Aswsome video as usual I have one quick question In one of your other videos in the hanger I saw you had for winter purposes that you can heat your engine from your iPhone what kind of make and model is that

[@martinpauly]

Thanks, Steve. The device I use (and love) which controls my engine heater is called SwitchEon. switcheon.com - Martin

[@davestarr7112]

Very informative and thought-provoking, Martin. Thanks

[@martinpauly]

Thanks, Dave. - Martin

[@christinewunder1672]

Thanks Martin! Stay warm…

[@martinpauly]

Thanks, Christine - I'll try! 😁 - Martin

[@johnfitzpatrick2469]

Hello Martin from Sydney Australia, I hope all is well. POH and AFM with suppliments. Firstly, thankyou for explaining the FAA required documents. After your training, I sat thinking why. What is the benefits and reasons, and why so important. I came to these conclusions; * Government control of modifications to plane type. * Maintenance of airworthiness safety. * Regulation of ownership between sales? Tallyho dear sir, 🖌️🧐🛩️

[@martinpauly]

Hello John, The distinction makes sense to me. There is a lot of good information in the remaining sections of the AFM (or POH). Giving the aircraft manufacturer the freedom to write that information somewhat freely while getting consensus from the authorities on the limitations seems like a good split. - Martin

[@mishmish1968]

Excellent review..

[@martinpauly]

Thank you! 😁 - Martin

[@endicot0195]

I also have been taught by every instructor I’ve ever had to square 25 on climb. I’m very interested to hear your explanation as to why this is incorrect.

[@martinpauly]

Sometime this year I will make a video on this subject. Until then: look at your POH or AFM, and try to find where it says you should use 2500 RPM/25" for initial climb. For most of the fleet you will not find such a recommendation in the book, let along a limitation. - Martin

[@jacksmith827]

Great Video!

[@martinpauly]

Thank you, Jack! - Martin

[@FarkyDave]

Love the Magnum Force movie clip!

[@martinpauly]

I couldn't resist using that little classic reference! 😁 - Martin

[@chrissanchez9935]

Sir, Thank You for the informative video.

[@martinpauly]

You are welcome, Christoph. - Martin

[@GKsGS400]

Did I ever tell you The Searchers is my favorite movie ever? 😉

[@martinpauly]

Oh yes, Martin Pawley... though I like my Bonanza better than his horse. 😁 - Martin

[@GKsGS400]

@@martinpauly Sure is easier on the backside, 😅 Love your vids. Keep up the good work,