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Power-off glide?

172driver

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The currently accepted practice in flight training is to teach the powered, stabilized approach to landing. In Cessnas, we chop the power to 15"MP or 1500 RPM abeam the numbers and add flaps abeam, base, and final. To carry this much power to the runway and be stabilized at the recommended approach speeds, we must come in fairly low compared to a power-off approach.

There is a definite period of time on downwind, base, and final where the a/c would never make it to the runway in the event of power failure. Even flying the tightest pattern possible and adding flaps at the last possible moment, if you're carrying that much power to the runway, you won't make it if the engine goes. You have to extend too far to get down in time with power. Especially true when practicing those landings on the numbers. Furthermore, Cessnas glide fairly well compared to other GA airplanes. At least you'd have a shot at it... but there's definitely a point of no return out there.

Do we just accept this risk in order to stabilize the approach? How would the FAA feel if we lost power turning final and didn't make the runway? Would they care that we were following their recommendation of a stabilized, powered approach? How do/did you teach it?

BTW, I am a big fan of teaching the power-off approach too but it is not used in day to day operations. More of an emergency or an abnormal procedure. It used to be the standard but I guess since engines became more reliable and too many pilots drove off the end of runways, we abandoned it.

Penny for your thoughts.
 
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TDTURBO

172driver,

You raise some good questions, I own a 182rg and I fly it both ways. If I am in IMC you must fly a power-on to maintain a 3 degree glide slope. I often chop power on downwind but need to be 1000ft minimum agl to make a tight final no flaps. As you know, you need to be extremely high on short final to make the runway in a 182 with full flaps (power off), it literally will drop like a rock. I think the FAA only cares about "stabilized" approaches when doing an IFR approach. Flying VFR you should always plan on carb ice or some other calamady and plan on making the runway power off. I have always been uncomfortable flying IFR approaches for that exact reason, you have to get down to MDA ASAP and that usually means if you lose an engine your in for a rough backyard crash. So, when I can, I always try to fly a pattern that will get me over the numbers power off. IFR we don't have that luxury.

BTW: Check the POH's and compare the Warrior to the 172, guess what, the Warrior will out glide the 172, most people don't know that.
 
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avbug

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172 driver,

If you are over the practice area and your engine fails, will you make the runway? Of course not. So what is the "risk" of flying in the pattern and not being able to make the runway? Hear this question all the time (could you make the runway if your engine quit right now?). It's ridiculous. Could you make the runway while flying a straight in approach from a mile or two out? Probably not. What about entering on a 45? Probably not. How about climbing out from the runway? Until close to pattern altitude, it's unlikely.

There is no need for conducting power off approaches. These should be reserved for emergeny proceedures training, but minimized in the interest of the engine. In a piston engine, reducing power rapidly, in great amounts, or for an extended period is a bad thing. Your aircraft engine is not an automotive engine; it is not liquid cooled. It is subject to rapid temperature changes with airflow and power setting. It is subject to unusual stresses and loads, and can be damaged if improperly handled.

Generally speaking, the power off pattern is poor form in a recip powered airplane. It's fine for shooting high key approaches in a turbojet, but not in a piston engine airplane. As a general rule, the power shouldn't be reduced so far that the propeller is driving the engine. This applies especially to geared piston engines, but also to direct drive engines such as the ubiquitous 0-320 in the Cessna 172.

There is no reason to make such a drastic power change in the airplane while flying the pattern. A gradual power change, or a series of settings as you hit abeam, key, base, and final, will work fine. You do not need to make a low approach to land. Extend a bit, reduce power a bit sooner, add flaps sooner; whatever. It's very possible to fly a normal pattern in the 172 or any other light airplane, without having to fly low, or reduce power to idle.

Cessna's glide about as well as any other GA airplane.

Pilots "driving" off the end of runways has nothing to do with teaching power on or power off approaches. It's not even a consideration. Respect for the engine that keeps you aloft has everything to do with it, however. One needs to learn to manage power; it's going to be a big part of your ongoing necessary flying skills.

The suggestion that power makes you extend farther and fly lower leads me to believe that a basic lack of understanding of the power/airspeed/descent rate relationship exists. As you are aware, it takes more and more power to fly faster and faster. As you should be aware, it takes more and more power to fly slower. Speaking in general terms, at airspeeds less than your best glide speed, which will be roughly equivilent to Vy, you will require more and more power to go slower and slower. The slower you go below this speed, the greater your rate of descent.

Accordingly, you don't need to fly a lower approach, pull power, or extend so far out that you can't make the runway if there is a problem. You need to manage your airspeed and power and aircraft configuration such that the airplane will do what you want it to do.

In a very short field approach, you may be close to full power during the course of your approach and landing. This may in fact be a very high approach. Don't assume that because you're having trouble making the runway with a normal pattern, while carrying power, that the power is the problem. Address the other variables. As you slow the airplane to a speed close to or less than Vx, you'll find that you'll be carrying a great deal of power to fly the same pattern, and will likely want to stay high. Experiment with different power settings and aircraft configurations; experiment with your timing in reducing power settings or making power changes. You'll find that the specific power settings change with the day and the aircraft weight, but you'll also develop a knack for making the approach turn out the way you want it to, without having to go to idle, or vary the power much at all.

Before long, you'll find that you can make the descent and landing with a single power setting, a single airspeed, and one configuration change. You'll also find that you can fly the same exact pattern with different power settings and configurations. It's all a matter of gradually learning to fly the airplane. It will come. Keep after it. Good luck!
 
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Fact: Most engine faliures occur at power changes.

Fact: Most power changes occur at or near an airport.

Fact: At cruise altitude most airplanes will make an airport.

Fact: If you do lose your only engine, you'll be doing a power off landing.

Fact: Shock Cooling an engine will shorten TBO in not a fact.

(Look at trainers, they get shock cooled all day and typically make TBO)

Observation: You better be good at power off landings and timing your flare as slow and accurate as possible, how else you gonna stay current?
 

172driver

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I agree with all that you've said avbug. Believe it or not, and I suspect you don't, we low-time guys aren't ignorant or stupid, just low-time. I suspect that I have as much time in a C172 as most, not that it matters.

I understand the power curves, Vx, Vy, Vg very well, at least for the aircraft that I have flown. I was also an automotive mechanic for ten years so I do understand engines. Granted, airplane engines do cool quickly and shouldn't be mistreated. I don't disagree with the powered approach. Just wanted to gather some opinions.

I guess the risk of engine failure on landing is the low altitude and terrain considerations. Usually alligator or shark filled wates in my case. In the practice area, at least you have some glide range and hopefully have picked a suitable field. On takeoff you're pretty much committed to the terrain in front of you. When landing, if I had my choice, I would reduce power more, add flaps sooner, and descend more steeply. Or fly slower and descend more steeply. Either way, I would make the runway.

FYI, Vg is well below Vy in the 172. I like your suggestion of flying "behind the power curve" to maintain both altitude and power on the approach. At least that way you're high and can pitch it over if anything happens. However, we are taught very specific airspeeds and power settings for base and final...standardization. Yes, these are only reference settings and will change with wind, temp, press, alt, etc. But on a 'normal' day,' this is what we must use. Using these will cause you to fly a fairly low approach...approximately 3 degrees. You would never make the runway.

I guess we can't always think about making the pavement in your opinion. That's valid and pretty much the approach I have taken throughout my training. I'm comfortable with it but is the FAA? Thanks for your opinion as always, but please try not to condescend to us low-timers too much. Some of us are fairly with it. Wouldn't pretend to be half the pilot you are but I will say that I can fly and teach Cessnas and everything associated pretty well.
 
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avbug

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TD,

Those are great wives tales, but tell me, how many engine failures have you had to get those stats? How many of them shook out the way you described? How many cylinders have you changed as a result of shock cooling?

On the east coast most airplanes might make an airport during an engine failure at cruise. Elsewhere...the odds aren't great.

What has TBO to do with the price of tea in china? Shock cooling doesn't affect TBO. Tea doesn't affect TBO. Nothing affects TBO. It's an arbitrary imaginary reference number assigned by the manufacturer for first-run engines.

However, shock cooling can drasticaly shorten the life of an engine. It's an age old debate, but generally those who are on the side of the fence that says it doesn't happen, have never been around to see it happen. I have, and I can assure you, it does.

172, I'm sorry you find my reply condescending. I stated nothing but facts. These are basic facts, and should come as no surprise, nor were they intended to be condescending. I know nothing of your experience, nor do I presume to know you or your expeience. I addressed only the words that were applied in the initial post of the thread.

I understand the common desire to reach the runway if something happens. However, I've spent a great deal of my flying career at low level (below powerlines, most of it), and there has been zero chance of reaching any kind of landing surface during most of that time. I find it humerous and incongruous, then, when it suddenly becomes so important to make the runway in the pattern. It isn't.

Most engine failures do not occur duing power changes. They occur arbitrarily. In some cases the failures are enhanced or manifest by the change; most failures are partial failures involving a component or system and not complete or catastrauphic failures. In many cases, such a failure may be undetected at a current setting but may be manifest at a higher setting, etc. I have seen this happen on a number of occasions.

Some of my most spectacular cylinder failures have been far from a runway and during constant power in cruise. Others have been during takeoff. I don't recall ever having one outside training during landing (with the exception of several successive failures during rollout in a C-130 due to a progressive leak onto the 5th and 10th bleeds), and on only two occasions have I had failures during a go-around (both in single engine airplanes). Looking back on various types of failures, I can't see any pattern that suggests they occured primarily during power changes, near airports, or any other consistent criteria.

If you do lose your only engine, you won't necessarily be doing a power off landing. You may very well be doing a partial power landing. I have had a number of these, because the most common type of engine failure is a partial failure. This may be a cylinder loss, carb or injection failure, induction leak, slipped mag or bad mag, or any number of other possible situations.

Certainly one should be familiar and conversant with emergency proceedures, as I detailed in my last post. However, as a matter of course, teaching regular landings without power in piston airplanes is bad form, and poor technique.

A 3 degree glideslope or approach angle is a standard glideslope, and is not a low angle at which to approach the runway.

There are other factors involved that make a power off approach unsatisfactory. Among them is backlash issues; something that's normally not a consideration for automotive engines. In aircraft engines, however, you should strive to never allow the propeller to drive the engine, if possible. All internal clearances, and even the mag settings, are predicated upon established backlash for proper spacing and timing. Stresses are typically incured in one direction. Excessive power off running of the engine, at manifold pressures insufficient for the engine to drive the prop, leave the prop to drive the engine and this reduces engine life. It also increases stresses and reverses loadings on critical components such as connecting rods.

A power off approach cools the engine, and reduces the level of carburetor heat available if needed. Most instructors seem to believe that the most likely time for carb ice to form is a power off descent. However, this isn't true. The most likely time for it to form is during the initial power up, following a power off descent. I've seen engine failures occur that can be directly attributed to this under the right circumstances. In such cases, often insufficient heat was available on short notice, due to an extended power off descent.

Many engines used for light training airplanes are reasonably tolerant of abusive power changes such as rapid throttle movement and power off descents. However, simply because one can get away with such abuse in these engines, does not make it acceptable in most piston aircraft engines. In many engines, it can lead to a rapid demise of the engine. Always treat the engine like your life depends on it, because it does.

Good habits in training airplanes translate to aircraft that are not training airplanes. Power off descents in turbocharged or geared engines, or in large bore engines and counterweighted crankshafts are to be avoided. Many airplanes shouldn't be brought below the bottom of the green arc on manifold pressure during the approach, and RPM and temperature must sometimes be carefully monitored. The time to instill these proper habits is during primary training, when the pilot should be taught to use proper power management and proper engine care.

I realized I had a lot to learn about engines as a student when I applied power in a J-3 to go around for some deer on the runway. The engine quit, and I quickly learned to baby the throttle. You can get away with rapid power application in an 0-235, but not in an A-65 or C-85. You shouldn't perform rapid power application, but many engines in use in light aircraft today, especially training aircraft, permit bad habits without much complaint. That doesn't make it right.
 

Timebuilder

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Last year, during the bulk of my instructing, I had two opportunities to teach engine out flying to my students working on private, instrument, and commercial.

1) Fly over a nice grass runway at 3,500 ft AGL and gently reduce power to idle, and coach the student to a full stop landing.

2) At our class D field, tower often asked me to "make short approach" from abeam the numbers for faster traffic inbound. Another teaching oportunity, including use of the slip.

Otherwise, we went to the standard method of a power on, stabilized approach. I taught them to look for alternative touchdown areas if the tarmac was too far away. Generally speaking, you don't want to make A LOT of radical changes, and I pointed this out to my students as the difference between "normal" and "training" operations.
 
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bobbysamd

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Power-off landings

You do want to teach a powered descent to landing as a normal procedure. You are teaching Cessna landings the way I learned and taught them. The same basic procedure also works in Piper. Teaching a power-off approach as a normal procedure is not correct.

I used to teach engine-out emergencies in the pattern, wherein I simulated an engine failure by applying carb heat and retarding the throttle to idle. At that point I taught students to stabilize glide and turn to the threshold. Of course, depending on where we were in the pattern, such as just turning downwind and with no other traffic and a long-enough runway, land downwind. Anything to get back to a runway, or taxiway, or, if necessary, an off-airport landing. But these were just emergency procedures.

I'd think the FAA would be more concerned about substance than form. Let's say you lose power on a two-mile final in a strong headwind. At that point I'd look for open areas where I could land the airplane and not necessarily worry if it'll be on a runway. It is more important to land the airplane as safely as possible someplace than to make the runway and not land safely.

Just my .02.
 
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avbug,
You are correct as usual and I do respect you opinions but I think shock cooling in normally aspirated engines is overrated though, here is a link too.

http://www.avweb.com/articles/shockcoo.html


What has TBO to do with the price of tea in china? Shock cooling doesn't affect TBO. Tea doesn't affect TBO. Nothing affects TBO. It's an arbitrary imaginary reference number assigned by the manufacturer for first-run engines.

Nothing affects TBO?

Not making TBO has allot to do with proper engine management, if you abuse an engine ie; cold starts without preheating below 40 degrees, over/under leaning, you wont see TBO without spending major cash. Also, new Continental engines have had serious problems with their top ends going south recently, their QQ has really gotten bad and their warranties suck. TBO is a guidline and is to be used as such.[/b]


I still feel staying current with power off landings is good.:p
 

Mickey

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I'm going to put my vote in for the power on approach as being the normal approach. Avbug pretty much covered most of it's merits. As far as shock cooling goes on the trainers vs. some of the larger and higher performance aircraft, I've noticed the trainers seemed to be a little more forgiving than some of the larger engines. I've come across several IO-360s that needed new cylinders in Arrows.
TDTurbo. At what point in a single do you think power off approaches might get a little scarey. Have you ever seen the glide profile of a T28. Maybe power off approaches weren't that far off of a powered approach in a light 150 or 172 but as you get to heavier aircraft you are asking for a more exciting approach. You can have the power reduced before you start adding drag to slow down in the pattern. If you loose your engine,stop adding drag or remove it and you can still make the runway from a well flown pattern (not a B52 pattern). In singles I don't usually add full flaps until the landing is assured.
Here's one profile: Plan descent with timely power reductions to arrive into the pattern at approx 14-15" MP; add flaps 10 on downwind, abeam the numbers gear down reduce 1-2" MP, flaps 20 before or after turning base as necessary, mixture and prop "in" or as necessary(mixture), turn final slow to ref +10 add full flaps so as to reduce to ref by threshold, ease pwr out, roundout and flare. This is just a rough profile but it works nicely in a lot of high performance aircraft. Obviously not all aircraft have these flap selections(ie Bonanzas, Mooneys,...) The point is you can fly powered approaches safely and without having to make large power changes while at low altitudes and airspeeds where your escape options are limited. In the above profile if the engine were to quit I would stop adding drag or and beeline to the runway, taxiway, highway, dirt road, field, etc. Usually the runway would be very doable. There a few variable to deal with yet but if you think deep enough you will figure them out. I don't like to type a lot so I skip some info.
Take care.
 

cvsfly

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What avbug said and the others for power on approaches. Practicing for power off glide/approaches is fine but not as a normal everyday approach especially in higher power/tubocharged aircraft. If you are nervous about single-engine aircraft there is always twins.
 

AWACoff

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TDturbo,
My major beef with that article is that the author repeatedly refers to his "gut feelings". In reality, he is just making a guess. He states he has no evidence, just theory. Let's look at another "guess". I fly 182s out of a small dropzone in northern Wisconsin. It wasn't unusual this winter to see temps around minus 20 degrees fahrenheit at 11,000MSL. On climb out I had CHTs around 400 degrees fahrenheit. Common practice is to allow no more than a 25 degree drop in CHTs per minute while descending. Imagine if I were to go to flight idle immediately and went into a descent with IAS around 160kts...the CHTs would drop like a rock (or skydiver even...). The front of the cylinder exposed to the relative wind is experiencing a much more rapid cooling than the rear of that same cylinder. The difference in contraction rates distorts the cylinder. It may not be much...but over time it would seem to matter a lot. This last winter with me flying...no jugs had to be replaced. Last year 3 jugs were replaced. The only major difference being that last year's pilot treated the airplane like a 2 dollar....(fill in the blank). The engine in the plane has over 2500 hours since overhaul. TBO is an arbitrary number set by the manufacturer. It does indeed not mean anything. You can use an engine past TBO all you want as long as it can pass an annual/100 hour. I think TGTBO should be the term (total guess time between overhaul). Just my opinion and a barbecue chicken san'wich.
 

surplus1

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AVBUG

I'm just curious. Does your background include extensive experience with large "round" engines?
 
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TDTURBO

If you have a 2000 TBO engine and your replacing the top end every 500 hrs thats normal? TBO gives you a baseline on what you SHOULD see out of the engine and IS an important number to judge engine treatment. If your engines consistantly need overhall before TBO, your doing something wrong. It maybe an arbitrary #, but it gives you an idea on what the engine life should be, thats all I'm saying.


AWAcoff,
Your preaching to the choir here, I never advocated chopping power from cruise in subzero temps to practice power off landings; I just believe you should stay current doing them in small planes weather permitting. As far as shock cooling in small engines, I don't buy it, that is only one of hundreds of articles debunking that wives tale. Sure, shock cooling a hot running big turbo is unwise, but the initial intent of the threads author pertained to training aircraft, hence the observation.
 
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avbug

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Surplus,

It does; it includes time spent flying them, and working on them as a mechanic. It also includes time spent working as a mechanic on turboprop engines, and turbojet engines, and even a short time as Director of Maintenance. I will never claim to be an expert, or even very knowledgable on most subjects.

On the subject of shock cooling, there have been some very good articles written each way. My favorite were written by Kas Thomas, who used to write for TBO advisor. I have several of his books right next to me. (Worth reading for those operating piston engines, incidentally; Fly the Engine, is a good one). He published a very convincing article years ago that seemed to disprove shock cooling, and then years later he published an equally convincing article that proved it.

I performed maintenance for a company one season that flew 30 piston airplanes. During five months, I changed 28 cylinders; cracked between the valves in the head, as a result of shock cooling. I'll grant that we felt part of the cause was due to some changes in Metalurgy that Continental had effected in their cylinder construction; the cylinders were without question more susceptible to damage, but the fact was it wasn't the metal that caused the cracking; it was the pilots.

I believe in the dangers overboost because I've changed enough cylinders on my own airplanes when the heads came off...after years of exposure. I believe in the dangers of shock cooling because of experience. That's not to say many more years of experience with diving airplanes at idle and seeing no damage couldn't convince me otherwise...but it's not a risk I'm willing to take, and it would take a lot of years to undo my belief in what I've seen to date.

On any given flight, I had 14 times the number of cylinders to deal with during the course of that flight, than most light airplanes today. It only took one to wreak havoc, and I was also responsible for maintaining those cylinders, and the rest of the airplane. I took them very seriously, and I got a LOT of exposure to the potential hazards that those engines might encounter. Shock cooling is real, as are the other hazards mentioned.

TBO is a misunderstood subject. TBO is a number. It doesn't change, except by ammendment. It's only purpose is a guideline, except for operators under Part 135. It is not a mandatory interval. It represents nothing but a best fair guess as to a general time that an overhaul *might* be recommended. Shock cooling doesn't affect TBO. Running sulfuric acid in the engine doesn't affect TBO. Shooting the engine with a .50 BMG round doesn't affect TBO. It stays the same.

An engine might not last until the recommended TBO. That still doesn't change the nature of TBO. It's an arbitrary number which is established at the time of manufactur, and is subject to ammendment. The amount of time an engine actualy goes before it's owner elects to have it overhauled has no bearing on TBO. Once an engine is overhauled, TBO no longer applies; it's based on a factory engine, first-run. For example, an engine with a 2,400 hour TBO gets overhauled at 1,800 hours. How many more hours does it have? Who knows?

Under part 91, engines are operated on-condition. An engine may be operated beyond TBO safely for many hundreds of hours, or it may require overhaul after only several hundred hours. There are many degrees of overhaul. An engine which has experienced several top overhauls, or cylinder changes during the course of it's life prior to "TBO" isn't working toward TBO any more...because it no longer applies. It didn't make it. This is very common. The engine is operated on-condition. The only time that TBO becomes mandatory is under 135, when an exemption is required to operate the engine beyond TBO.

I have worked for operators that were responsible for extending TBO by 300-400 hours, by submitting the data from our programs to the manufacturer. Accordingly, we were able to operate aircraft far beyond the origional TBO, while flying under 135. One employer single handedly extended TBO on a common piston recip by 400 hours, 50 hours at a time. We did complete teardowns, split the case, and everything. Lots of documentation and testing. Then we rebuilt the engines and moved on.

An engine may not make it to "TBO", but it doesn't change TBO. what prevents an engine from getting there may be a number of factors involving mechanical defects and operating conditions, but by and large the biggest single factor is the pilot. Metal remembers; any stresses placed on a metal part are there for the life of the part. Stress is cumulative. You may do power off descents and rapid changes until the cows come home and see no damage. However, the stresses are there, much like bending the tab on a pop can back and forth many times. Sooner or later something will give. Is it the last 50 renters that abused the airplane to this point, before you take it, or the next one? Will it be you?

Treat those engines like your life depends on them. They're your best friend. Treat them like your life depends on it, because...it does!
 

norskman2

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Power-off landings

Back to the original idea: power-off landings.

I've had some instructors who expoused this idea mentioned earlier that every approach should be set up so that if you should lose power at any time, you should be able to complete the landing.

I understand this business about the back-side of the power curve etc., but still not sure if I get it. If I were to suddenly lose power completely, wouldn't my rate of descent increase? Or does the back side of power curve mean that I extend my glide path. Does this require a different airspeed from the approach speed?

Thanks for helping clear this up.
 

avbug

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TD,

What make/model engine are you flying that's only getting 500 hours? What kind of overhaul is being performed? What type of aircraft, and what kind of operation? What experience level for the pilots?
 
T

TDTURBO

Good post avbug,

I think were getting caught up in semantics instead of substance. I understand what you are saying and feel we are on the same page.
 
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T

TDTURBO

avbug,

I was using the 500 hrs an example of a common TBO for a small plane. If you're not making it to TBO and replacing major components at 500hrs on a 2000hr TBO engine, somethings wrong. When I say shock cooling doesn't effect TBO in a normally aspirated small engine, I realize it doesn't change the factory given #, only the ability to reach that #, therefor effecting that engines TO BE OVERHAUL time. Like I said, semantics.
 

FlyinBrian

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I think AvBug's point is that "making TBO" is not necessarily a good measurement of how well an engine is cared for. TBO is fairly arbitrary, and a well maintained and properly operated engine can make it well beyond TBO.
 
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