PA-34 power setting sequence
 

Hello there,
I have just started flying on a multi engine Piper Seneca V and recently I've been having a discussion with my instructor about engine power setting for the climb. In all manuals it is advisable to retard the throttles first and only then to decrease the RPM. However, my instructor has been insisting to decrease the RPM before adjusting the manifold pressure. The rest seems to be alright. Could anyone help please? Is there a possibility that my instructor might be wrong?

Usually procedure is to throttle back before reducing rpm to reduce the chance going 'over square'. Eg 27" 2700 rpm on take off reducing to 25" 2500 rpm for the climb. ( 27" 2500 rpm being over square)

But if you look at the book over square is often allowed. Eg 27" 2500 rpm

So to reduce the fiddling of reducing throttle, setting rpm then increasing throttle your instructor is probably just short cutting the exercise. I used to do this in the bulldog and the Pitts all the time.

Read the manual , as I recall the bulldog would allow 29" 2500 rpm at sea level.

It is common practice to reduce RPM after take off for noise abatement. 2500 RPM is a common choice, and then maintain full throttle throughout the climb.
This should not result in an 'overboost' otherwise fixed pitch props wouldn't be possible (29" and 2300 rpm is typical of FP at take-off).

However normal practice would be throttle-prop-mixture when decreasing power and mixture-prop-throttle for increasing. The only difference is that there is no need to throttle back when decreasing RPM in the climb.

The "oversquare" issue is actually a load of rubbish, I think it applies to some historic aircraft, but you should know the limits for what you are flying. Certainly the normal thing is to throttle back first to avoid overboosting, if you know the limits then OK you CAN do it the other way round - but I can see no argument FOR it.

Verrrmin

I have around 2000 hours on piston engine twins such as the PA-34. I have always reduced power after take-off using the throttles before bringing the RPM back - that is how I was taught and it seems to have worked over the years. One type I flew had geared and supercharged engines and your instructors technique would have seen some scary indications - just before the loud bang :eek:

As the others have said, it's 'throttle first'

Enjoy. https://www.google.co.uk/url?sa=t&rc...bERsClepz1RwbA

Above all else, operate the engines as the Flight Manual and engine Operating Manual state. Thereafter, it is most common to reduce throttle, then propeller RPM, rather than RPM then power. If noise abatement requires, reducing from 2700 to 2500 RPM first after takeoff is probably okay - if that is the reason for doing it. Other than that, I would not touch the props until the power was reduced.

Reducing RPM while the power is up, is reducing your detonation margins. I have done detonation testing on several Continentals. I have made these engines detonate - it can be done.

If you make the engine detonate, it will be damaged. You have seconds to undo it (though you won't know it's happening) before pistons start melting. Engine temperatures will affect the detonation margins too. Hotter is worse.

Simply put, detonation takes time and power to happen. You give it the power, by having the throttles open. You give it time by slowing RPM. So the faster the engines are turning, the less time (per engine rotation) as available for detonation to occur, before the next cycle begins, and removes the opportunity anyway. So if you reduce power while leaving RPM faster, you are giving more margin for detonation - good for the engines. Opposite, is not so good.

So, I operate engines that way. If someone tells me different, I ask them to show me in the Flight Manual, but I won't do it differently from that, without a darn good reason. The NIMBY's who want quiet around the airport, 'cause they did not notice it there when they bought the cheap house, don't care about your engines detonating. Even your instructor might not be really worried, as long as the engines keep running for them.

Some Lycomings, and all turbo'd engines I've flown allow some over square, so do what the manual says. The best thing you can do is to understand yourself, and make your own informed decision. So good on you for asking, and welcome to PPRuNe......

The Seneca 5 is turbocharged so all the wive's tails re "square" go out the window! :hmm:

Your instructor is wrong. Go read the POH it gives the recommended climb power setting and the correct way to go from max power to climb power is to reduce the MP first with the throttle and then set the correct RPM with the prop levers. As Step pointed out reducing RPM without reducing MP is inviting detonation particularly in turbocharged engines.

This is such a basic engine handling fact that it makes wonder how qualified your instructor is to be teaching on this aircraft.....

why ?
 

Of all the statements above I agree with the last line form big Pistons the most !

The light twin is overpowered on two engines so that it can (just) perform on one engine if required to do so, my view is that you should be as gentle as you can with both engines to preserve them so that if you need full power from one engine it will be able to make full power.

Tankengine while from an indication point of view you are utterly correct about the oversquare thing, however due to the temp rise of the charge I doubt if the absolute volume of the charge reaching the cylinders is very far adrift from a normaly aspirated engine on a ISA day, of course supercharged engines can maintain ISA conditions in the manifold during conditions far above ISA.

Well, I simply cannot disagree with all the statements mentioned above - I was of the same opinion when discussing it with the instructor. Anyway, take -off power setting for this a/c is 38 inHg and 2600 RPM, so it wouldn't be so dramatic when reducing to 2500 RPM first, as it's not so big difference. The wisest thing would be though, to leave this power setting sequence the way my instructor wants and after the exam just do as the manual and the common sense says. I just wanted to be sure I wasn't missing anything :) I believe that my instructor is extremely professional and maybe he has a bit different considerations, which I don't understand at all :D

Talking about a NA engine now, my own is an IO520.

The last few percent of the throttle movement opens a 'fuel enrichment valve' that allows extra fuel to the engine. If you reduce MP just a bit, say from 29" to 25", you will see the CHT start to rise quickly because this valve closes. Keeping the throttle full open during the climb in an NA aircraft will keep CHT lower and the MP will fall by itself.

Reducing MP is totally unnecessary and even harmful in my situation, reducing RPM is optional and only for noise reduction on the ground. Once established in the climb, say from 4000 ft onwards I will lean to keep a constant EGT during climb.

There is a lot of bad or simplified advice floating around from old days when training had to be 'expedited' or due to lack of proper engine instrumentation.

I have flown a turbocharged twin for 15 years and there was a published climb power with given MP, RPM and fuel flow in the POH.

Hang on - we might be missing something here, and the OP's last post gives a clue.

After departure, leave the throttles but reduce RPM from 2600 - 2500. There's nothing wrong with an adjustment this small for noise reasons without touching the throttles. I was taught this, no doubt many others have been as well.

HOWEVER - it is the only time you do it. Thereafter it's always throttles back first, then Props back. Going the other way Props forward first, then Throttles.

And for the "over square" gang - go fly a Turbo people and discover that it ceases to exist. Even a normally aspirated IO540 is flown "oversquare" on departure and you just let the MP come down naturally, whilst leaving the RPM at 2500 for the climb.

(I delivered a Seneca V from Europe to Brazil - 2200 RPM / 28" MP was a nice cruise setting for that trip)

What Chilli Monster said...

I was taught exactly that. The PA34 POH does say in the climb out section to reduce the throttles first when reducing power from climb, however.

I had the same confusion as the OP, and it was hard enough trying to remember to do everything in the right order at first without figuring out when the rules were being slightly bent.

What you can and can't do with the engine has to be backed up by data from the engine or airframe manufacturer. Many turbocharged engines require that full redline RPM be set when using the maximum available MP.

The allowable relationship between MP and RPM is model specific and will vary even for the same basic engine.

So to answer the OP's question I would ask your instructor to show you where the 38 in 2600 RPM power setting is authorized. Some POH's have a chart of allowable MP vs RPM which will provide the necessary data, however for most engines you have to go to the engine operation manual provided by the engine manufacturer.

Sadly the practice described seems to be the all to common flying school practice of procedures mindlessly passed down from instructor to instructor with no critical thought and unsupported by any actual data.

At 38" on the Seneca V you're not at Max MP (40") so Prop Red line is not required, a slightly lower figure is permitted.

According to the EASA Seneca V Type Certificate Data Sheet, the maximum allowable Manifold pressure is 38 inches at 2600 RPM. I stand by my contention that a power setting of 38 In at 2500 RPM is not an approved power setting.

At a large well known school in Spain using Seneca 3

It was taught at Top of Climb Level off bring props back to 2400 adjust throttle to give 32" then lean

For descent leave props 2400 reduce power top 22" and descend

Is this the wrong way of doing lever moving ?

Personally, I would not do it that way, unless the Flight Manual specified that as the procedure. Reasons as previously stated.

Remember that the fact that someone flies and aircraft a certain way, and then perhaps teaches someone else, who then teaches someone else, does not necessarily make any of it right. Go back to first principles, what does the Flight Manual, or engine manufacturer say to do?

Can we all agree on RTFM? Which is, oddly, usually the answer in this sort of discussion.

In fact I had an interesting time reading the POH for a C152 yesterday and comparing the power settings I was taught to the ones in the POH. (The Cessna isn't my usual aircraft.) I've made myself a chart of RPM vs altitude for 65% and 75% like the one which is placarded in the PA28 I usually fly.

At the risk of sticking my neck into somewhere I haven't been for a while, might I suggest that power settings used at a twin training school are not necessarily those that would be used in real time by a charter pilot quite correctly handling the engines on the same machine.

My day job is spent sitting between a couple of TIO-540-J2BD engines, each capable of pushing-out 350HP. The thought of reducing RPM before MP scares me. :eek::eek::eek:

Flight training procedures should be 100% in accordance with the POH. An important part of all training, but especially, advanced training is inculcating students with the importance of understanding all of the aircraft systems and knowing what the expected performance should be for every phase of flight.
The only way this can be achieved is by integrating the POH into every lessons.

With respect to piston twins and charter flying I have had to beat out the stupid " flight schoolism " from numerous new hires. Why can't the school just teach it the right way from the beginning ? :ugh:

Rev up. Throttle back

In my turbocharged commander 114tc (se) max take off power is 39/2575.

Once airborne over 1000ft agl I will go to high climb power 35/25 .. Later I will go to normal climb power 30-31/24

Cruise is 29-31/22-24.

Square on a turbocharged piston in this category? I think not.. Only at approach speed. 23/23 gives 120kts.

I allways reduce throttle first. The reason is that when you reduce rpm the mp will always rise a bit. You do not want to do that when you are already at max.

The other thing is that its the easy way to work the flow from left to right .. First throttle, then rpm, then the fuelmix

Same in my commander, except O/B is 42" MP.

So I normally use 40" on take off (which can be fun jiggling the throttle on the take off roll as you normally get an increase in MP as the roll gets faster), once the flaps are up reduce to 36" and leave prop full forward for the climb (as per the POH). At TOC MP back to 31" and RPM back to 24" for a fast cruise.

Seneca V
 

Hello guys

Taken from Seneca V POH:


4.29(k)
When power is reduced for descent, the mixtures should be enriched as altitude decreases. The propellers may be left at cruise setting; however if the propeller speed is reduced, it should be done after the throttles have been retarded. Cowl flap should normally be closed and the T.I.T should be maintained at approximately 1300F or higher to keep engines at the proper operating temperature. Set the altimeter. Adjust the windshield defrost as required during descent.

Hope it helps.

Wow, that's awkwardly worded! As all Piper Seneca engines are direct drive, the propeller speed is the engine speed. As you retard the throttles, and the propellers reach the fine pitch stops, they [engines] will slow in relation to the throttle position.

Yes, you can further reduce RPM by selecting a more coarse propeller pitch, though I cannot think of any reason to do this in a twin, other than to stretch a glide after a dual engine failure. Having the props selected to coarse with the power is very low is generally harmless, unless you suddenly advance the throttles (like to go around). If you advance the throttles with the prop levers well back, it will be very quiet still, and rapidly become very expensive.

I can't imagine what Piper envisioned when they wrote that, but I would avoid doing it without a profound understanding of engine operation, and a compelling reason. An engine operating at maximum RPM is noisy, but it is protecting itself from detonation. Reducing RPM while maintaining the power setting is taking you toward detonation conditions, and you will only find out afterward that you were there, with a maintenance bill or engine failure.

Throttle back, rev up except for;

setting climb power
levelling off in the cruise

Why not ask him? Refer him to this thread if needed.

I'm with tmmorris -.RTFM. I really don't care what schools teach unless they teach what is in the manufacturer's AOM. Far too often rumour, heresay, personal foibles and hobby-horses and down right ignorance get in the way of the correct way of doing things.

PM

Been a while since I have flown PA34 but could the reduction of rpm to 2500 cause the MP to drop to a suitable climb MP setting like 35".
I know when I first went on the PA34 throttling back would drive me nuts as reducing the rpm caused the MP to drop and needing resetting. (Turbo slowing down).
I know we teach TPM to reduce power but does reducing rpm first to 2500 cause the MP to drop say 35".
If throttle comes back to 35" first, then pitch to 2500 does the MP drop below 35 requiring resetting?
Can't remember since not having taught on them for 15 years since the CAA killed of multi rating to mortal souls.