ENGINE TOPPING

[offshoreigor]

Made ya look!

But seriously, I've just had a discussion with our Engineering Folks and there seems to be a difference of opinion about what a Topping check is actualy checking.

When I was a "Baby Pilot" I learned that "Topping" was a function of fuel flow. Now I understand that it is a total performance check of the engine(s) wrt to N1/Ng, T5/TIT/TOT and Q available, but isn't it really checking the limiting factor of fuel flow in a given DA condition?

Your thoughts are most welcome.

Cheers :eek: OffshoreIgor :eek:

[BIT]

[Health Warning]I am aware that there may be different interpretations on the word "topping" so if you dont agree that may be the reason :)
I would have thought that you would not want to be fuel limited to check the absolute performance of an an engine as in a fuel limited condition the engine clearly has more to offer. My experience of engine performance checks has been to pull to a first limit (Ng or TIT/TGT/)and note the parameters before them checking against manufacturers engine data to determine if the engine is still performing to an acceptable level. If there is a marked (notwithstanding static droop) drop in Nr before the first limit is reached then you are probably in a fuel limiting condition so you would need to change DA before re-testing.

Any engine gurus out there care to add more I'm interested to here.

Be Safe

[Nick Lappos]

The topping check is a catch-all for lots of possible checks. If you are simply making sure that you can squeeze out a max N1 or fuel flow, it is simply a check that you can get to maximum power, without necessarily checking what that max is.

If you compare measured N1, temp, or torque at an ambient pressure altitude and OAT against a chart for the minimum spec engine, then you are doing a one point power check, and measuring the engine against the standard one used to calculate the performance of the aircraft. This topping power check also proves the other topping check at the same time.

A part power "power assurance" check is used to measure the engine against a standard one at low power (which saves wear on the engine). This confirms that nothing bad has happened, and that the engine should still produce full rated power when called. If the engine type can have less margin at high power than at lower power, the power assurance check may have a minimum power margin as called out. This is a clue that the engine might naturally have lower margin at full power, so the extra margin is tacked on to the lower power.

What goes wrong is usually that the engine gets dirty, eroded, fodded, or a bleed leak so it can't produce the power it should, and maintenance must be performed. Also, if the pilot choses to do the check with a crosswind or downwind, some exhaust might be sucked in that spoils the readings by heating up the inlet air and making the power drop.

[CTD]

igor....By definition, topping is indeed checking a fuel flow limit against power achieved, but that limit may be preselected on the fuel control to protect an engine from Ng overspeed rather than an inability to produce further flow. It depends on the engine.

With a PT6T for instance, topping is adjustable, and a topping check is performed to ensure the Max Ng is attainable. On engines where Max Ng is not attainable without exceeding another limit (like on the PT6T-3D where the Ng limit is 109.2, and you'll ALWAYS hit Q or ITT first no matter how high you go), a part power trim stop is used to check calculated topping at a much lower value, but this test can't be done in flight.

Hope that helps a little....

[tgrendl]

Great postings/questions.

My bit to add would be this;

Topping, (the point at which the ng section is providing the absolute maximum power for the ambient conditions)is your guarantee that the engine will provide all of the flight manuals performance criteria listed.

The various daily health checks provide information on engine trends, not absolute power output. While they usually correlate they don't give the guarantee.

Any factor that might influence maximum power output is being screened on a topping check. Damaged turbine blades, bad rigging, burnt case liner, fuel control scheduling, Np feedback loop etc.

If it tops sooner then the charts say it will then you start the toubleshooting.

Hope to hear more answers too..

[IHL]

For the Rolls Royce Allison powered SK-76 , engine topping/limiting check is performed to ensure that the engine will deliver 2.5 minute power without excessive droop. It is usually performed after a 150 hour inspection, engine installation , or FCU/governor maintenance. There are a couple of different limiting checks for the 76A, the one I prefer is found on page 2-5 of the Flight Check Procedures Manual(FCPM).

For example; referring to figure 2-1 of the FCPM, on a standard day 15 C @ SL the target NR would be 101.5%. With the engine to be checked set to 107% NR and the other engine at idle the collective is raised to achieve 111% torque or 826 C. T5 and maintained until NR stabilizes , usually 5 s. Nr is then recorded and compared to the target value.

If the recorded NR is = or > than the target value of the chart, in this case 101.5 % then no further maintenance is required. If the NR is less than target, then maintenance is required. Usually the engineers will then tweak the FCU or governor.

Important note, in cold climates; make sure that the aircraft is heavy so that it won’t hover on one engine.

[ShyTorque]

And I thought "engine topping" was that creamy stuff I found inside the rocker cover on my old car engine in the winter months....

Maybe it was mayonnaise instead?

[Thud_and_Blunder]

CTD,

My thanks for your posting - it helped me to understand a little better the difference between our ground topping and air topping procedures. I still have a problem with the local pilots though; one of them has interpreted a maintainers manual comment along the lines of "..if the RRPM droops below 97% in flight.." to mean that on the B212 it is perfectly legit to bring it down to 94%! He's unable to show me anywhere that positively allows it, and I've shown him plenty of statements that the min limit is 97%, but I have the nagging feeling that he still does it his own way when he's flying. Ho hum.

[CTD]

Thud,

As you know, and have obviously shown your friend, the Limitiations section of the B212 RFM says...
Rotor RPM Power On, Min 97%, Max 100%
Rotor RPM Power Off, Min 91%, Max 104.5%

There should be no further discussion. Furthermore, the old 204 / 205 habit of drooping the rotor to get more power doesn't work on the 212 (or the 205 with 212 blades for that matter). The 212 rotor works best at 100% Nr, and loses efficiency below that number power on.

If your friend would like something a little more official, that can be arranged as well.....straight from the horse's mouth so to speak. ;)

[tgrendl]

CTD,

Great posts. How do you check the topping (not just a health check but topping) on the B212?

Thanks :)

[CTD]

tgrendl:

Topping on the 212 / 412 can be done in flight, or on the ground to a lower value using a part-power trim stop.

I'm not a huge fan of the in-flight check as it requires one to pull into the transient Ng range (101.5, 102.4 or 103.4 depending on the set-up) and that gives me the willies. As I said above, the later engines like the 3D in the 412EP cannot be topped in flight, and must use the trim, as the max Ng is a screaming 109.2, and you can't get there without getting you name in the book.

At any rate, to answer your question.....basically you roll one back to idle, and increase power on the other until the topping value is reached, ensuring no other limits (Q, ITT) are exceeded. At the target value, the rotor should droop with no further Ng speed increase. If the rotor doesn't droop, or droops too early, adjustment is required.

On the 212 / 412, topping serves both sides of the power equation.... it ensures the power is available for OEI events, and protects the engine from Ng overspeed.

Hope that answers your question.

[212man]

CTD,
what Nr do you allow during OEI training then? In the real case of an engine failure you can droop to 91% Nr, surely the 97% is a 'normal' limitation? A topping check is not a 'normal' operation and consequently I would have thought it was acceptable to droop below 97%. The reason being that if, for some reason, you have slightly wrongly adjusted beep ranges, lazy N2 governors or a misadjusted droop compensation cam (or all 3) it is easily possible to reach 97% Nr on one engine without achieving topping. Pulling to, say, 94% will confirm without doubt whether the engine is indeed topping at the correct figure or less. It is also possible to achieve the correct N1 at 97% Nr, but not be sure whether in fact further application of collective will increase the N1 further ie it is not topping.

I stand back awaiting correction.

[RW-1]

Gee ShyTorque, wasn't that a bit "over the top?" :D

[ShyTorque]

RW-1,

The sauce of it!

ShyT

[tgrendl]

:D

212man,

Excellent catch. Additionally the reason we operate at specific airspeed ranges (around lowest total drag speed) is to reduce the possibilty of flapping to equality becoming a danger with reduced (out of the norm) rotor rpm. :eek:

[Thud_and_Blunder]

212 Man,

Fascinating post. During OEI training we only allow 64.9% Tq/ 97% N2/Nr, and would only allow the RRPM to go as low as 91% during autorotation. You say you go that low after "engine failure"; do you really mean OEI or double-donk-stop? I've certainly seen nothing that permits power-on below 97% in the Bell books.

My understanding of Bell's limits is that they are either "normal" op-applicable or "emergency". Topping isn't usually an emergency, at least not with our engineers! Your other topping points all appear valid to my limited brain, but surely they'd be better covered during ground topping?

[212man]

I'm suprised to hear that you can reach 64% TQ in the far east, in our environment that would mean encroaching the OEI N1/ITT inter-con limits which is obviously undesirable.

For Training we 'top down' the good engine to give Nr droop such as to allow a brief lift off from the ground before settling back down. This normally equates to about 96-7%N1 and about 760 ITT with 55% TQ. This is at 90% or MATOW for the conditions (usually gives 10,000lb).

For the exercises we allow the actual Nr limits specified for the emergency case, so for a continued take-off you can droop to 91% Nr, and the rejected case allows 95 % Nr. It works well and is representative of the real case.

You may be right that it is not correct, but we've only been doing it for 30 years so perhaps we've missed something. certainly the Uk CAA are quite happy with the away we do it. (No sarcasm intended BTW).

Must go, I'm on my hols to Nice!

[CTD]

212Man,

You are correct of course, and I guess I didn't say it very well. As I read Thud's question about Nr, I now realize he was refering to drooping during a topping check, not operationally. Allow me to clarify...... My point about RRPM being most efficient at 100% is for AEO, normal operations. Steady state, the 212's asymmetrical rotor works best at 100%, and drooping it (to lift a heavy load for example) accomplishes nothing once the stored potential energy is depleted.

For topping, I agree that sticking with 97% may not always be practical, and one really wants to see evidence of a positive Nr decay. If that requires drooping below 97 then so be it, but refer to the caution at the end of this post.

Similarly, things are a little different in an OEI landing event. In this case we are not concerned with the flight manual steady state, power-on limit of 97%, and we droop the rotor to whatever is necessary to soften the blow. In doing this we simply consume potential energy stored in the rotor, as one would at the business end of an autorotation.

A word of caution about low RRPM at high power: Here you're in No-Man's-Land with respect to yoke, pitch link, or blade loads. The RFM limitation of 97% power on is intended to prevent prolonged operation in this area. Ideally, one should carry as much RRPM on the approach as possible, and delay drooping the rotor until you need it.

I hope this clears the mud a little. Have a good holiday.