Robinson Safety Notice SN-25 (carb ice) - additional 1.5 in. HG MAP?
 

Robinson just released updates to the R22 and R44 POH, main points of interest being changes to the clutch light emergency procedure (now 10 seconds and land as soon as practical unless other indications) and the section on use of carburetor heat/assist.

I am trying to fully understand the implications of the new Safety Notice SN-25 on carburetor ice, where it talks about an additional 1.5 in. Hg of MAP. Read the SN here:http://www.robinsonheli.com/srvclib/rchsn25.pdf

The final paragraph reads (I added emphasis):

Carburetor heat reduces engine power output for a given manifold pressure. Approximately 1.5 in. Hg additional MAP is required to generate maximum continuous power (MCP) or takeoff power (TOP) with full heat applied. The additional MAP with carb heat does not overstress the engine or helicopter because power limits are still being observed. Since the engine is derated, it will produce TOP at lower altitudes even with full heat. However, avoid using more heat than required at high altitudes as the engine may reach full throttle at less than MCP or TOP.

Does that mean that the pilot can pull an additional 1.5 MAP over the determined MAP limits when operating with full carb heat? Or is it saying that the additional 1.5 in. Hg will be applied even though the manifold pressure gauge shows you are at normal MAP limit for the day?

Any thoughts?

Many thanks in advance.

Jay (R22/R44 pilot)

I believe that is the correct interpretation.

The power produced by the engine is function of how much air and fuel is in the cylinder. Make the air hotter (and less dense) you need to get more in there for the same power. So for the same actual power in the engine (and hence pressure/force which is the true mechanical limit) you need the throttle open more and that gives a higher manifold pressure.

Now ducking head under pillow .....

I agree it does read like that assuming the coffee break experts agree, however it will be a very hard sell to the students which would go along the lines of:

"now lets work out your 5 min takeoff and continuous power limits, and remember they arent really limits, because if the carb heat is out/on you can pull 1.5 inches over the limit"

no room for misunderstanding or confusion at all I think, just wait till they get on a turbine with the "limits arent limits" methodology.

Yep, easy to understand and delivered in the practical direct manner, as utilized by many clever yanks.

I would clarify it thus:-
'you need more "volume or air if it is at a higher temperature to get the same weight of air" in there'--

You will have less power by 1.5 MP,and expect the throttle to be at max stop sooner limiting power avalable- It does not state you can pull more than the limit.:= IT WON'T HAVE IT.:rolleyes:

It suggests it will 'have it' at lower altitudes - evident by the fact that before you start the engine the MAP is reading about 30" at sea level, and so full throttle should be somewhere close to this (some 3.5" above the MTOP) but the derating of the engine and torque through the drive train is the limiting factor until the altitude at which full throttle will be less than this torque/power limit... I interpret as saying if the max MAP is 24.5 then in practice the max power allowable with C/heat is 26", which at lower altitudes is safely available, but caution at higher altitudes...

I don't get the 10sec for clutch light either.
Especially in the 'light' of the guy flying the R44 low level at night on 4th July. Sure, he could have done some things differently, but we 'walked' (swam) away from it. Even 6-7 secs would be enough and then I'd pull it.

I was putting the new pages in the POH myself recently and besides bits of different wording to stress some stuff, these 1.5in manifold and 10sec for clutch light were the stood out.

I have been asking RHC for sometime to clarify the situation when the pilot uses carb' heat. I am very pleased to see that they have. You might find my technical document "No Ice Thank You" interesting as it fully explains the effect of carb' heat v BHP. You will find the above on my website Mornington Sanford Aviation.

"Does that mean that the pilot can pull an additional 1.5 MAP over the determined MAP limits when operating with full carb heat?"
Yes, I had Robinson tell me this spicifically when i went to RHC school about 5 years ago. I think Id something to do with the power charts going off of OAT vs. CAT

What the pilot must understand is that the limit is a BHP limit which is fixed. The point on the MPG at which that limit is reached is variable. The MP Limit Chart requires OAT and Press' Alt when determining the MP limit, this is ok until you add carb' heat as this will directly effect the engine efficiency thus more MP to achieve the same BHP. Think HP Limit and not so much MP Limit - when is my engine developing R22 124BHP/131BHP - R44 205BHP/225BHP/245BHP.

The way it was explained to me at the factory, was that normally you use the OAT to determine your MP limit, but once you use carb heat, then you use carb temp to determine what your limit is since the engine doesn't know the difference. For example, say your OAT is 20C, but with carb heat, the air entering the engine is now 30C, you would now use the MP limit for 30C and that is the additional power they are referring to.

Great replies
 

Thanks for all the great replies. I would say the consensus is that we pull an extra 1.5 over the limit on full carb heat. I have emailed Robinson for clarification.

My only other thought is the MAP limits were not just about engine performance limit, they also take in to account the air density to affect the amount of pitch you can safely pull (how much up collective) as that would affect drag (so rotor RPM) especially if there was an engine failure --> If you were pulling so much pitch and had an engine failure, you might never be fast enough to lower the pitch before a critical (80% + 1% per 1,000) rotor RPM was reached.

I would say it is a shame they don't release a draft to forums like this to deal with any ambiguity before publication...

I'll post the reply from Robinson when I get it. In the meantime, I'll hope the temp & dewpoint spread stay out of carb heat range so I don't have to consider if the extra 1.5 is the right thing to do...:rolleyes:

Confirmed by Robinson - Yes pull extra 1.5 in Hg
 

A friend at the Robinson Safety Course just got confirmation from Tim Tucker (Chief Instructor at Robinson) that YES we can pull the additional 1.5 in. Hg (if it is there - see the bit in the SN-25 about high DA).:ok:

Useless gauge?
 

I very timely thread as I am fairly sure I had carb ice ontake-off in a R 22 two weeks ago. It was a training flight with a new student. Half tanks, just after sun up, OAT 14°C and the dew point was 10°C (later noted from a METAR issued an hour before sun rise). All was good in the hover, and the gauge was out of the yellow arc. Immediately after transition while crossing a road and fence the aircraft started vibrating quite badly and the climb became very sluggish. There was an open area after this and I almost opted to put it down there, but as it was not getting worse and there was a little bit of climb performance I kept it going instead. I was using all the power it had and therefore the throttle was wide open. About 20 seconds later the vibration was gone and performance restored.

I then started second guessing myself, thinking that I hadnot chosen the safest option. Yet it seems if I had lowered the collective witha partial carb ice blockage it is possible that the engine could have died froma rich mixture cut out. Our height and speed would have made this a tricky situation at best.

I did some research and found some interesting information especially Richard Mornington-Sanford’s excellent ‘NO ICE, THANK YOU’

http://www.morningtonsanfordaviation.com/pdfs/MSA-No-Ice,-Thank-You.pdf

Most disturbing of all though was Robinson’s own SN 25. Thiswas revised in July 2012. Sure it is on their website but they don’t seem to be shouting it from the rooftops exactly and they certainly did not tell me about it.

In it all reference to the Carb Heat Gauge has been removed. The notice now simply says:

‘When in doubt, assume conditions are conducive to carburetor ice and apply carb heat as required’

So what the hell is the gauge for then?!

Another case of Robinson covering their ass? :ugh:

So we can pull the extra 1.5.... BUT
 

A second question arises that I don't have an answer for - what about the redline limit on the MAP gauge - can you exceed that in meeting the 1.5 additional? :confused:

Frog Pan - pleased you found my " No Ice Thank You" useful. Reference your experience - if when hovering you are over wet grass or any wet surface without sufficient carb heat applied, you will have ice forming in the carb'. On an R22 you need about 15 deg C indicated, just out of the yellow is not sufficient. One of the most important gauges in the cockpit (apart from the RPM indication) is your carb' heat gauge, however so many pilots do not, or do not know how to check its accuracy. Pleased that the outcome of your experience was positive. Regards Dick Sanford.

Useless Gauge
 

Hi Dick, its Frog Fan but nevermind :cool:
I get your point about wet grass except in this case it was concrete. The dew point was noted later by referring to a METAR issued at 6 am, an hour before sun rise so had to be a degree or two lower by then.
I don't recall that the gauge was touching the yellow line either but concede it must have been fairly close.

What do you think about Robinson removing all reference to the gauge from SN-25?
Surely it implies that the gauge is not to be trusted. Where in the POH is there anything about testing the gauge also?
It kind of looks like Robinson knows that the gauge is inaccurate, suspect that it may have caused accidents (the evidence after all disappears in seconds) and are covering their ass with the amended notice so their lawyers can point and say 'we warned you, so no our fault'. :mad:

Sorry Frog Fan, it was a bit early in the morning.
In my experience the Carb heat gage like another similar gage (fuel gages for example) should if possible be checked/referenced against a known measurement. With the carb' heat gage it is easy as you have an OAT gage and or your met' info (prior to first start of the day of course).
"Dare I say it" but A SN is just a SN the helicopter should be operated IAW the relevant current revision of the POH and in section 4 page 11 (which is an FAA approved page) in which RHC have not changed the reference to the gage.

Unfortunately there is no reference to or how to check the carb' heat gage in the POH, for what it is worth I have been trying to get it included for years, however I have been teaching the procedure for as many years.
I have had the pleasure of lecturing the carb' icing section of the RHC factory pilots flight safety course when my annual visit coincides with a safety course as I conduct the European Robinson R22/R44 pilot flight safety course.

For what it is worth:

I have been involved with Frank Robinson for some 32 years and have a unique relationship with the factory as a maintenance course instructor and an accident/technical investigator (I do not work for RHC, I am independent). From a personal point of view (yes, it is limited), in all my years working with Frank/RHC I have never seen the factory do anything other than deal as openly as possible with problems as they arise. The litigious world that they and other aircraft manufactures have to swim in would have caused a lesser man than Frank to give up.

That is not to say i agree with everything RHC does and sometimes it is easier for "no bodies" like me to publish things like "No Ice, Thank You"

Ok, this has gone on for to long, I do not usually get involved but this is a pet subject.

Best wishes, fly safely. Dick Sanford.

Thanks for the clarification Dick, and I will always do the check you suggested in future.
Also good to know that you have faith in RHC, it has helped renew mine a bit too :)

Pleased to help Frog Fan. "it is not the gun that kills you, it is the person holding it" The Robinson is a great product and I have been fortunate to have been involved (in a very small way) from almost day 1.

If this is the intent of SN-25, then another amendment of the POH is required. As it stands, manifold pressure limitations are published in section 2, and placarded in the cockpit. As long as this doesn't get amended, you can still not legally use more MAP than what the limitation chart tells you, carb heat or not.

Lelebebbel (or something along those lines)
Correct, that has been the problem from the start. The Limitation is a BHP limitation, therefore referring to MAP as a limitation is not the best way to convey the information to the pilot (in my view).
There will be situations where given certain atmospheric conditions where a reasonable amount of carb heat has to be applied to keep the CAT needle out of the yellow will result in the MAP indication being greater than the MAP limit - I cannot fly, even if I know the actual BHP is not being exceeded.

Keep them in the green, best wishes

Dick Sanford

So this turns into a different kettle of fish, or is that barrel of monkeys?

I'm perfectly happy with the physics and had pretty much figured this phenomenon out for myself. I also go to great lengths to make sure students understand the difference between over boosting (exceeding BHP limits through the gearbox), over pitching (pulling so much pitch that engine goes beyond full throttle so that rrpm droop ensues) and the difference that DA makes to the situation.

With the issue of the SN and, as Lelebebbel has observed, no change to POH or placards, I have to ask what is the motivation of the SN? Are we to try and teach students about something that is legally unusable? Is putting the information in a SN a hope that the flying community will run with it without Robinson having to formally change performance and perhaps certification data?

Please don't get me wrong, love the Robbos, lots of hours in 'em but this seems to be a tip and a wink rather than information we can actually use to educate pilots about their machines.

Thoughts anyone?

I am not sure why RHC issuing a SN should be treated with suspicion? For RHC to change the required pages in the POH, which have to be FAA approved takes time. Issuing information via a SN does not. Sounds like "damned if you do, damned if you don't" RHC are stating in a publication that the pilot can take into account the reduction in engine output power when using carb' heat. Not sure where the "legally unusable" come from.

I am not sure what is meant by over boosting through the gearbox. Over boosting is usually something the pilot might do to a turbocharged or supercharged engine (you cannot over boost a normally aspirated engine as in the Robinson R22, R44) Exceeding the BHP limits will increase the drive train load per cycle thus subjecting the components to greater loads than RHC applied during their fatigue testing - the pilot becomes a test pilot.
Full throttle on your engine is a physical stop, after this point the engine will not produce any more HP.

We are not talking about the risk of exceeding the limited BHP but getting across to the pilot the effect of the application of carb' heat on the power output of the engine.

Confused student.
 

So for a R22 Beta.
OAT 25C.
CH Cold.
Max continuous: 23.75
5 min take off: 24.75
CH Hot (+1.5").
Max continuous: 25.25
5 min take off: 26.25

So its then OK to exceed the MP red line at 25.2 since the whole scale has effectively shifted? Red line is now at 25.2 + 1.5 = 26.7.

Working things backwards from the CH hot figures above to use the CAT gauge to base calculations upon, the CAT I would be looking for to support this would be off the scale, which terminates at +40C.

OAT 25C.
CH Hot (+1.5").
Max continuous: 25.25
25.25 - 23 = 2.25
2.25 * 100 = 225
225 / 3 = 75C

:confused:

Hi Dick,

My post was not intended to imply suspicion I promise. Merely to try and garner opinion on to what practical use this information can be put. Combined with your own excellent document the SN does explain the effect of carb heat on engine power and MAP indications.

What does not appear to be stated however is what you can do with this knowledge in the cockpit. We are obliged to fly within the published (POH) limits which do not currently reflect any allowance for using carb' heat. Happy to be corrected here.

Therefore the debate I'm inviting is "How do we put this knowledge to use?".

The document says that for a given BHP output the application of full carb' heat will increase the MAP reading by 1.5" (providing of course you don't reach full throttle first). What it doesn't say is that it's therefore legal to use an amended MAP limit when flying with full carb' heat.

Maybe that's not the intent of the SN and I'm missing the point but the questions are already being asked, see LOZZ's post above.

Perhaps amendments to the POH are in the pipeline?

Disclaimer: It doesn't seem to matter what you type it always seems to come over as confrontational on here! Really not the case here. I welcome the published information, I'm just interested in opinions on how we put it to use.

@LOZZ - YES
 

@LOZZ: In your example, according to Robinson Safety Course this week, yes you could exceed the redline, because the hot air reduces the air density and therefore means you are pulling less BHP, the extra 1.5 in. allow for that, even if it means exceeding the redline. You are NOT exceeding the engine BHP limit. I think Dick explained this well.

Now if you are at operating high DA you may not be able to get there and may suffer a drop in engine/rotor RPM!:eek:

Ambiguity
 

@HeliChopper:

I wish Robinson would release a draft of changes to POH to forums like this to get feedback on any ambiguity. Would save themselves a ton of emails and questions at every RSC.

"Approximately 1.5 in. Hg additional MAP is required to generate maximum continuous power (MCP) or takeoff power (TOP) with full heat applied."

The above is taken from the subject SN. The intent seems to be clear to me but I would agree that it would be absolute if it was ratified in the POH limitations section, however as explained, this takes time.

I cannot and do not speak for RHC but I would think that a POH limitation section amendment is on it's way.

I for one, will be very happy to see such an amendment as those people that know me or have attended my European Robinson Flight Safety Course will know that I have been teaching/discussing/explaining the subject of Carb Icing in the Robinson Product for more years than I wish to remember.
In particular this current subject of Carb' heat -v- BHP.

I apologise if I have come across as 'confrontational' as that was not the intent and is unhelpful, however, we should be able to have a "Frank" !!!!!!!!!!!! discussion.

Dick, No apology required, just poor grammar on my part, I should have written "whatever one writes on here....". Seems so easy to get into a bun fight.

Careful though, after the "Frank" gag you could be labled a pundit!

Leave it on?
 

Does this mean that if I lose 1.5 inches MAP with full carb heat applied and I fly around at 2000ft above sea level most of the time that I could leave the full carb heat on permanently and just pull 1.5 extra inches so long as I don't run out of throttle which I know here in Scotland at these altitudes I won't? Why bother with the on and off and continually checking if I'm reading this correctly?

bvgs:
Well, yes, you could, however why would you want to unless the conditions required you to?
The engine is less efficient, that is why we need to pull more indicated MAP, it's running rich, therefore your fuel consumption is increased.
Under certain atmospheric conditions you could be actually inducing carb' icing by having it applied. That is why you have a lock on the carb' assist system.

Regards Dick Sanford

Yes I can see how that makes sense, always tend to run with it on more than is required anyway just to be on the safe side. Question? Why when hovering at almost max weight so the mp is right up on the limit, the throttle pretty much approaching its limit does the carb temp gauge show its very cool, to the point of needing heat applied to keep it safe. I thought that the venturi would be well open and therefore the air not accelerating as quick etc etc...just wondered ? Loved your article on the icing, perhaps if I read it again I might be able to answer my own question but the pub calls for a pint :)

bvgs:

Pleased you found my carb' icing document interesting. Yes, you will find the answer in the document, even if you are demanding your 131 BHP the butter fly valve is nowhere near fully open. The R22 Beta II is a bigger problem than an R22 Beta as the 0-360-J2A BII engine is more de-rated than the 0-320-B2C Beta engine, take a look at the butterfly angles I have presented in the document i.e. MAP -v- butterfly angle.
Please to hear that you use a little more than a little less, that is the right way to approach the setting. The manufacture has given the pilot a method of preventing the ice build up in the carb', it is the pilot's primary responsibility to prevent the engine from stopping by using it.
Depending on the flight exercise I was flying and the atmospheric conditions I very often left the car' heat in the full hot position so as not to have to disturb Blogs.

As I tell the pilots on the flight safety course; I have had the R22 engine stop on me due to carb' icing and the scary thing was, we did not know it had stopped! Work that one out!

Keep them in the green.

Regards Dick

The CAT probe is located before the throttle valve in the R22 carburettor. The throttle position therefor shouldn't affect indicated temperature much (hence the need for the "below 18in apply carb heat" placard).
It can imagine that you would see a slight decrease in indicated CAT because more fuel is being injected at higher power.

R44 Carb Ice
 

I totally get the carb ice thing when we are coming into land, lowering the lever and creating a Venturi causing the air to speed up and subsequently get colder thus allowing ice to form from in the carburettor due to the moisture in the air. So all good with that. However, I was hover taxing the other day at just under 25 inches of manifold pressure (almost at full weight) and the gauge was going well into the minus. No big deal as I pulled carb heat to ensure it was above zero but of course subsequently lost some power and had to “cushion crawl” and use the down draft. I’m just wondering why the carb gets so cold when the butterfly valve is almost fully open. Is it simply the vast amount of air being sucked in? Thanks for any insights👍🏻👍🏻

It's a lot of air being sucked in through relatively small opening. Venturi is still prety much in effect.

In order to atomise the fuel and turn it from liquid into gaseous form, energy is required (gaseous state having a higher energy than liquid state). This transfer of energy from the air to the fuel 'sucks' heat out of the air and cools it down slightly. There's far more thermodynamics than I can comprehend going on in reality, but that's one mechanism at work explained in a simple way that can help explain the temperature drop from ambient even at full power.

Would carb icing even occur at 24-25" ?

Yes, but it is unlikely. While OP saw temp decrease, icing is not very likely at high power.
Excerpt from Wiki:
The venturi effect can reduce the air temperature by 39 K; 39 °C (70 °F). In other words, air at an outside temperature of 38 °C (100 °F), can drop to −1 °C (30 °F) in the carburetor. Carburetor icing most often occurs when the outside air temperature is below 21 °C (70 °F) and the relative humidity is above 80 percent.[1] The risk of carburettor icing is significantly increased at partial power settings (such as when power is reduced during descent), due to the cooling effect of the partly-closed throttle.[2]