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

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

Old 9th Sep 2012, 22:50
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Question 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)
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Old 10th Sep 2012, 00:18
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Does that mean that the pilot can pull an additional 1.5 MAP over the determined MAP limits when operating with full carb heat?
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 .....
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Old 10th Sep 2012, 00:28
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Does that mean that the pilot can pull an additional 1.5 MAP over the determined MAP limits when operating with full carb heat?
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.
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Old 10th Sep 2012, 00:31
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Does that mean that the pilot can pull an additional 1.5 MAP over the determined MAP limits when operating with full carb heat?
Yep, easy to understand and delivered in the practical direct manner, as utilized by many clever yanks.

Make the air hotter (and less dense) you need to get more in there for the same power.
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'--
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Old 10th Sep 2012, 01:16
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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.
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Old 10th Sep 2012, 01:32
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It does not state you can pull more than the limit. IT WON'T HAVE IT.
Since the engine is derated, it will produce TOP at lower altitudes even with full heat.
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...

Last edited by Aucky; 10th Sep 2012 at 01:33.
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Old 10th Sep 2012, 05:03
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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.
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Old 10th Sep 2012, 06:37
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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.
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Old 10th Sep 2012, 23:41
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"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

Last edited by 500guy; 10th Sep 2012 at 23:44.
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Old 11th Sep 2012, 00:28
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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.

Last edited by Dick Sanford; 11th Sep 2012 at 00:29.
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Old 11th Sep 2012, 05:31
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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.
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Old 11th Sep 2012, 22:49
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Thumbs up 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...
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Old 12th Sep 2012, 00:08
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Thumbs up 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).
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Old 12th Sep 2012, 17:29
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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?

Last edited by Frog Fan; 13th Sep 2012 at 07:36. Reason: METAR time
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Old 12th Sep 2012, 23:27
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Question 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?
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Old 12th Sep 2012, 23:34
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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.

Last edited by Dick Sanford; 12th Sep 2012 at 23:34.
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Old 13th Sep 2012, 07:18
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Useless Gauge

Hi Dick, its Frog Fan but nevermind
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'.

Last edited by Frog Fan; 13th Sep 2012 at 07:39.
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Old 13th Sep 2012, 08:13
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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.

Last edited by Dick Sanford; 13th Sep 2012 at 09:32.
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Old 13th Sep 2012, 09:23
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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
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Old 13th Sep 2012, 09:37
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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.
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