Robinson Safety Notice SN-25 Carburetor Ice - Clarification of the additional 1.5 in. HG additional MAP

[Jay Bunning]

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)

[eagle5]

I guess some things have changed in the year since I was at the RHC course, since there was no mention of this stuff then?

 

One thing I do remember though, was that they said when using carb heat, to get the temp. off of the CAT gage to determine MAP, which would (most likely) give you a higher number than what you determined using the OAT or ATIS temp.,...although it looks like I'm going to have to call and order the updates?

 

They could have explained it better, but I'm putting my money on; You, the pilot, will have to pull 1.5 in. Hg above what the chart says in order to get MCP/TOP when using full carb heat!

Edited September 10, 2012 by eagle5

[eagle5]

Hmmm, a thought just occurred...

 

I wonder if this change to the carb ice safety notice is to keep pilots from pushing the carb heat back in on short final (a practice that many CFIs have told me to do, but at RHC was told not to do)?

[Jay Bunning]

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).

[brettjeepski]

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).

 

Let me see if I am understanding what you are saying. If my flight conditions are flying at 2000' PA and 20 degree Celsius Dew point 10 Degree Celsius, that my MCP would be 22.2 and TOP is 23.1 (calculated off of MAP chart)...so I would have to use Carb Heat. Since I am using carb heat I can pull now 23.7 MCP and 24.6 TOP as long as I don't hit full throttle?

[zippiesdrainage]

We've been agruing this at my school for a few days now and asked a DPE their opinion and he seems to agree with Robinson that pulling carb heat increases the pressure inside the engine but doesn't create the additional break horsepower that you would get at that manifold pressure setting without carb heat. So we can use the additional 1.5 because essentially the MAP gauge is no longer accuratly representing the power you're getting out of the engine for your MAP limit.

[Jay Bunning]

Let me see if I am understanding what you are saying. If my flight conditions are flying at 2000' PA and 20 degree Celsius Dew point 10 Degree Celsius, that my MCP would be 22.2 and TOP is 23.1 (calculated off of MAP chart)...so I would have to use Carb Heat. Since I am using carb heat I can pull now 23.7 MCP and 24.6 TOP as long as I don't hit full throttle?

 

Yes. 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?

[ridethisbike]

Yes. 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?

 

No. The redline indicates a limitation. Going above that would mean you're no longer operating according to the POH and the aircrafts limitations.

 

On a side note, it will DEFINITELY be nice to be able to pull that extra bit of pitch!

[Mikemv]

No. The redline indicates a limitation. Going above that would mean you're no longer operating according to the POH and the aircrafts limitations.

 

On a side note, it will DEFINITELY be nice to be able to pull that extra bit of pitch!

 

RTB,

 

I agree with you but think of it this way if you will. When you exceed a limit listed in the POH you are operating within the POH as that is where the limitation, Chap 2. is stated, but you have exceeded the stated limitation and that must be logged.

 

Mike

[gary-mike]

Dang it, now I have to order the subscrition for updates, my POH is no longer current.

[Jay Bunning]

According to Tim Tucker at the Robinson Safety Course this week: Yes you can exceed the MAP redline to meet the extra 1.5 in. The limitation is on the engine BHP - that doesn't always equate to a set MAP as it depends on air density. The redline on the MAP was an FAA requirement, but does not show a BHP limitation.

[lelebebbel]

According to Tim Tucker at the Robinson Safety Course this week: Yes you can exceed the MAP redline to meet the extra 1.5 in. The limitation is on the engine BHP - that doesn't always equate to a set MAP as it depends on air density. The redline on the MAP was an FAA requirement, but does not show a BHP limitation.

 

From a technical point of view that may be right, but legally you can not. No matter what Tim Tucker or this SN say. What counts is in "Section 2 - Limitations" and placarded in the cockpit. The Manifold Pressure Limitations chart limits your MAP based on OAT and Pressure Height alone. There is no allowance for carb heat on this chart, and therefore you can not, legally, exceed the numbers on this chart, SN25 or not. The red line on the gauge is based on this chart, it simply shows the 5-min MAP limit at 40C OAT and sea level.

 

I would like to see a proper amendment of the POH, now we have a confusing SN that conflicts with the limitations, and a lot of confused pilots. Maybe the MAP limitations chart should be based on CAT rather than OAT, wouldn't that be more accurate?

Edited September 14, 2012 by lelebebbel

[Jay Bunning]

I hear what you are saying Lele, but think why that limitation was created - in part to protect the engine. And if the reality is that you will still be below the BHP limit and therefore not over-stressing the engine - respecting what the limit was created for. Plus if you have an extra 1.5 in. to pull to get the BHP that you should get and to hopefully stop you hitting the ground - I know what I would do, 'legal' or not. I'd argue the words of Tim Tucker at an official Robinson Safety Course have weight too...

 

Agree to disagree on this one I guess...

Edited September 14, 2012 by Jay Bunning

[eagle5]

From a technical point of view that may be right, but legally you can not. No matter what Tim Tucker or this SN say. What counts is in "Section 2 - Limitations" and placarded in the cockpit. The Manifold Pressure Limitations chart limits your MAP based on OAT and Pressure Height alone. There is no allowance for carb heat on this chart, and therefore you can not, legally, exceed the numbers on this chart, SN25 or not. The red line on the gauge is based on this chart, it simply shows the 5-min MAP limit at 40C OAT and sea level.

 

I would like to see a proper amendment of the POH, now we have a confusing SN that conflicts with the limitations, and a lot of confused pilots. Maybe the MAP limitations chart should be based on CAT rather than OAT, wouldn't that be more accurate?

 

That's interesting since I was told to use the CAT gage for determinimg MAP while using carb heat. Until you mentioned it I never thought to look, but it does say OAT on the MAP chart, so, if you're right, then even using the CAT gage would also be illegal?

 

Its also interesting that, according to the OP, there was a recent POH update! I wonder why they didn't go ahead and include a carb heat/MAP change?

[lelebebbel]

I hear what you are saying Lele, but think why that limitation was created - in part to protect the engine. And if the reality is that you will still be below the BHP limit and therefore not over-stressing the engine - respecting what the limit was created for. Plus if you have an extra 1.5 in. to pull to get the BHP that you should get and to hopefully stop you hitting the ground - I know what I would do, 'legal' or not. I'd argue the words of Tim Tucker at an official Robinson Safety Course have weight too...

 

Agree to disagree on this one I guess...

 

There is nothing to disagree over here - from a legal point of view, the situation is clear as day. Section 2 counts, nothing and no one else. If they told you that you can only fly with pink underwear in section 2, that, too would be a limitation and any other color would be illegal, no matter if it had a negative effect on the aircraft or not.

 

As I said, from a technical point of view of course it is right, you aren't overstressing the drivetrain because you aren't actually producing any excess torque, but that still doesn't make it legal.

 

The real problem I see here is that this whole situation is sending the wrong message to students learning on RHC products. "It's a limitation, but because someone told me that it is technically wrong, it's ok to exceed it" - doing that might not be a problem in this case, in this helicopter, but what is the student actually learning here? Maybe a few years down the track, they'll be flying a turbine, the PIC overtorques the thing, but makes a compelling argument that 10% over the red line is ok... see what I mean?

Robinson needs to fix the MAP limitations in the POH.

[Mikemv]

Knowledgable Robby guys,

 

When Carb heat is applied is there an indicated decrease in MP? Possibly about 1.5"?

 

Just wondering?

 

Also, most power setting limitations deal with transmission limitations and not engine damage considerations.

 

Mike

Edited September 15, 2012 by Mikemv

[Goldy]

I would like to see a proper amendment of the POH, now we have a confusing SN that conflicts with the limitations, and a lot of confused pilots.

 

Here's a statement that I think we all agree would solve the issue, so I contacted RHC......stay tuned.

 

Sorry Mike, I havn't flown a carb version in a while so can't confirm what I believe would happen with full carb heat applied.

[lelebebbel]

Knowledgable Robby guys,

 

When Carb heat is applied is there an indicated decrease in MP? Possibly about 1.5"?

 

Just wondering?

 

Also, most power setting limitations deal with transmission limitations and not engine damage considerations.

 

Mike

 

Yes, pretty much exactly 1.5 on the R22 Beta. I think it's about the same on the Beta 2 and Raven 1 but haven flown either in a while.

 

Another interesting twist is that the Beta 2's Engine is derated by Lycoming to 24.6" MAP at sea level (see TCDS here, O-360 J2A http://rgl.faa.gov/R...$FILE/E-286.pdf)

It is then further derated by RHC to whatever the chart in 2-11 says, but adding 1.5" over the charted takeoff limit would put it over Lycomings limit of 24.6" in some cases.

 

Again, I am not saying that you are actually producing too much HP. But until RHC amends section 2, you are exceeding aircraft- and engine limitations (from a legal point of view).

Edited September 15, 2012 by lelebebbel

[ridethisbike]

In response to MikeMV, I never noticed. I just know we started to drop and I pulled a little collective and watched the MP to make sure I was where I was supposed to be.

 

 

THIS IS HYPOTHETICAL BASED ON A DROP IN INDICATED MP

 

IF the MP indication DOES drop when full carb heat is applied, there's really no need to amend the POH because even if you do add that 1.5" (from its new indication), you're only bringing it back up to where it was before you added the heat and therefore not exceeding the limitation with the extra 1.5"

 

Pictures? Ok...

 

MP before carb heat = 20.0

MP WITH carb heat = 18.5 (this assumes a drop in indicated MP)

MP with extra 1.5" pulled in = 20.0

 

I think this was what Mike was getting at.

[iChris]

There is nothing to disagree over here - from a legal point of view, the situation is clear as day. Section 2 counts, nothing and no one else.

 

Robinson needs to fix the MAP limitations in the POH.

 

 

In general, the Type Certificate Data Sheet (TCDS) and the RFM, which may be required by the TCDS, provides information necessary for the safe operation of the helicopter based on the helicopter’s type certification (CFR 21.5). TCDS and RFM information is specific to a particular aircraft type and the components therein. Thus, any inquiry into whether the additional 1.5 inches of MAP in SN-25 may be used must include an examination of the particular aircraft’s certification bases on the applicable TCDS.

 

No person may operate a civil aircraft unless it is in an airworthy condition (CFR 91.7). The FAA defines airworthiness as an aircraft meeting its Type Certificate. The guidance in Safety Notice 25 with respect to the additional 1.5 inches of MAP, which observe TCDS power limits, is in compliance with Type Certificate H10WE; therefore, the aircraft is being operated and maintained (legally) in an airworthy condition. TCDS H10WE references the following Takeoff & Maximum continuous ratings:

 

Type Certificate No. H10WE (page 3):

“Maximum continuous (124 hp.) 2652 rpm (104%)

Takeoff (5 minutes) (131 hp.) 2652 rpm (104%)

See RFM for maximum manifold pressure corresponding to hp. rating and ambient conditions.”

 

The reciprocating engine is considered primarily as an air pump with the pump capacity directly affecting the power output. Any engine instrument, that relates factors affecting airflow, can indirectly reflect engine power output. Manifold pressure is a convenient, indirect, way for the pilot to comply with the 131-hp. takeoff and 124-hp. maximum continuous rating.

 

The MAP limits and MAP charts in the RFM are for the convenience of the pilot in estimating horsepower output. MAP limits or MAP references are not listed in the TCDS; however, the RFM is not in contradiction since it does reflect, indirectly, the horsepower limits prescribed in the aircraft’s type certificate.

 

Safety Notice SN-25 “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.”

 

91.9 [a] Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.

 

21.5 Airplane or Rotorcraft Flight Manual: [b]. The Airplane or Rotorcraft Flight Manual required by paragraph [a] of this section must contain the following information:

 

[1] The operating limitations and information required to be furnished in an Airplane or Rotorcraft Flight Manual or in manual material, markings, and placards, by the applicable regulations under which the airplane or rotorcraft was type certificated.

 

 

In any case, the type certificate is the basis andr foundation for the RFM.

 

 

 

 

Edited September 17, 2012 by iChris

[Mikemv]

In response to MikeMV, I never noticed. I just know we started to drop and I pulled a little collective and watched the MP to make sure I was where I was supposed to be.

 

 

THIS IS HYPOTHETICAL BASED ON A DROP IN INDICATED MP

 

IF the MP indication DOES drop when full carb heat is applied, there's really no need to amend the POH because even if you do add that 1.5" (from its new indication), you're only bringing it back up to where it was before you added the heat and therefore not exceeding the limitation with the extra 1.5"

 

Pictures? Ok...

 

MP before carb heat = 20.0

MP WITH carb heat = 18.5 (this assumes a drop in indicated MP)

MP with extra 1.5" pulled in = 20.0

 

I think this was what Mike was getting at.

This was the point of my question exactly. Now at 20.0"MP, what happens when you remove carb heat? 21.5"?

 

Do not worry as iChris has addressed this as not busting a limitation! Do not read this as selectively disregarding Sect. 2. Limitations.

 

Mike

[lelebebbel]

This was the point of my question exactly. Now at 20.0"MP, what happens when you remove carb heat? 21.5"?

Manifold pressure does not drop when carb heat is applied - it increases. Carb heat reduces engine power, which reduces engine RPM, causing the governor (or pilot) to open the throttle, thus increasing MAP. Manifold pressure increases by about 1-2" when full carb heat is applied. In your example, MAP would drop from 20" to around 18 or 19" when carb heat is removed.

 

iChris

 

The MAP limits and MAP charts in the RFM are for the convenience of the pilot in estimating horsepower output. MAP limits or MAP references are not listed in the TCDS; however, the RFM is not in contradiction since it does reflect, indirectly, the horsepower limits prescribed in the aircraft’s type certificate.

 

Hmmm.. so the RHC TCDS does not directly contain MAP limits, the limitations section of the flight manual - and the placards in the cockpit - do. The newest safety notice instructs us to ignore these limits under certain conditions, but so far no mention of this is made in section 2 or on the placards in the cockpit..

I am not a lawyer and I can't even begin to guess how all this would hold up before a judge - or not. Regardless, I still find this a rather unsatisfactory situation.

Edited September 17, 2012 by lelebebbel

[ridethisbike]

Manifold pressure does not drop when carb heat is applied - it increases. Carb heat reduces engine power, which reduces engine RPM, causing the governor (or pilot) to open the throttle, thus increasing MAP. Manifold pressure increases by about 1-2" when full carb heat is applied. In your example, MAP would drop from 20" to around 18 or 19" when carb heat is removed.

 

 

You say that the MP actually increases with carb heat applied and then go on to say that it's the pilot causing this increase. If the pilot is the one CAUSING the increase, then logic points to a decrease from initial MP with the introduction of carb heat.

 

 

Reasons I agree that the MP decreases with the introduction of carb heat PRIOR to pilot input? Well what do we know about cold air vs hot air? Cold air is more dense and therefore has a higher pressure. Hot air is thinner and therefore has a lower pressure.

 

Now I know the first thing everyone is going to say is "nuh uh! When you warm air up it has a higher presure!" When you have a sealed container of air and heat it up, it will in fact exert a higher pressure on the container. This isn't because it actually has this kind of pressure, it's because the air is trying to EXPAND. This expansion is what exerts the pressure on the container.

 

None of the intake tract is a sealed container. The carb heat is being pulled from outside where there are no constraints on it's ability to expand so it isn't going to magically start building pressure. In fact, the opposite is true since we know that Bernoulli's Principal will affect that hot air as it passes through the carburetor. The velocity of the air will increase and according to said principal, the pressure will decrease even further. "But that's only at the venturi" Also true, but remember, a naturally aspirated engine pulls its air in via a vacuum in the cylinder. The air being pulled in by the cylinder increases its velocity through the intake tract, thereby decreasing it's pressure.

 

So, with a given MP and the introduction of carb heat PRIOR to pilot intervention, the MP indication should drop. Then it'll rise when the pilot pulls in more collective.

 

 

PS I hope that all makes sense... haha. I know it's broken down about as barney style as it gets, but it (hopefully) helped you understand what was going through my brain. It can be hard for me to explain things without breaking it all down sometimes...

[Lindsey]

Ridethisbike--

 

Lelelebel's point (I believe) is that if you are in a hover with no carb heat applied, and then you apply carb heat, you will be seeing approximately a 1.5" increase in indicated MAP *to maintain that same hover*. Which, in my experience, is true.

[Mikemv]

Ridethisbike--

 

Lelelebel's point (I believe) is that if you are in a hover with no carb heat applied, and then you apply carb heat, you will be seeing approximately a 1.5" increase in indicated MAP *to maintain that same hover*. Which, in my experience, is true.

 

I think everyone is in agreement that applying carb heat decreases MP. Lele's point was that after the decrease in engine RPM and (unstated but corresponding) MP that the pilot or governor will increase RPM & therefore MP.

 

I will ask what will happen to MP if the helo is at full throttle during a hover and carb heat is applied? (I know the answer but throwing it out for consideration) Let us make it any recip. helo just for understanding the principle of physics and operation. Now we can also be R22 specific.

 

Mike