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  • Carb heat rpm drop ?

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Technical Chat Tips and Advice, including flying the Luscombe

Moderator: HTB

 #7949  by Stuart Jones
 Sat Apr 20, 2013 10:05 pm
Hi, i have recently completed tail wheal conversion and have been out in JA, What kind of drop in Rpm would you expect with an 0-200 when carb heat is applied ? I was only getting around 50rpm today, that doesn't really strike me as being enough ? Thank you.
 #7951  by Phil Laycock
 Sun Apr 21, 2013 7:37 pm
Hi Stuart

50 RPM does sound a bit low .... I would expect something like 75 - 100 RPM. However it might be the Tacho which doesn't show the necessary accuracy ......does it sound like a decent RPM drop ? . If not I would investigate whether the carb Heat box flap is operating the full range when you pull Carb Heat control...it might be the cable is stretched or the connection is mis-rigged or worn. Then check the flexible "Scat" hoses for integrity and finally the actual heat shroud around the exhaust itself........these get a beating with vibration and wear the edges so that a lot of warm air escapes .
let us know what you find
regards

Phil
 #7952  by Stuart Jones
 Sun Apr 21, 2013 8:27 pm
Hi Phil, thanks for the info. There are big gaps around the exhaust pipe and the shroud, I can get the tip of my finger between them. I'm thinking this may be the cause ?
Stuart
 #8133  by Nige
 Fri Aug 30, 2013 11:14 am
Hi all,

I've been recently training guys on a couple of Luscombes (and a others) and an unusual trend has crept in regarding the operation of Carb heat prior to t/o and on the app to land. This is pertinent to the Continental in particular.

In these cases carb heat has been applied (pulled?) whilst taxying and right up to the point of departure, in one instance for over 4 minutes! that's a lot of unfiltered air! I'm sure most of the Luscombe chaps apply carb heat mainly during power checks and (strongly recommended in high medium to high risk conditions) just prior to applying full power to get airborne.

In the other cases, carb heat is being applied correctly when below 2000 rpm, say on base but then removed (pushed in?) a long way out on finals - more of a Warrior habit (Lycomings ). Our beautiful Continental is now purring along at around 1500 rpm on finals with a good chance of collecting that nasty ice once again. :shock: in one case the time between pushing the carb heat in (removing it) and landing was over a minute and a half - long enough to make your heart miss a beat should you want to go around.I'm not saying the old girl will stop but, on demand, she may well take valuable seconds to
recover and to the uninitiated, this could lead to feeling of a potential engine failure at the very least. I's no big deal if you leave the carb heat out right up to landing. If it's a touch and go, then that's the point to remove the carb heat , or, at the very least remove the application on very short finals, although in testing conditions this is messy and distracting. If carb heat is left out,(applied) over the touch and go/ go around, again, it's not the end of the world either. About 2 HP. Sort strips could be a problem though.

All is good with my colleagues now, and my primary objective is providing guidance and safe operation of these old girls ( the Luscombe!), ensuring Luscombes are preserved (along with their pilots) that bit longer! :wink: Just thought I'd mention it as I'm sure the vast majority of luscombe pilots operate the carb heat system correctly. :D

Sadly, Autumn is not far away.. :( carb ice is sneaking up..

Happy flying all.

Nige.
 #8173  by JEZZA
 Mon Sep 30, 2013 3:01 pm
interesting subject- generally i tend to on G- AKVP post start, leave my carb heat on - it takes a lot of time to warm upthe venturi/ throttle gate area, & icing can start to form due to the temp dropin the venturi, no matter what oat is..also during taxiing - particularly on wet grass, its still on.then after @ 10 mins the carb ht goes in & i commence run ups, this also gives time for your engine to get to run up temp go ahead- .Mag drop chk also best checked when eng is now at temp, Obviously, care needs to be taken, as quite rightly the carb air is unfiltered- so watch out the Newly mown grass- for cuttings being re sucked back/in to the upflow of air. my SOP also on Landing/ go Arnd , is to put carb ht back in over the Hedge . some landings though one has to be cautious as on the roll out/ to taxiing speed/ turn off is has been known to get a icing prob as the plugs ,also ,arent getting , the usual temp, & can foul up carbon wise & on some A/C the engine can faulter / stop . so i usually always do a quick run up / mag chk then Shut down. clearing your plugs for your next start. regards jezza.Just Back from the blakesburg/ Ottumwa , Iowa U.S.A 40th Ann vy, & luscombe Meet- what a sight to see- luscombes Galore!!. Temp 101 degrees!!
 #8176  by Keith Old
 Tue Oct 01, 2013 6:41 am
Nige always has valuable advice on many subjects
Agree that this is always one to watch , especially as we move into autumnal weather
At White Waltham we have an overhead join , so once overhead and I pull the carb to on , then throttle back for the descent and leave the carb heat on all the way to touch down.
If joining downwind anywhere , I would pull the carb heat on quite early on the downwind leg and again leave on until just at the point of touchdown
Keith GBSSA
 #8177  by Steve Martin
 Tue Oct 01, 2013 1:12 pm
Having seen a good friends engine (continental C90) splutter and stop 200ft up :shock: , shortly after take off due to carb icing, the last thing I do before letting the brakes off and pushing the throttle in is a Full Carb heat check whatever the weather is. I tend to leave the carb heat hot from base leg thro' to landing.
 #8206  by Rob
 Sat Nov 09, 2013 5:55 pm
In humid conditions (which is nearly always this time of year) I keep the carb heat on from just prior to descend point right up to applying power for the GO. I just keep my thumb sticking out when pushing the throttle forward. Reading through the NTSB database is quite sobering (ice can form really quick as little heat is produced). Also found that on a C-95 powered Cub that keeping the carb on while applying power takes away the faltering when doing touch and gos in colder temps.

Robert
 #8713  by Johan Whiteman
 Tue Sep 23, 2014 8:35 am
Hi Guys, I fly the Luscombe 8A with a 65hp engine and the fuselage tank. According to SOP's one has to do the t/o and app with the carb heat on to reduce the fuel flow due to the high nose attitude as the fuel tank sits quite low and it is gravity fed. Any comments. My density alt at my field is around 8000' thus giving around 48hp but she flies beautifully with 2 adults up and 1/2 fuel, nice a/c. (she has a ragwing)
 #8714  by Johan Whiteman
 Tue Sep 23, 2014 8:35 am
Hi Guys, I fly the Luscombe 8A with a 65hp engine and the fuselage tank. According to SOP's one has to do the t/o and app with the carb heat on to reduce the fuel flow due to the high nose attitude as the fuel tank sits quite low and it is gravity fed. Any comments. My density alt at my field is around 8000' thus giving around 48hp but she flies beautifully with 2 adults up and 1/2 fuel, nice a/c. (she has a ragwing)
 #8720  by Robert Lees
 Fri Sep 26, 2014 6:10 pm
Johan Whiteman wrote:...<snip> According to SOP's one has to do the t/o and app with the carb heat on to reduce the fuel flow due to the high nose attitude as the fuel tank sits quite low and it is gravity fed. Any comments. My density alt at my field is around 8000' </snip>


Hello Johan: Something not quite right there. Can you quote the source of your "SOPs"? Written down somewhere, or verbal?

Carb heat richens the mixture. At 8000'DA, that's the last thing you need. What carb is fitted, and do you have a proper mixture control?

My gut feeling is that you have been misguided.

Rob
 #8721  by dmcneil
 Fri Sep 26, 2014 7:41 pm
Greetings All,

It is indeed SOP to apply carb heat on take-off in carbureted A-65 powered Luscombes equipped with a fuselage tank. From Aircraft Specification A-694 8A Section:

Fuel capacity 14 gal. (+40). Placard required: "Full carburetor air heat required for takeoff and
landing." The reason for this placard is that during takeoff acceleration and initial high angle-
of-attack climb, the fuel flow may not be adequate for proper operation.
Application of full carburetor heat in this case helps overcome the possible deficiency of
fuel flow during takeoff. Carburetor ice is not a basic consideration in requiring this
placard. (See also items 101, 102, 106, 109, 111, 112 and 115 for optional tanks).


This originally became an issue back in the late 1930s-early 1940s. Engine stoppage on take-off became enough of a concern for the CAA to issue an Airworthiness Maintenance Bulletin in 1940-1941. See attachment (hmmm.. the site software isn't letting me upload the attachment).

Earlier this year the FAA issued a Special Airworthiness Information Bulletin about the same issue. Link:

http://rgl.faa.gov/Regulatory_and_Guida ... 4-09R1.pdf

This new bulletin was the result of another engine stoppage on take-off in an 8A. Here's what the FAA Aero Engineer in charge of the bulletin said when asked about the genesis of this new bulletin:

"The SAIB was prompted by an accident that involved a loss of power on take
off / climb on 8A airplane. The carburetor heat was in the cold position
on take off and the pilot was not aware of the reason for using it on take
off. The appropriate placard was installed."


So the problem still crops up. I can't imagine that this would be a problem for Johan with an 8000' DA at take-off! But it is definitely a potential problem for those operating at or near sea level. That A-65 is just too powerful at sea level. :)

Dan
 #8723  by Johan Whiteman
 Sat Sep 27, 2014 8:21 pm
Hi Dan tks for the info, I do have the vented fuel cap but usually don't apply the carb heat. (For the past 5 years) however in the last month I had an aborted t/o due to power lost and also had an engine cut after landing as the tail came down, in both cases the fuel tank was about 1/4.

Will do the placarding and use the carb heat as from now.

Tks again.