Carb Heat Causes Power Loss

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pelmet
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Carb Heat Causes Power Loss

Post by pelmet » Tue Nov 19, 2019 11:09 am

This is a strange one. The pilot selected carb heat and there was a significant power loss. Either there is a mechanical problem, ice melting causing a temporary loss of power, or an unlikely coincident loss of power for other reasons. Typically, if one does something and the engine loses power, you put things back the way they were and see if the power comes back. Carb heat causing a power loss due to melting ice is a situation where you will likely have to either keep it on and accept the power loss until all the ice is melted or perhaps try some sort of repeated on and off applications of the heat. But in this case, the boost pump was turned on. I am curious if that would typically help, make things worse, or have no effect.

"C-FEMM, un Grumman Traveler exploité par Ottawa Biplane Adventures inc. effectuait un vol
selon les règles de vol à vue depuis l’aéroport d’Ottawa/Rockliffe (CYRO), ON pour un vol
touristique local avec un pilote et 2 passagers à son bord. Alors que l’appareil était établi à 2000
pieds au-dessus du niveau de la mer et se dirigeait vers le centre-ville d’Ottawa pour un tour de
ville, le pilote a actionné la manette du réchauffe-carburateur. Presque immédiatement après, les
révolutions du moteur sont passées de 2300 à 1600 tours par minute accompagnée de fortes
vibrations. Le pilote a immédiatement activé la pompe électrique d’alimentation en carburant et
ouvert la manette des gaz au maximum sans aucune amélioration. Le pilote a déclaré une
situation de détresse à la Tour de contrôle d’Ottawa et s’est dirigé vers le parc de la Baie pour y
effectuer un atterrissage d’urgence. Une fois atterri, le pilote a appliqué les freins au maximum,
mais n’a pu éviter la collision avec les arbres devant lui. La radiobalise de détresse a été activée
par l’impact. Le pilote et les 2 passagers ont pu évacuer sans difficulté et n’ont pas été blessés.
Les services d’urgence avaient été avisés et sont arrivés peu de temps après. Aucun incendie ne
s’est déclaré."


Translation with CARB HEAT entered by me in place of throttle-carburetor throttle

"C-FEMM, a Grumman Traveler operated by Ottawa Biplane Adventures Inc. was flying
according to visual flight rules from Ottawa / Rockliffe Airport (CYRO), ON for a flight
local tourist with a pilot and 2 passengers on board. While the device was set at 2000
feet above sea level and was heading to downtown Ottawa for a tour of
city, the pilot operated the CARB HEAT. Almost immediately after,
engine revolutions went from 2300 to 1600 revolutions per minute accompanied by strong
vibration. The pilot immediately activated the electric fuel pump and
open the throttle to the maximum without any improvement. The pilot declared a
distress situation at the Ottawa Control Tower and headed to the Bay Park for
make an emergency landing. Once landed, the pilot applied the brakes to the maximum,
but could not avoid the collision with the trees in front of him. The emergency locator transmitter has been activated
by the impact. The pilot and the 2 passengers were able to evacuate without difficulty and were not injured.
The emergency services were notified and arrived shortly thereafter. No fire
declared himself."
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CpnCrunch
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Re: Carb Heat Causes Power Loss

Post by CpnCrunch » Tue Nov 19, 2019 11:27 am

That kind of power loss is fairly typical with bad carb ice, but it would normally clear up in about 10 seconds if it is carb ice (or at least, significantly improve). Leaning the mixture might help. Sometimes I have to leave carb heat on for the whole flight, although that is unusual.

The Grumman Traveller seems to have a Lycoming engine, so I wouldn't expect it to get serious carb icing like that.

Anyway, I can't really think of any mechanical or weather related problem which could possibly cause the engine to have such a serious reduction in power with carb heat both off and on (assuming it has been left on for 30 seconds to melt any ice).
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rookiepilot
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Re: Carb Heat Causes Power Loss

Post by rookiepilot » Tue Nov 19, 2019 12:21 pm

Wouldn't the boost pump on richen the mixture, which carb heat also would richen? Leaning would seem to be the answer. (assuming carb ice)
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photofly
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Re: Carb Heat Causes Power Loss

Post by photofly » Tue Nov 19, 2019 12:38 pm

If an engine has a carburettor, the boost pump simply fills up the float chamber in it, if the engine driven pump is failing to do so. It shouldn't make any difference to the mixture if the float chamber isn't empty. (And if the float chamber is empty... the mixture is too lean already :-) )
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Squaretail
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Re: Carb Heat Causes Power Loss

Post by Squaretail » Tue Nov 19, 2019 3:37 pm

Anyway, I can't really think of any mechanical
I have seen once when upon application of carb heat, the flap within the carb heat box broke loose and caused a substantial loss of power for the remainder of the flight.
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CpnCrunch
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Re: Carb Heat Causes Power Loss

Post by CpnCrunch » Wed Nov 20, 2019 11:07 am

I have seen once when upon application of carb heat, the flap within the carb heat box broke loose and caused a substantial loss of power for the remainder of the flight.
How did it cause the power loss? (I'm assuming you mean more power loss than simply having carb heat on all the time).
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pelmet
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Re: Carb Heat Causes Power Loss

Post by pelmet » Wed Nov 20, 2019 3:55 pm

CpnCrunch wrote:
Wed Nov 20, 2019 11:07 am
I have seen once when upon application of carb heat, the flap within the carb heat box broke loose and caused a substantial loss of power for the remainder of the flight.
How did it cause the power loss? (I'm assuming you mean more power loss than simply having carb heat on all the time).
Flap blocks intake air.
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Re: Carb Heat Causes Power Loss

Post by CpnCrunch » Wed Nov 20, 2019 5:22 pm

pelmet wrote:
Wed Nov 20, 2019 3:55 pm
Flap blocks intake air.
I don't see any reports of that ever happening, and it seems somewhat unlikely given the physics and the dimensions of the internal parts. There are a few cases of the flapper breaking and bits of it blocking the carb inlet. The only actual forced landing in Canada from a broken carb heat flapper is this homebuilt:

https://wwwapps.tc.gc.ca/saf-sec-sur/2/ ... d2001A0738

Although even that doesn't say what exactly caused it, and it's unclear if the pilot leaned the mixture.
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Re: Carb Heat Causes Power Loss

Post by corethatthermal » Wed Nov 20, 2019 7:03 pm

and it seems somewhat unlikely given the physics and the dimensions of the internal parts
It is ALSO somewhat unlikely that a pig can fly , UNTIL you strap a mini hang-glider onto betsy the pot bellied pig and shove her off the cliff! Now you learn that pigs can really fly !!! :lol:

Carb heat boxes are up there with landing gear retracting systems that are neglected by AMEs in the industry ! Who wants a $20,000.00 annual every year anyways LOL
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Re: Carb Heat Causes Power Loss

Post by cncpc » Wed Nov 20, 2019 11:20 pm

There is a point where adding carb heat will not recover power. I'd be fairly sure a 700 rpm drop in RPM is well past that point. At best, you might have some ice chunk(s) come loose and block the mechanicals inside the carb throat.
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Re: Carb Heat Causes Power Loss

Post by Squaretail » Thu Nov 21, 2019 10:07 am

pelmet wrote:
Wed Nov 20, 2019 3:55 pm
CpnCrunch wrote:
Wed Nov 20, 2019 11:07 am
I have seen once when upon application of carb heat, the flap within the carb heat box broke loose and caused a substantial loss of power for the remainder of the flight.
How did it cause the power loss? (I'm assuming you mean more power loss than simply having carb heat on all the time).
Flap blocks intake air.
It did happen, in this case as I recall the actual shaft that the flap is on sheared off, which a SDR was filed over it, the shop doing the work thought is was such a bizarre failure. That said, while the power loss was substantial, the airplane still was able to make it back to the field. I doubt there was an incident report filed over it.
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Re: Carb Heat Causes Power Loss

Post by CpnCrunch » Thu Nov 21, 2019 10:39 am

Squaretail wrote:
Thu Nov 21, 2019 10:07 am

It did happen, in this case as I recall the actual shaft that the flap is on sheared off, which a SDR was filed over it, the shop doing the work thought is was such a bizarre failure. That said, while the power loss was substantial, the airplane still was able to make it back to the field. I doubt there was an incident report filed over it.
It doesn't seem to be possible to view SDRs, but it sounds like this CADOR:

https://wwwapps.tc.gc.ca/saf-sec-sur/2/ ... d2003C1254

No mention of any blockage though, just carb heat stuck on and minimal impact.
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Re: Carb Heat Causes Power Loss

Post by hangarline » Thu Nov 28, 2019 12:07 pm

A possible cause for excess RPM drop with carb heat applied is a hole in the muffler, which is allowing hot exhaust entry into the carb. Sound far fetched? No, because I had it happen here. Customer complained of excessive RPM drop with carb heat applied. Excessive was an understatement. When I ran the aircraft, RPM went from 1700 to almost stalled (300) when the carb heat was applied. Started to inspect the engine and decided to pull off the heat muff shroud and saw the problem immediately. Look at the attached photo. Don't even get me started about the number of years that it took to get to that stage without anybody pulling the shroud to perform CF90-03. Customer had just purchased the aircraft with a "fresh annual".
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