Some Cherokee Q & A

[Pe11e]

- I flew total 25 hours, and I guess time for oil change has come, since I saw in hangar that oil and fuel filter are yellow. That means it's the time to change the filters? Oil is still original brown color, but it's low, a little above first line (can't remember the number). The plane is not flown daily, one/two times a week. What will happen if I prolong changing of fuel/oil filters? I just want to know what will happen, but I don't wanna crash on my next flight lol.

- What is the philosophy of carburetor heat? How can I know when to turn it on, except when the engine sounds rough even with good mixture?

- I usually wait for engine to warm up before taxi/takeoff, needle to be close to the middle line. But when the weather is cold, it takes 10-15 minutes for engine to warm up, but needle is still not at the ideal temp (on the middle line). What are consequences if I set the take off throttle on a cold engine or when the needle is below the middle line?

- Start up procedure in my case is: master on, magnetos both, mixture full, beacon on, primer two times, fule pump on for few seconds, starter on, throttle to 20-30% then when engine starts, close to idle to keep rpm under 1000. I keep fuel pump on while flying, is that ok? Is my start procedure correct?


Thank you in advance!

[AKar]

Well, my personal opinions follow, though they are formed mostly based on others' advices.

The plane is not flown daily, one/two times a week. What will happen if I prolong changing of fuel/oil filters?

In real life, the oil filter should be changed and inspected during oil change (generally recommended each 50 hours or 4 months, whichever comes first). The biggest issue, again in real life, would probably be that if there were any abnormal wear within the engine, it would probably get spotted during oil filter change. Hardly any conditions develop fast enough to completely kill the engine within 50 hours that don't show up somehow, any metal debris found during oil filter change is a sign of that. If you don't replace the oil filter, the risk is that the filter gets so clogged that the bypass opens on some engines, releasing the unfiltered oil into the circulation greatly accelerating the wear. With a good engine, that would probably take hundreds of hours, but with any abnormal wear condition, it could happen rather quickly.

The Cherokee doesn't really have a replaceable fuel filter in that sense, but there is a screen under the engine. It should be drained and cleaned each 50 hours in this model. Also there is a screen at the carburetor, but it is usually checked only if there is a problem suspected. The fuel screen could, in principle, get clogged if ignored, with obvious consequences.

- What is the philosophy of carburetor heat? How can I know when to turn it on, except when the engine sounds rough even with good mixture?

Well, there is not really much else. A one advice is to turn it on when carburetor icing is suspected, so you could monitor the conditions. A low throttle setting, and high humidity make a clue, though you can find all kinds of charts that show you how susceptible the conditions are for icing.

What are consequences if I set the take off throttle on a cold engine or when the needle is below the middle line?

It won't have to be that high. On the green arc, or above that 75F mark, is enough. Just warm the engine for a few minutes at least and until the needle starts rising, and you're fine! Of course, having a nice, warm oil may reduce the wear a bit, though I'd say that at take-off rpm's, too thin oil is as big of an issue as having slightly thick oil is.

- Start up procedure in my case is: master on, magnetos both, mixture full, beacon on, primer two times, fule pump on for few seconds, starter on, throttle to 20-30% then when engine starts, close to idle to keep rpm under 1000. I keep fuel pump on while flying, is that ok? Is my start procedure correct?

Yeah, that's fine on the A2A Cherokee with impulse couplings on both magnetos...I don't think in percents of throttle, so I don't comment that part. I just push it in slightly, and adjust while she fires. Leaning during ground running is recommended, btw, and somewhat important in A2A Cherokee. Priming depends on temperature, but if you've got it running, then it should be fine. And no reason whatsoever to keep the fuel pump on while flying...not sure if it would actually harm anything, but generally the engine fuel pump outlet pressure should be within optimum range for the carburetor inlet on it's own - adding the electrical pump to the parallel of it won't improve anything at least!

BTW, I'm not sure why it is recommended to have the electric fuel pump switched on during the tank switch... I don't think it does any good in Cherokee's fuel system, but perhaps there are practical reasons for the recommendation.

-Esa

[Oracle427]

Anytime you reduce power below 2000 rpm it would be safe to open the carb heat for a couple of seconds and observe if the engine continues to run smoothly. At low power settings you may ice up and not realize it until you try to add power. The reason to wait before reducing power is that melting ice may make the engine run rough. At low power settings the heat may not be sufficient to be effective if the carb is iced up.

Icing can form in the middle of summer on humid days.

[Great Ozzie]

I keep fuel pump on while flying, is that ok?

If the engine driven pump fails in flight... idk... may not recognize that failure with the boost pump on. If the engine driven pump fails, I would want to land when practical (find an airport and land asap).

Newer Cherokee checklist you may find the boost pump is turned off during the runup to check the engine driven pump. Then the electric boost pump on for takeoff and off at "desired altitude" - which to me, a couple thousand feet agl seems reasonable.

I want the boost pump on for the pattern / takeoff & landing. Running around with it on "all the time" (cruise) may use up its life... hate to lose it then not have when it could be needed.

[Nick - A2A]

- What is the philosophy of carburetor heat? How can I know when to turn it on, except when the engine sounds rough even with good mixture?

Pe11e, I was also a bit unsure about carb heat application when I began using the Cherokee. A bit of digging around on the internet led me to a few sources, including the report on a fatal PA-28-140 accident in the UK last year in which carburettor icing was implicated.

I won't discuss this tragic incident further here. However, the AAIB report did lead me to a further publication which discusses the use of carb heat: it's the Civil Aviation Authority Safety Sense Leaflet 14: Piston Engine Icing.

The following extract from this leaflet is also reproduced in the accident report on G-ATRR.

‘j) Descent and Approach
Carb icing is much more likely at reduced power, so select carb heat before, rather than after, power is reduced for the descent, and especially for a practice forced landing or a helicopter autorotation, i.e. before the exhaust starts to cool. A full carb heat check just before selecting hot air for the descent is advisable.) Maintain FULL heat during long periods of flight with reduced power settings. At intervals of about 500 ft (or more frequently if conditions require), increase power to cruise setting to warm the engine and to provide sufficient heat to melt any ice.

k) Downwind
Ensure that the downwind check includes the cruise carburettor heat check at paragraph 6(i) above. If you select and leave the heat on, however, speed or altitude will reduce on the downwind leg unless you have added some power beforehand.

l) Base Leg and Final Approach
Unless otherwise stated in the Pilot’s Operating Handbook or Flight Manual, the HOT position should be selected well before power is reduced and retained to touchdown. On some engine installations, to ensure better engine response and to permit a go-around to be initiated without delay, it may be recommended that the carb hot air be returned to COLD at about 200/300 ft on finals.’

Note the final sentence above, which mirrors the advice given in the A2A Cherokee PIlot's Manual.

"Carburetor heat should not be applied unless there is an indication of carburetor icing, since the use of carburetor heat causes a reduction in power which may be critical in case of a go around. Full throttle operation with heat on is likely to cause detonation."

All the best,
Nick

[Pe11e]

Thanks guys! Every bit of every answer is really helpful!

Anyway, all that carburetor heat and engine roughness mumbo jumbo made me horny just before my next Cherokee flight. Why? Because next time when the engine start to stutters a bit and sound rough, I will turn on carburetor heat with such a pleasure and watch Accusim doing its magic!

I must study the anwers a couple of times more to be sure I'm doing all the thing properly.


Cheers!

[Pe11e]

I had a pretty strange moment last night with the Cherokee. Outside temp was 7C, I did a standard start up procedure, but without primer at that time. Engine started pretty easily, but couldn't mantain steady RPM. It was losing revs just like that. Opened carb heat a bit and it revs up to expected 1000RPM. So there was ice in the engine, and I'm sure I've read in the guide that carb heat is not advisable after start up while parked, and even while taxing. Is it connected to the primer or the outside temp was too low for fuel pump only start?


Also I have a pretty rough time landing this plane. I always land with full flaps, keeping 70-80kts on final, but soon as I get close to the runway at around 30-40ft agl keeping 60kts, I have trouble to land this thing on main wheels! If I pull down to get close to the runway it sill land on the nose gear since when I pull up gently the plane will climb like mad even at 55-60kts at full flaps. How do you flare this thing, it climb like crazy! My best landings are around 55kts, floating about 10ft agl because I can't flare and suddenly nailing it down on the runway a bit hard. Is my landing speed too high (I doubt)? Is there any specific techniques?

Oh yes btw, I mantain approach speed and angle with a throttle and a trim, trying to avoid corrections with an elevator. I guess this is a good technique?


Thanks once again!

[AKar]

Is it connected to the primer or the outside temp was too low for fuel pump only start?

Once the engine is running, it shouldn't matter how it was started. Typically, the engine needs to be turning for the moisture to get sucked in to the carburetor and for the temperature to drop so that ice can form. I don't think the starting method should matter any way in practice on whether the ice forms in the carburetor or not.

The issue why the use of carburetor heat is discouraged on the ground is that it bypasses the engine air filter, thereby increasing changes of debris getting sucked into the engine, in turn accelerating the wear.


About the landing technique, there are many around here that can give you better advice than I can. But regarding the elevator, I'd recommend exactly the opposite: you should actively use it, that's why it is there for. Just coordinate its use with power for best and smoothest control.

-Esa

[Oracle427]

You are going to fast on short final. The approach airspeed is the speed you should use for a stable approach until you get to the last 50 or so feet. From that point onwards you should be at idle power if you aren't already at idle power and you should be leveling off just off the surface and holding the airplane off the ground until stall and touchdown.

On days where carb icing is likely you may want to run up the engine with carb heat to ensure smooth operation and no icing right before takeoff. On really bad days for icing it might even be necessary to use carb heat during takeoff and accept the reduced performance.

As Esa said opening the carb heat on the ground had the drawback of drawing in debris. However an engine failure due to carb ice on takeoff will draw in much larger debris by forced induction!

[AKar]

As Esa said opening the carb heat on the ground had the drawback of drawing in debris. However an engine failure due to carb ice on takeoff will draw in much larger debris by forced induction!

Yep. An important point in any field, especially including aviation, is that you should not be afraid to use the tools at your disposal when necessary - no matter what the recommendations or written procedures say! The reasons behind the recommendations and limitations should therefore be understood by heart so you can make informed decisions. If conditions require the use of carb heat on ground, then you shall use it. But when considering that, I might want to avoid using it in the dust cloud blown up by another airplane for example.

Some folks actually recommend that carb heat should be on for every take-off in conditions that are most susceptible for icing. I personally (based on nothing ) think that's a bit of an overkill, especially considering the margins available at many airfields, and the fact that fully open throttle is not that prone to ice, but clearing the carburetor at moderate power with carburetor heat on just before fully advancing the throttle while closing the heat for take-off sounds like a good recommendation.

-Esa

[Nick - A2A]

How do you flare this thing, it climb like crazy! My best landings are around 55kts, floating about 10ft agl because I can't flare and suddenly nailing it down on the runway a bit hard. Is my landing speed too high (I doubt)? Is there any specific techniques?

There's some good discussion/advice on flare techniques in this thread, along with some videos etc.

Following Dudley's advice in post 3 in that topic, I tend to keep my speed at around 80mph until very short finals. With reduction to idle power I've always found the airspeed bleeds off rather quickly. Here's a video I made of an approach and flare. It isn't intended as an example of ideal technique by any means (I don't fly for real and any constructive criticism is gratefully accepted ); just a way of showing that even with a rather brisk arrival over the threshold, excessive ballooning or 'floatyness' shouldn't be an issue.

Cheers,
Nick

...By the way, the video was shot with a camera phone, so afraid the quality isn't great - especially streamed via DropBox.

[Lucevan]

I was taught to use carb heat for a count of ten as part of the Power checks prior to take off.

Use it when descending with reduced power and every so often on route (at waypoints for example).

It's used for a check on downwind and then cold for approach.

That's what I was taught anyway.

Cheers

[Great Ozzie]

Some folks actually recommend that carb heat should be on for every take-off in conditions that are most susceptible for icing. I personally (based on nothing ) think that's a bit of an overkill, especially considering the margins available at many airfields, and the fact that fully open throttle is not that prone to ice, but clearing the carburetor at moderate power with carburetor heat on just before fully advancing the throttle while closing the heat for take-off sounds like a good recommendation.

Not sure I have heard that (yes have heard "use on every landing")... but I certainly have heard the opposite i.e. "never use on takeoff" - to which I submit the following AVweb article: Eye of Experience #8: Carb Ice Demons Refer to the sections, My Experience and Carb Heat on Takeoff

I was taught to use carb heat for a count of ten as part of the Power checks prior to take off.

When you do this check, "time" isn't so important as is "what", as in what happens.

You want to see an rpm drop.

If the engine were to "stumble" (cough, gag, choke etc.) then I am going to wonder if I am getting carb ice (experience / familiarity with the airplane helps here). Always going to check my rpm on the takeoff roll to see if I am getting what I expect to be getting.

Carb Heat has been discussed in another thread here at A2A... A couple of NTSB .pdfs I'll recommend: google "ntsb carb heat" or direct links below.

Engine Power Loss Due to Carburetor Icing (direct download)

NTSB Recommendation on Carb Heat (direct download)

[Great Ozzie]

Note the final sentence above, which mirrors the advice given in the A2A Cherokee PIlot's Manual.

"Carburetor heat should not be applied unless there is an indication of carburetor icing, since the use of carburetor heat causes a reduction in power which may be critical in case of a go around. Full throttle operation with heat on is likely to cause detonation."

Ok, this can be found e.g. in a Cherokee 180 "E" manual. However, my problem with it is the statement, "unless there is an indication of carburetor icing". Your first indication that there is carb ice could be when you go to apply power... *and nothing happens*.

Cessna took a different tack with this. If you look (for example) in a 172RG POH... which uses an O-360 (Lyc. 180hp) you will find in the BEFORE LANDING checklist:

Carburetor Heat -- ON (apply full heat before closing throttle).

And in the BALKED LANDING checklist:

1. Power -- FULL THROTTLE and 2700 RPM.
2. Carburetor Heat -- COLD.

One can also find this in other Cessnas with Lycoming engines e.g. the C172 "N" with the O-320 (Lyc. 160hp).

I prefer Cessna's advice. When in conditions that could produce carb ice (large range of temps btw) I prefer to add Carb Heat anytime I am below a cruise power setting. I think there is good basis / reasoning behind it. I also disagree with the concern of having carb heat on during the go-around, should that become necessary. To me, Carb Heat is one of those "clean-up" items, just like flaps & gear. And certainly, even if removing carb heat e.g. a couple hundred agl (short final) it still needs to be taught as a clean-up item.

The NTSB takes the same side as the Cessna procedure. Here's a quote out of one of those links I provided above.

Most publications advise pilots that carburetor ice can be detected by a drop in engine rpm for airplanes with fixed-pitch propellers, or a drop in manifold pressure for airplanes with a constant speed propeller. Although the use of this method of detecting carburetor ice might be practical for the steady state conditions of normal cruise flight, the method sometimes becomes less practical during descent. Experience has indicated that the distractions to a pilot during descent to enter a traffic pattern, which include preoccupation with traffic awareness and radio communications, can preclude the pilot‘s detection of a drop in engine rpm. Additionally, the aural detection of carburetor ice indicated by a change in engine sound is not as easily recognized as during the stabilized conditions of cruise flight.

Accordingly, the Safety Board believes that pilots need more definitive guidance from airplane manufacturers concerning the use of carburetor heat on the descent and/or before-landing checklists. Descents in most carburetor equipped general aviation airplanes can be conducted without adverse consequences to the powerplant with full carburetor heat on when power is reduced below the normal cruise flight engine power setting (that is, below the green arc on the tachometer or the manifold pressure gauge). The use of this procedure would preclude the formation of carburetor ice when detection of such formation could be difficult, in lieu of more specific guidance or procedures from manufacturers concerning designated airplane models.

Therefore, the National Transportation Safety Board recommends that the Federal Aviation Administration:

Amend, as necessary, in conjunction with the General Aviation Manufacturers Association and the airframe manufacturers, the descent and before-landing checklists in the pilot’s operating handbooks and airplane flight manuals of carburetor-equipped airplanes to require the use of full carburetor heat when engine power is reduced below the normal cruise power range (the green arc on the tachometer or the manifold pressure gauge)...

[Caldemeyn]

I tried to lean the mixture when using carb heat but couldn't get any RPM's back this way. I don't know if the carb heat is touched by accusim goodness yet on this plane, the way it richens the mixture when applied and you need to lean it to compensate for the RPM drop. Maybe it's really subtle here ?

Plus it would be nice to have the negative effects of the primer being not locked, when flying.