IAS vs GS on C206

[northern33]

Hi there,

I have an odd question for you... On a recent flight I observed something that I hadn't seen before (considering my meagre 85 hours of flying time).

I flew out on a 100NM sightseeing trip at about 6500ft and back home at 6000ft.

Going out I experienced some pretty decent tailwinds as the ground speed showed 170mph while the IAS indicated 145mph. Everything seemed normal. CHT's in the 380 range. Running ROP.

The odd part was the return trip. I turned back and descended down to 6000ft and noticed that my ground speed dropped to 128mph which pretty well matched the IAS. I expected to fight a headwind coming home, but also expected to see the IAS in the 140 to 150mph range with a ground speed of only 110mph. The CHT's dropped to 340 as well. I tried a few different altitudes (5500ft) to see what difference the density would make, but none really.

Everything else seemed unremarkable. The CHT's also dropped from 380 down to about 340.

Engine conditions during both legs of the trip:
20"MP and 2500RPM. Mixture ROP. Cowl flaps open.

[iflyforpie]

Do you have an actual temperature readout on your EGT? If so, what were the actual numbers instead of degrees ROP?

Air that is less dense will make your peak EGT lower since you are burning less fuel, resulting in power and speed loss. This would explain your lower CHTs as well.

I am also interested at your power settings. We usually run our 206 22/23 or 21/22. This brings the fuel burns down and keeps good pressures in the engine without running it too hard (Continental IO-520-F). If it is a turbo or Lycoming you can run it at even higher MP vs RPM (oversquare) and still have no ill effects. 20/25 is basically like going 100 in 3rd gear.

[lost in the north]

We run our 520's at 24 sq.,f*** thr fuel burn.We want the speed................

[Meatloaf]

I'm curious why you were flying at 6000 feet?

The only other things I can think of are what was your fuel flow at the different altitudes (which is almost insignificant) and what was the OAT. Obviously you were near lateral wind shear and maybe an inversion.

Iflyforpie, he was saying his CHT dropped when he descended

For such a small change in altitude, I can only think of the two questions above for why there would be such a large difference. That or there is a mistake with the numbers in the first place.

[northern33]

Or I have a mag going south on me... That would explain the lower CHT's and the lower airspeed. Damn, wish I had paid more attention to the EGT's though! The more I think of it, the more I wonder if it is a mag issue. I had to adjust the mixture on the way back. Adjusting the mixture ending up with a lower fuel flow (ROP), but relatively higher EGT's than I had only minutes before in the opposite direction.

Iflyforpie, I now run my 206 at higher power settings as per a bulletin from Continental last fall. They do not recommend cruising with the IO-550 at power settings less than 2400RPM. Running at less than that could cause issues with the harmonic balancer. As luck would have it, I had to tear down my engine this winter because I couldn't keep alternator belts on the engine. The 6th order balancer pin had developed some play in the bushing. I can only speculate that my old habit of running 22sq contributed to that. Who knows. For the extra fuel burn I will take the speed it gives and hopefully save another engine tear down. Sorry for the long winded answer on the power settings iflyforpie!

[iflyforpie]

Sorry for the long winded answer on the power settings iflyforpie!

You had me at 550.

[northern33]

lol, fair enough

[Meatloaf]

20" of manifold pressure is 20" of manifold pressure, regardless of whether a mag is running right or not. It might increase your fuel flow to maintain 20" but won't affect your airspeed, especially not to the degree you suggest.

[cdnpilot77]

I'm curious why you were flying at 6000 feet?

Can I re-ask this question?

[Edi]

I'm curious why you were flying at 6000 feet?

Can I re-ask this question?

He filled IFR.

[cdnpilot77]

I'm curious why you were flying at 6000 feet?

Can I re-ask this question?

He filled IFR.

Thats what I think we are hoping, but he went out at 6500'...was there a stop and file or file while en-route for the return trip? Presumably IMC was not prevailing during a sight seeing trip or did he just file for practise/fun?

[northern33]

cdnpilot, The altitude I posted was an example that I was trying on the return leg to rule out anything different on the trip other than direction. If route details are what you are most interested in; I went out at 6500 and returned at 5500ft. I experimented with a few different altitudes to rule out density altitude as the winds certainly did not change from my 180 degree turn back. The area near where I was experiencing the issue has a 7000ft ceiling due to a military base nearby. I was more concerned about having the extra altitude if I needed it and really wanted to rule out anything that had to do with density altitude. Thanks for focusing on the route data though and not the mechanical information I'm looking for advice on. Managing an a/c issue that I had not yet experienced over nothing but bush remained my primary concern.

I truly believe that forums like this one are invaluable for fellow pilots or those that are considering a life in aviation, even recreational. As our numbers are going down and not up, I think we need to support each other so we can all continue to share it. In the example I posted there are probably a whole list of things a veteran pilot could have done to troubleshoot the issue and understand what is happening under that cowl. I am trying to get there. Flying as often as I can and asking folks with a whole bunch more experience than I have is how I'll get there. For those of us that don't have local flying clubs, this is the next best thing (AV Canada).

Any other novice pilots wishing to share their experiences will likely think twice if it just turns into entertainment for folks with more experience. To those that took the time to help troubleshoot I truly appreciate the advice. They don't teach the aircraft ownership stuff in ground school!

As it turns out there is more to the story. I went out to the field today to check the mags, but the electric fuel pump is not functioning. More troubleshooting to go.

[mag check]

Maybe you lost an intake valve on one jug, this would cause a drop in power/speed, raise your manifold pressure, since it is open to atmosphere, which would cause you to pull back the throttle to maintain the desired mp, reducing power further, thus dropping your temps.

How does it run on the ground? MP better be around 10-11" at idle, or you got a problem.

[northern33]

Hi Mag check. There was no change in MP. My money is on a failed mag now.

I just re-read my original post and can see that I did a poor job explaining my return flight and the question I had about the flight. My flight out was at 6500, and the return at 5500ft. I follow VFR altitude rules.

The issue I experienced was on the return trip home. At 5500ft I seemed to really struggle with performance. At first I thought maybe the wind direction was different or the density actually was significantly different. The IAS and GS just didn't make sense. I briefly experimented with a few different altitudes with a keen interest in duplicating the altitude East to eliminate the wind direction and density as the reason. The small altitude differences on a route that I just made minutes before really made no difference as you would expect.

To be honest a dead mag never really crossed my mind at the time. I am a bit embarrassed now that I didn't think of that at the time. I even went through a bit of a checklist to see if I had inadvertently changed something. Even if I had thought of it, I would likely not have tested the mag theory in flight as I was about 75nm out from the nearest airport. And where I fly there are no fields or roads; just bush, muskeg and small lakes. Not the place I want to execute an emergency landing when the engine is still running, albeit down on power. Reflecting on it now I suppose it's no different than the run up checks I do on the mags or the pre-shutdown check I do on the mags.

On a slightly different note, what is the 'average' life span of a magneto?

[iflyforpie]

On a slightly different note, what is the 'average' life span of a magneto?

Average lifespan of a mag is the overhaul time of the engine. 500 hour inspections will usually change the points, rotor, and gears depending on the model and condition. A bad condenser or carbon tracing is usually what takes a mag out of service early.




When troubleshooting mag stuff I always go cheap to expensive. This is assuming that a live check hasn't exposed a completely dead mag (where I would remove the P lead and see if it fires up again, if not, replace the mag, if so, look for the short).

Run the engine on each mag and see if one is running rough. Run it on the rough mag for a while with the cowls off, and then shut off the engine and take a cheap screwdriver handle and run it on each exhaust. If one stack doesn't take material off the screwdriver, you've got a bad plug, HT lead, or carbon tracing on the rotor.

Dirty, improperly gapped, or worn plugs can cause you all kinds of grief. We clean ours every 50 hours and combined with ruthless leaning has removed pretty much all bad mag drops.

Some harness problems can be seen visually (loose end, torn shielding and insulation) and can usually be confirmed by turning the mag switch on and snapping through the impulse couplings (I never tried it with booster coils or shower of sparks) with the leads within 1/4 inch of an exposed ground. If this doesn't expose anything, you need to test each lead for leakage with a lead tester.




If is it just a generally bad drop or excessive difference in drops but without vibration I will go straight to timing. I've seen mag nuts back off and move the timing out of whack. I've also seen excessive wear on the cam follower do the same thing. A quick check with the synchronizer will confirm the timing. Retarded timing will cost you power. Excessively advanced timing is worse for your engine than running full throttle with the prop pulled all the way back--you get more power, but you can destroy your engine within a matter of hours.

The final thing is the mag itself and this is a case of pulling it and sending it away since most shops aren't equipped to do 500 hour inspections anymore. Some Bendix ones you can open the breaker cover and see if the breakers are excessively pitted which means a bad condenser.


For the electric fuel pump, check the breaker/fuse first, but also check the throttle switch. These cheap plasticy gizmos often either go out of rig so the throttle cam doesn't push it, or just decide to bust.

Maybe you lost an intake valve on one jug, this would cause a drop in power/speed, raise your manifold pressure, since it is open to atmosphere, which would cause you to pull back the throttle to maintain the desired mp, reducing power further, thus dropping your temps.

How does it run on the ground? MP better be around 10-11" at idle, or you got a problem.

I wouldn't think so. It is the throttle plate that sets the MP and MP is read before the intake valve (otherwise the MP would be all over the place). I've never actually seen a stuck intake valve, but I would imagine it would vibrate nearly as badly as a stuck or broken exhaust valve.

[cdnpilot77]

Hey Northern,

My question or re-asking of another posters question, was not meant to be confrontational. I am not an experienced pilot either with just over 300hrs. I was trying to find out why you would have done your return leg at that altitude where there could have been a potential conflict with IFR traffic. It obviously makes a more sense after expaining it. I was unable to contribute to the technical aspect of the thread but thought we might be able to also cover the neuro aspect of the thread at the same time as it raised a concerning question for obviously more than just me. I apologize if I was incorrect. As you say we are all here to learn and I was wanting to know what your thought process was for that particular instance. I appreciate you sharing your reasoning and hope that you are able to get those snags squared away.

CP77

[angry inch]

There are many areas of western Canada where you can fly vfr at 6000' ASL without contravening the cruising altitudes order.

[Meatloaf]

There are many areas of western Canada where you can fly vfr at 6000' ASL without contravening the cruising altitudes order.

That part is true but the poster is from Fort McMurray. The elevation in that area is about 1300 feet ASL. I have flown IFR through there at 6000 feet before. I also have flown over 1000 hours on a 206 alone. I am positive that the issue the poster is referring to is not a dead mag. He set 20" of power for both legs. By setting the power he will have erased the effect of any mag issues on his airspeed.

[northern33]

cdnpilot, my apolgies for jumping on you. Just trying to figure out a flight that was a real drag.

Meatloaf, I know it's likely a dumb question, but how do you know so quick it's not a dead mag?

[iflyforpie]

Manifold pressure doesn't equal power. The MP gauge on the 206 in the hangar is reading 27 right now.

I haven't tried it, but this is what I would imagine a dead mag would do to your engine readings:

Mag dies, power drops. Prop goes to finer pitch to keep RPM the same. You don't need to touch the throttle since the MP will stay the same as long as RPM is the same (normally aspirated). Fuel flows are the same since you haven't moved the throttle or mixture.

The only things that would be different are your airspeed, and your EGT and CHT.

I wouldn't rule out a dead mag yet, but there are still other things that could cause this problem (blown muffler baffles for example).

Comments?

[square]

You guys are making a mountain out of a molehill here!

Indicated airspeed is called indicated for a reason boys, it's not accurate. The hotter it is outside and the higher your pressure altitude is, the lower your airspeed will indicate. True airspeed will be roughly the same, a little slower than normal as it gets hot out and a little faster than normal in the higher altitudes. I believe ASIs are calibrated to be accurate at 15 degrees at sea level at a certain angle of attack, but if it's 20 degrees at 6500 feet your TRUE airspeed will be much higher than your indicated airspeed. As for a 206 doing 145 mph at 20" and 2500 rpm, you probably had some updrafts I don't think a 520 would go that fast.

Try doing the IAS vs TAS calculations, then the TAS vs GS calculations should make sense. I'm guessing winds were about 10-15, sounds like everything's normal to me. The other weird thing you'll find is that when it's really cold out your indicated airspeed will be higher than your true airspeed.. I used to fly a 206 around in the winter and I'd be 150 KIAS and 150 KT GS on the way out then 150 KIAS and 120 KT GS on the way back, goes the opposite way when it's low & cold.

Oh yeah, and 2400rpm 24" MP for me, runs about 15 GPH with no problems. I say ABOUT 15 GPH because that changes with temperature too, 50 degrees rich of peak is about 15.5 in the winter and 14.5 in the summer.

[square]

Ok I will do some calculations then.. Don't have an E6B but I found one online! Very neat app. http://www.csgnetwork.com/e6bcalc.html

Ok, so assuming it's around 25 degrees on the ground in YMM, that's about 11 degrees above standard temp which puts it to about +15 at 5500 and +13 at 6500.

Density altitudes

5500' pressure altitude at 15 degrees = 6762' DA
6500' pressure altitude at 13 degrees = 7760' DA

True airspeeds

145 mph IAS at 7760' density altitude = 165 mph TAS (for 20" of power you now own the fastest 206 in the world.)
128 mph IAS at 6762' density altitude = 143 mph TAS ( = 129 kts, still a good speed for that power setting.)

Winds aloft

170 mph GS - 165 mph TAS = +5 mph tailwind
128 mph GS - 143 mph TAS = -15 mph tailwind

As for the CHT difference, it could be chalked up to the fact that you had just climbed for about 10 minutes to get to 6500 at lower speeds with less engine cooling, so it's going to take time to cool from about 430 in the climb back down to a cruise temperature. By the time you're turning around again it's cooled down and you start a descent at higher speed with better engine cooling, bringing you down to around the 340 range which is just about what I'd expect with the cowl flaps open at low power, if it was a bit warm out.

I haven't flown a 206 with a 550 engine though so my expectations might be a bit off, but my understanding of them is that they're derated to provide 300 BHP at full power just like a 520-F is, but 1) you're allowed to run full power continuously in them and 2) it will hold that power better over the length of the engine, being the only differences in performance.

I'm guessing you were typically cruising a little slower on the way out with a 15 mph tailwind and that would about sum it up.