Question for instructors

[Check Airman]

Hi,

I'd like to have a debate settled. Some time ago, I was on a pretty short (~80nm) VFR cross country flight in a 2003 172. I had full tanks and the weather was perfect VFR. I saw that my TAS came up about 7kts slower than the speed i had used in my calculations, even though the power was set correctly and the OAT was corrected for. I got that speed from the cruise tables in the manual, of course. I increased power a bit and gained 3 or 4kts. I was happy with this and continued the flight with the new power setting.

When I mentioned that to a couple of my friends last night, they were adamant that I should not have increased the power. They claim I should simply have continued the flight at the lower TAS, and that the move was poor judgement.

What dou u guys think?

[FlyingForFun]

What I would want to know is why was the speed you observed lower than that in the manual? Was it because of a mis-reading ASI? Mis-reading RPM guage? Aircraft overweight? General wear+tear on the aircraft causing a gradual degradation in its performance over its short service life?

How well do you know this aircraft? Does it normally fly at this speed, or did it slow down just for this flight?

I don't think there is a right or wrong answer to the question. The fuel consumption may well have increased by quite some margin when you increased your speed (do all new C172s have a fuel flow guage? Those I've flown all do) but you had plenty of fuel to cover this increase.

FFF
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[Check Airman]

Originally Posted by FlyingForFun

What I would want to know is why was the speed you observed lower than that in the manual? Was it because of a mis-reading ASI? Mis-reading RPM guage? Aircraft overweight? General wear+tear on the aircraft causing a gradual degradation in its performance over its short service life?

How well do you know this aircraft? Does it normally fly at this speed, or did it slow down just for this flight?

I don't think there is a right or wrong answer to the question. The fuel consumption may well have increased by quite some margin when you increased your speed (do all new C172s have a fuel flow guage? Those I've flown all do) but you had plenty of fuel to cover this increase.

FFF
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I was quite familiar with the plane and the gauges were all read properly. Everything else was "normal" (weight etc.). The plane had fuel flow gauges.

Typically, I select a power setting and altitude from the table, so there's no one speed i usually get on my cross country flights.

My friends were quite passionate about their opinion that it was wrong to do this (we spent about an hour arguing it...I'm sure u can just imagine 3 pilots arguing on the phone). What do u think? Was I indeed wrong? Is there any reason i should NOT have taken this action?

[hugh flung_dung]

You planned a certain TAS so it's reasonable to set the power you need to get that TAS, providing you still have sufficient fuel reserve and the engine is operating within limits - if not you need to adjust your ETAs and headings (stating the obvious).
It's rare for aircraft to achieve flight manual performance when they're middle-aged (- humans are the same :-) ), one that's only 6 years old should be better. Out of balance? T&S not correct in panel. Something out of trim?

HFD

[Dude~]

The removal of spats usually costs about 7kts, that could explain it but you'd probably have noticed during the preflight.

[Check Airman]

Thanks...I know my judgement is still sound. The plane was pretty new, only 2-3 yrs old, so the numbers in the manual are still fairly reasonable.

What are spats? Are those the speed fairings on the wheels? If so, when removed, the manual says there's a 2kt penalty, which I had corrected for.

[Dude~]

Yes I meant spats as in wheel fairings. I'm surprised they only save 2 kts as on the PA28 I fly you lose 7 kts with them off.

[Cobalt]

How did you obtain your TAS? Whizz-wheel? Or does your ASI have a sub-scale for temperature and altitude so you can read it of directly?

Did you use the exact power AND MIXTURE setting indicated by the manual? For one aircraft I fly regularly, the airspeed and fuel flow data is for leaning peak EGT, there is a second data set for leaning 100F rich EGT, which gives you 10% higher fuel flow and 7 kts higer airspeed...

And of course the flaps were fully retracted after take-off...

C.

[DFC]

Are you comparing the altitude you plan to fly at (based on QNH) to the "Altitude" in the cruise tables which is based on 1013 / 29.92?

Also have you allowed for a non-standard temperature at that level?

If the actual conditions are different from standard atmosphere there will be a difference in planned and actual TAS.

As to what to do for range?

Actually, that really depends more on your groundspeed/fuelflow relationship thatn TAS/fuelflow relationship.

Regards,

DFC

[Check Airman]

Originally Posted by Cobalt

How did you obtain your TAS? Whizz-wheel? Or does your ASI have a sub-scale for temperature and altitude so you can read it of directly?

Did you use the exact power AND MIXTURE setting indicated by the manual? For one aircraft I fly regularly, the airspeed and fuel flow data is for leaning peak EGT, there is a second data set for leaning 100F rich EGT, which gives you 10% higher fuel flow and 7 kts higer airspeed...

And of course the flaps were fully retracted after take-off...

C.

The ASI has a subscale.

The engine was leaned as recommended in the POH.

[Check Airman]

Originally Posted by DFC

Are you comparing the altitude you plan to fly at (based on QNH) to the "Altitude" in the cruise tables which is based on 1013 / 29.92?

Also have you allowed for a non-standard temperature at that level?

If the actual conditions are different from standard atmosphere there will be a difference in planned and actual TAS.

As to what to do for range?

Actually, that really depends more on your groundspeed/fuelflow relationship thatn TAS/fuelflow relationship.

Regards,

DFC

I'm not sure what u're asking about the altitude, but the manual covers temperature deviations, which were factored.

[foxmoth]

At the end of the day I thought the question was not "why was my speed wrong?", but "Was I wrong to increase speed?" - IMHO I would say that as long as you allowed for the higher fuel burn then there is no reason not to do this if you want to - if not then your drift/time calculations will be wrong (though to be honest, not enough to really be significant).
I would be interested to know their arguments for not increasing speed.

[B200Drvr]

I agree with Foxmoth, First of all your ability to asses and correct the situation shows good judgement, I would like to know from your friends how they think pilots operate on schedules. I know this sounds extreme but what we learn on the journey is what we will apply at our destination. If I depart a point and am expected/have a slot time at a destination, I dont just leave it up to the Gods to get me there on time, I do what is required within the bounds of safe operation/POH/SOP's etc to stick to the schedule. I think THEY show poor judgement, not having the ability to adapt to a changing situation.
Good on you.

[FlyingForFun]

At the end of the day I thought the question was not "why was my speed wrong?", but "Was I wrong to increase speed?"

That's very true, Foxmoth, but the point I was trying to make in asking the question "why was my speed wrong" is that the answer to the second question almost certainly depends on the answer to the first.

I'm still not quite sure what the "correct" answer to the original question is. I can think of a few scenarios (i.e. a few answers to the question of why the airspeed was wrong) with definite answers. For example, if the aircraft is known to have an RPM guage which misreads slightly, then there is no doubt at all that it would be appropriate to increase the power to get the desired speed. On the other hand, if the aircraft was a little older and had simply become less efficient with time, then to increase the power above the POH manual would quite likely result in a disproportionate increase in fuel consumption - most likely not a major issue if Check Airman had full tanks of fuel for a short trip, but probably not the most appropriate course of action to use on a regular basis.

Without knowing what the reason for the reduced speed, all I can say with absolute certainty is that maintaining the slower speed would be the conservative option, but increasing the speed to the planned speed is unlikely to cause any problems if you can take the increased fuel consumption, and saves the small hastle of altering your ETAs on your plan. Personally, if I have an unexplained problem I'd probably go for the conservative option, but the problem as Check Airman describes it is so minor that being conservative is possibly over-cautious.

FFF
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[DFC]

Originally Posted by Check Airman

I'm not sure what u're asking about the altitude, but the manual covers temperature deviations, which were factored.

The Altitudes quoted in your POH are altitudes in the standard atmosphere. That means the height is referenced to 29.92 / 1013.2

If your altimeter was not set to 29.92 than the altitude you are flying at may not be the same as the altitude in the POH.

From a practical sense, unless you are flying with 29.92 on the altimeter, you have to make allowance for the difference between the Altimeter setting you are using and 29.92 to see how far you are away from the level you planned for.

What was the Altimeter setting you used and the OAT if you can remember?

regards,

DFC

[Check Airman]

Originally Posted by DFC

The Altitudes quoted in your POH are altitudes in the standard atmosphere. That means the height is referenced to 29.92 / 1013.2

If your altimeter was not set to 29.92 than the altitude you are flying at may not be the same as the altitude in the POH.

From a practical sense, unless you are flying with 29.92 on the altimeter, you have to make allowance for the difference between the Altimeter setting you are using and 29.92 to see how far you are away from the level you planned for.

What was the Altimeter setting you used and the OAT if you can remember?

regards,

DFC

1. The question was "was I wrong" not "why was the speed off".

2. The altimeter setting wasn't too far form standard, so i doubt that this may have been a factor (+/- 0.03 inches)

3. the plane was pretty new (~2 yrs old)

[Check Airman]

Originally Posted by foxmoth

I would be interested to know their arguments for not increasing speed.

They climed that a Cessna is build to crz at a partucular RPM (75%, or whatever RPM the owner stipulates), and increasing power above the planned crz setting puts undue stress on the engine, thus higher maintenance costs, increased risk of failure etc...

[Check Airman]

Originally Posted by B200Drvr

I think THEY show poor judgement, not having the ability to adapt to a changing situation.

I told them I thought this was pretty obvious...you make a flightplan, every effort should be made to stick to it, within the bounds of safe operations. Otherwise, the navlog is just anothe bit of paper that could have been left on the ground.

[foxmoth]

They climed that a Cessna is build to crz at a partucular RPM (75%, or whatever RPM the owner stipulates), and increasing power above the planned crz setting puts undue stress on the engine, thus higher maintenance costs, increased risk of failure etc...

Others may disagree, but as long as the engine is operated inside the limits set in the POH I would say that is a load of codswallop. Often you operate at an RPM lower than max, but this is mainly due to the way the fuel consumption goes up, also of course, if you operate at your max RPM it only needs a slight descent or updraft before you exceed it so a good idea to have a bit of a buffer.

[Check Airman]

Originally Posted by foxmoth

Others may disagree, but as long as the engine is operated inside the limits set in the POH I would say that is a load of codswallop. Often you operate at an RPM lower than max, but this is mainly due to the way the fuel consumption goes up, also of course, if you operate at your max RPM it only needs a slight descent or updraft before you exceed it so a good idea to have a bit of a buffer.

That's my stance...the band wouldn't be green if causes damage, and i'm not talking about operating at the very edge of the green. I keep all my needles well inside the green band.