Extrapolation of graphs

[JeroenC]

Hi all,

i am working on the flightplanning for my USA trip. The plane will be a PA-28-140 160 HP Cherokee.
Now, in some places i will encouter a density altitude >>>> 7000 ft.

The POH graphs don't go above 7000ft and state "extrapolation of chart above 7000 ft is invalid".

Of course the service ceiling is also way above 7000 ft, so what does this mean? How can I make a flightplanning in this way?

The strange thing is, that some graphs do go up to 14,000 ft., e.g.: Climb performance, engine performance, cruise performance, glide performance.

It basically is only the take-off and landing which goes up to 7000 ft.

Thanks in advance.

[Floppy Link]

Max operating alt 14000
max alt for t/o and landing 7000

?

[JeroenC]

Is there such a thing as a "maximum certifeid landing/TO altitude"?

[Floppy Link]

certainly there is for bigger aircraft. The first 2 limitations in the 767 manual state:

Max operating pressure altitude: 43,100ft

Max take-off and landing pressure altitude: 8,400ft

which is why I jumped in there, maybe smaller aircraft are the same, need to check the Manufacturers Aircraft Flight Manual

:ok:

[Gertrude the Wombat]

Extrapolation of graphs is always invalid. And as Feynman pointed out you're usually better off ignoring the last point on the graph as well - if the last point was any good they'd have been able to make further measurements, so the last point is probably lousy.

[djpil]

The POH graphs don't go above 7000ft and state "extrapolation of chart above 7000 ft is invalid". ...How can I make a flightplanning in this way?

There's your answer - don't plan on going to an airfield where the density altitude will exceed 7,000 ft. I've spent a lot of time flying in the Rocky Mountains and the PA28-140 (has it really got 160 hp?) would not be my choice. You'll find it frustrating at least, dangerous at worst at a high altitude take-off and mountains to avoid after take-off, or a go-round.

[RodgerF]

Quote:

I've spent a lot of time flying in the Rocky Mountains and the PA28-140 (has it really got 160 hp?)


It is possible to fit a 320 (160Hp) engine in a PA28-140, there are examples on the US register.


However when I was mountain flying in Switzerland the instructors would not use anything less powerful than a PA28-180.

[Mark 1]

I've had the same problem flying PA28s in S.Africa with 5400' elevation and 30 degrees OAT (ISA+25, work it out).
Most of the parameters affected by air density are smooth functions, so a moderate extrapolation would probably be accurate.

But as has been said, this isoutside the range of the factory test data, so you become the test pilot and, strictly speaking, your CofA is invalid (it usually says that a flight manual forms part of the certificate).

Obviously a lot of people work in the 'grey' bounds of the graphs. I'll leave the choice to you.

[JeroenC]

Thank you all for your replies.

Yes, it does have 160 HP.

Basically what is mean is:

Nowhere in the POH does it state it max. certified landing alt.
So does the "extrapolation is invalid" implicitly mean a max. alt., or does it merely state: there is no data for that alt.?

I personally think that a max. density alt. of 7000 ft is low, the plane surely must be able to do higher?

It's like the demonstrated xwind. It is not illegal to fly above the dem. xwind, just very stupid.

Thanks again.

[IO540]

"Extrapolation of graphs is always invalid"

That may be legally true, but practially it is true only if one has no idea of the underlying physics.

[djpil]

I personally think that a max. density alt. of 7000 ft is low, the plane surely must be able to do higher?
JeroenC, I don't know exactly where you'll be flying but consider Jackson Hole, Wyoming. 6500 ft elevation and 6300 ft long. Density height, say, 8500 ft. In a Cessna 172 for a local flight. The POH showed that the runway length was more than adequate and that the rate of climb was OK - I was not extrapolating in this case (and I am, or was, a flight performance engineer). I'd flown the aeroplane several times recently, slightly lower weight though. Light wind. After take-off the modest climb rate became a steady descent when a slight wind shear occured due local terrain - a little canyon was funneling the wind. I was within seconds of setting up for a landing in a field. Eventually got it up to 11,000 ft - the minimum needed to go somewhere.
I much preferred the Husky - it'd go straight to 12,000 ft with no doubts. In that part of the world 12,000 ft is normally where I'd cruise, with the occasional step up to 14,000 to clear a ridge.

[RichyRich]

Extrapolation of graphs is always invalid. And as Feynman pointed out you're usually better off ignoring the last point on the graph as well - if the last point was any good they'd have been able to make further measurements, so the last point is probably lousy.

So, if we ignore the last point, that makes the second-to-last point the real last point, and we start again. Eventually we have no graph :D

Out of interest, do you have a reference to your Feynman 'quote'? Thanks. I once started to plough through his 'Lectures in Physics'. Probably do so again sometime in the future. Never read anything else but I believe that he's quite a good novelist.

[slim_slag]

People extrapolate all the time, high airports in the Rockies would be very quiet in the summer if they didn't. Experienced people tell the old joke about being a 'test pilot' and then go off and safely fly.

Note the word experienced !

So find somebody who knows what he/she is talking about and get some dual. Best people are the crusty old freight dogs and charter pilots who hang out at mountain airports. Lowland instructors will do at a pinch.

Tend to agree with the Swiss instructors who want 180HP, more is better, and not much more expensive in the US.

Take a look at the AIM 7-5-5 and 7-5-6 (http://www.faa.gov/ATpubs/AIM/Chap7/aim0705.html#7-5-5)

[18greens]

Listen to these people and get an instructor to take you into some of the higher airports then you will understand.

I took off in a 172 at DA 7000' ish. I checked the books , did the sums, didn't beleive it was going to take 5000' of the 9000' to reach rotate speed but it did and then climbed at 150fpm. On another trip I took off from sandiego to go to Little bear (the one at 6200'). The plane was so tired it barely made 400fpm at SD so I scrubbed the flight and went up thecoast instead. I doubt it would have ever reached rorate speed at 6200'.

If you are going to extrapolate be sure of your capabilities. Its challenging work flying up there but the views are great. Its an experience I remember frequently.

[Gertrude the Wombat]

Out of interest, do you have a reference to your Feynman 'quote'? It's in one of the "autobiographies", I think "Surely you're joking?".

[englishal]

And if in the SW of the USA, you could always take off at the crack of dawn to help with the old DA.

Myself, a 180HP PA28, 2 up at say Big Bear (6700') is pushing it in the summer - in fact I wouldn't go unless at the crack of dawn or dusk.

Now stick a turbo on it, and its a different matter....:D

[JeroenC]

Thanks for all the answers.

The plane i'm gonna fly has a 160 HP engine. The general POH i have for the model is for the 150 HP model.
The take-off perfromace graph sohw a straight line, e.g. linear relation bewtween distance and DA. Therefore, I am assuming a can make a extrapolation.
So, at DA of 9,000 it will use 1750 ft of rwy with paved, level, dry rwy with no wind. I am assuming I won't take-off with a tailwind.

This is of course for a new machine. So lets double this figure: 3500 ft. By the way, it is exactly for the weigt I have planned to take.

So i have a couple of places with doubts:
Sedona, AZ; 5132 ft rwy, expected DA: 7500 ft
Monument Valley, 4000 ft, DA: 8700
Jackson Hole, WY, 6300, DA: 9000
Big Bear: 5850, DA: 11,000!!!!

Big bear in the summer is indeed a little over limits, i see now ;)

I don't know about these. It is above 3500 ft, but it's close. Any ideas?

Grand Canyon will ahve 9000 DA as well, but also 9000 ft rwy, so should be OK.

And I have a stunning 10 HP extra in respect to the POH.

Climb performace will degrade to 200 ft/min @ DA 9,000, again, for a new plane.

Can you please continue to give me some valuable input?

Thanks in advance,

Jeroen

P.S.: I checked www.weatherunderground.com for average temperatures the ;ast years, to calculate DA. It showed me also that going at the crack of dawn onlyreduced the DA between 500 and 1,000 ft. Will that really make that much of difference?

[Dale Harris]

Well, it all boils down to this I guess....... The chart says "Not to be extrapolated". You decide to extrapolate. Fair enuf, your decision. Ask yourself this tho, if you f*ck it up, and survive, who they gonna come after????? Don't get me wrong, I'm all for people making their own INFORMED decisions, as long as they are prepared to wear the consequences of THEIR OWN actions. Especially in the old US of A. It is, after all, your decision.

[Flyin'Dutch']

Jeroen,

I have about as many hours in my logbook flying in the mountains as you.

From your postings I gather that yours is further a bit thinner than mine and a lot of others on these shores.

There is absolutely nothing wrong with that or the wish to go and have a good time and at the same time expanding one's experience envelope.

I do have to share with you though that the combination of things that you are willing to undertake seems pretty ambitious and that I am concerned that you are overstretching yourself a bit.

So that is of my chest.

On a different note I think that there may well be something wrong with your take off calculations as for my Maule with the 235hp Lycoming the take off would require 1450ft in no wind conditions at Big Bear with an ambient temperature of 60degrees F. It is generally well accepted that a Maule has a better take off profile than a Cherokee and would be surprised if you would only 300ft more than the Maule.

Go and fly with an experienced instructor over there and have fun, make sure it does not end in tears. Those mountains have propensities that flatlanders like you and me know very little about.

Frank