Lack of density alitutude in Cessna POH.

[DanWEC]

I'm sure 99% of us are familiar with the Cessna takeoff performance chart, but I have a question- why doesn't the Pressure Altitude column factor Density Altitude instead? It's a simple extra step for the pilot to calculate DA or use a Koch chart, but is there a reason why it's left like that?

Seems like an absurdly silly question, but as I finish up the Class IV rating, I am discovering things I don't know the proper answer to, and could be asked by students.

Cheers,

Dan

[Shiny Side Up]

It's a simple extra step for the pilot to calculate DA or use a Koch chart, but is there a reason why it's left like that?

Because Cessna is covering their asses. They know that most pilots @#$! up something like a density altitude calculation.

[Meatservo]

Temperature is a variable in the chart, so that's how they take into account density. Check out density altitude and you'll see that your "density altitude" is the altitude you would have to be at in the "standard atmosphere" to experience the same air density you are seeing in real life at your actual altitude. The variables are pressure altitude and temperature. Since temperature is accounted for in the chart, then you only need pressure altitude. You don't need to account for temperature twice.

At least that's what I choose to believe. I'd probably @#$! up a density altitude calculation.

[DanWEC]

Temp is a mention in the Time, Fuel, Distance to Climb chart, and accounted for in a Note on the chart to increase them all by 10% for 10* over standard temp, but I was referring to the Takeoff Distance chart. My mistake- I should have been more clear on which chart, sorry.

[iflyforpie]

Seems like an absurdly silly question, but as I finish up the Class IV rating, I am discovering things I don't know the proper answer to, and could be asked by students.

The thing is, there will probably always be things that you won't know. I'm willing to bet that even Cat Driver or Colonel Sanders come up with that very occasionally.

As an instructor, you are not simply an information dispensary. Use some of that developmental teaching and get the student to figure out why they did it like that.

As to your question, I am not entirely remembering what the takeoff distance chart looks like, but I believe it gives pressure altitude from sea level and up on the vertical axis, and temperature horizontally in ten degree increments on the horizontal axis with takeoff run and 50 foot obstacle data below each. This is your performance compensated for density altitude even though it doesn't say it explicitly.

[DanWEC]

IFP, sounds like a Piper chart that you are recalling. I've found them to br more exact. Cessna charts are usually columns and interpolation. No mention of temperature on it.
You're completely right about the teaching aspect. Part of teaching is also the intrinsic benefit of grasping the material itself so much more when you teach it as opposed to just initially learning it. I'm enjoying really absorbing everything I can.

Cheers.

[iflyforpie]

What vintage of Cessna? It changes quite a bit from the care free performance suggestions of the early 60s to the liability laden health and safety aircraft of the late 70s. My '66 Skymaster manual is half the thickness of my '76 172 manual. My 206 manual doesn't give glide speed or distance or demonstrated crosswind.

I know for sure that the mid to late 70s at least have temperature listed. You are right, the Piper charts are much better... more like a high performance turbine aircraft.

[Colonel Sanders]

I am guessing Dan is referring to something like this:



In his shoes, I would simply phone up Cessna and ask.

Of course, there's no one still at Cessna who wrote the
POH - they're all long since retired. I suspect the answer
Cessna might give you is that they thought it was probably
the easiest way for a pilot to calculate the takeoff distance.

Contrary to what you might have heard - Cessna doesn't
actually hate pilots and owners. Don't confuse them with TC.

[Meatservo]

That's the chart I had in mind there. See, there's different columns for different temperatures. This is how density is accounted for.

[Shiny Side Up]

Contrary to what you might have heard - Cessna doesn't
actually hate pilots and owners. Don't confuse them with TC.

True, but they certainly don't have a high opinion of pilot and owner ability. I'm pretty sure they weren't too fond of them either when they decided to stop making little airplanes in the eighties either.

[x-wind]

Good question, I remember thinking about it while I was learning.

The question is: how do you determine density altitude to enter the chart with?

Answer: It requires a circular slide ruler, graph or formula & calculator; as Cessna probably considers it, an extra tool.

A Cessna comes with a altimeter & OAT so your all set for the chart and that's the purpose behind the design I figure.

With that said, they could have a density altitude chart in the POH and then cut down on the size of the POH because, as I recall, all the performance information is laid out In this long winded fashion and it eats up quite a few pages especially in the cruise flight figures. It would make interpolation easier.

No absurdly silly questions ever. Learning those learning factors should make you aware that the learning process is often precarious, encourage the questions always and remember that graph in the FIG about reviewing before it becomes long term memory.

[Rowdy Burns]

Nothing really to add to the discussion other than the attached file.

RB