Here's a really dumb question

HOTDOG

HOTDOG

New Member
OK, this probably a really stupid question...but I'll ask anyway:

Pressure Altitude is altitude corrected for non-standard pressure.
Density altitude is pressure alt. corrected for non-standard temp.

So, I'm trying to calculate my time, fuel, dist. to climb. but all altitudes on that chart are listed as pressure alt.

If my airport is say 1000 MSL and I want to climb to 8000 MSL. Do I just use my airport altitude as the starting point or do I have to figure out the pressure altitude at my airport for that day as the starting point?

I'm so confused....Someone please help.
 
note that the charts colums are pressure altitude and a temeprature (10*, 20* etc)

Temp + PA = DA

:)

you should calculate your PA for that day to give you the starting point.
 
I take the local altimeter value and find the coresponding value (in feet)that either needs to be added or subtracted to the airport elevation to find the PA for that day. That PA value used in conjunction with the temp will give me the DA. I hope I'm doing this right :)
 
Interpolation my good friend.....You can also use your e6b to do the pressure/density conversion.
 
L00king4traffic said:
I take the local altimeter value and find the coresponding value (in feet)that either needs to be added or subtracted to the airport elevation to find the PA for that day. That PA value used in conjunction with the temp will give me the DA. I hope I'm doing this right :)
Click to expand...

So, if the altimeter value is 3037 and the airport elevation is 1000, what calculation do you have to do to determine the PA?
 
Man, doing those calculations...all that preparation to take the ole 172 out for a 3-pointer...those were some fun days! :rolleyes:
 
HOTDOG said:
So, if the altimeter value is 3037 and the airport elevation is 1000, what calculation do you have to do to determine the PA?
Click to expand...

Generally, each 0.01 of altimeter difference equates to 10 feet of PA. SO--for a higher altimeter setting, you subtract from the field elevation, or for lower than standard you add. Here's your example:

30.37 Alt Setting
- 29.92 std day
--------------------
.45 difference x 1000 = 450 ft DECREASE (since pressure higher than standard)

1000 - 450 = 550 ft PA

This is not ABSOLUTELY exact, but it will be close enough for any chart you care to work.
 
Yeah, interpolation is accurate enough. Negligible difference.

But look at those tables... a difference in 10 degrees C is only a minuscule change in the number you seek. Like I tell my students when it comes to compass correction card... Can you hold 1 degree correction on this DG?
 
Hey Kristie, you in Scottsdale? I just moved to Glendale to instruct. Where can i get a sweet JC shirt?
 
Fly4Pay said:
Generally, each 0.01 of altimeter difference equates to 10 feet of PA. SO--for a higher altimeter setting, you subtract from the field elevation, or for lower than standard you add. Here's your example:

30.37 Alt Setting
- 29.92 std day
--------------------
.45 difference x 1000 = 450 ft DECREASE (since pressure higher than standard)

1000 - 450 = 550 ft PA

This is not ABSOLUTELY exact, but it will be close enough for any chart you care to work.
Click to expand...

Thanks! :)
 
Fly4Pay said:
Generally, each 0.01 of altimeter difference equates to 10 feet of PA. SO--for a higher altimeter setting, you subtract from the field elevation, or for lower than standard you add. Here's your example:

30.37 Alt Setting
- 29.92 std day
--------------------
.45 difference x 1000 = 450 ft DECREASE (since pressure higher than standard)

1000 - 450 = 550 ft PA

This is not ABSOLUTELY exact, but it will be close enough for any chart you care to work.
Click to expand...

:yeahthat:

There's also a DA chart which you could use to come up with very similar (close) PA/ DA values. That could need some interpolation though. I don't see that chart on the POH, however, it's on the Jepp book for Private Pilot (Chap 8 Sec A pg 9). Using your example above, the value I came up with using the chart was 588 ft PA (i.e. 1000-412).
 
Hotdog, I take it that you are in your primary training and are beginning to work on cross countries. Given that, I would start with asking your CFI just how much detail he wants you to go into with that table.

Notice that, in addition to being in pressure altitude, the table does not take into consideration desnity altitude (which, to be accurate, you would have to calculate for each of the altitudes), winds (as they maight change during the climb), the fact that you may have to maneuver in the traffic pattern, the point at which you choose to lower the nose and decrease your climb for better visibility, etc, and the usual cross country calculations don't suggest you try to plan for those.

Bottom line, no matter =how= tightly you calculate it, this calculation, far more than other cross country calcs, will end up as an educated guesstimate.

The result is different CFIs approach this issue in different ways. My CFI simply told me to forget about it and add 1 minute for each 1000' of climb to my first leg calculation (ended up being incredibly accurate). I have my students use the PA numbers based on the airport elevation without correction and tell them that's it's really just a training exercise (for all the reasons I mentioned). But I know that others have completely different approaches, which is why I suggest you ask yours.
 
MidlifeFlyer said:
Hotdog, I take it that you are in your primary training and are beginning to work on cross countries. Given that, I would start with asking your CFI just how much detail he wants you to go into with that table.

Notice that, in addition to being in pressure altitude, the table does not take into consideration desnity altitude (which, to be accurate, you would have to calculate for each of the altitudes), winds (as they maight change during the climb), the fact that you may have to maneuver in the traffic pattern, the point at which you choose to lower the nose and decrease your climb for better visibility, etc, and the usual cross country calculations don't suggest you try to plan for those.

Bottom line, no matter =how= tightly you calculate it, this calculation, far more than other cross country calcs, will end up as an educated guesstimate.

The result is different CFIs approach this issue in different ways. My CFI simply told me to forget about it and add 1 minute for each 1000' of climb to my first leg calculation (ended up being incredibly accurate). I have my students use the PA numbers based on the airport elevation without correction and tell them that's it's really just a training exercise (for all the reasons I mentioned). But I know that others have completely different approaches, which is why I suggest you ask yours.
Click to expand...

Thanks midlife...I'm getting ready for my commercial written exam and I've always done those calculations as you tell your students, just using the PA based on airport elevation....but, as I was studying, I'm thinking I needed to get more precise with these "guesstimates" as a commercial pilot. I will definitely check with my CFI. Thanks for your advice.
 
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