Different scales on E6B for PALT and Temp
Posted by vintageripstik@reddit | flying | View on Reddit | 10 comments
I am struggling to understand what the two different pressure altitude and temperature windows are for on a mechanical e6b. I can confirm that pressure altitude equals density altitude at 15C on both windows, so it stands that the two should give a consistent reading.
However, when I try to apply a practice problem of finding density altitude I het different answers depending on the window I use.
Given pressure altitude 933 ft
temperature 7°C
The right window is correct, the left window is wrong by many thousands of feet. I can't wrap my head around why the scales can give a consistent value at standard temperature but do not agree at non-standard temperatures. My assumption is that the left window with pressure altitudes between 0 and 3000 would be more useful for calculating your density altitude while on the ground, where the right window with pressure altitudes going up to 50,000 ft would be more appropriate for computing density altitude while in flight. But that doesn't seem to be the case. What am I missing here? Question
Avaricio@reddit
Well, you're at 9000ft in the last pic, not 900 for starters. You also don't use that scale for density alt, it's for converting true to calibrated alt.
vintageripstik@reddit (OP)
Ok that is fair ...🤦🏻♂️
Fixing that, the two still disagree. Maybe it's not worth understanding why the two scales differ, I suppose they are just representing two different formulas that have similar starting points for DALT=PALT at 15C
Avaricio@reddit
They aren't meant to agree, they're computing different things. The left window is for computing true altitude above MSL, correcting for temperature because non-standard temp means non-standard pressure variation with altitude. You don't read the density altitude window at all with that one. They both agree at 15C and 0ft because that's the ISA reference temperature at sea level.
vintageripstik@reddit (OP)
Yeah that makes sense now. The scales are just encoding different functions. Fair enough, thank you!
jet-setting@reddit
You’re not looking at the scales correctly. The pictures you show are not lined up on the numbers you’re trying to set.
vintageripstik@reddit (OP)
Thank you I see that I was off there
However, even if I set the values accurately on the left hand window I get a DALT of ~ -1,000 which is still quite inaccurate. It just seems that the two scales are different for a reason, because they are used to make computations for airspeed and altitude on the outer scales.
Ok_Witness179@reddit
You got it lined up for about 9000 feet PA on the third image there.
vintageripstik@reddit (OP)
I agree, but when I use the right window the temp and PALT on the left hand window still do not align
MeadyOker@reddit
This might help:
https://youtu.be/_UU1XeWwNxw?si=iwZo9NgK6h28faXV
rFlyingTower@reddit
This is a copy of the original post body for posterity:
I am struggling to understand what the two different pressure altitude and temperature windows are for on a mechanical e6b. I can confirm that pressure altitude equals density altitude at 15C on both windows, so it stands that the two should give a consistent reading.
However, when I try to apply a practice problem of finding density altitude I het different answers depending on the window I use.
Given pressure altitude 933 ft
temperature 7°C
The right window is correct, the left window is wrong by many thousands of feet. I can't wrap my head around why the scales can give a consistent value at standard temperature but do not agree at non-standard temperatures. My assumption is that the left window with pressure altitudes between 0 and 3000 would be more useful for calculating your density altitude while on the ground, where the right window with pressure altitudes going up to 50,000 ft would be more appropriate for computing density altitude while in flight. But that doesn't seem to be the case. What am I missing here? Question
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