Density Altitude Effects on our Plane - Page 2

pscraig · 12-04-2007, 10:40 PM

They've made it much more difficult to properly do this calculation. The correction chart USED to be on a laminated copy of the W&B form in the FO side in the glareshield, but with the new W&B forms a couple of years ago it's gone. Then they got rid of the QRH and its useful charts, including this one. I've suggested to several people that we put that chart back in there, since it is now buried in the volume 1. Not a good place for a chart that should be consulted before every flight. The end result: nobody does the correction anymore. The anti-QRH squad in the training center evidently thinks that crews pull out their volume 1 on every quick turn. Some very enterprising FOs have copied and laminated the chart themselves for quick reference, but it should be more accessible.

VH · 12-05-2007, 11:51 AM

Quote:

Originally Posted by pscraig

Some very enterprising FOs have copied and laminated the chart themselves for quick reference, but it should be more accessible.

Me me me! Guilty.

IHateBAMA · 12-05-2007, 05:06 PM

Quote:

Originally Posted by pscraig

They've made it much more difficult to properly do this calculation. The correction chart USED to be on a laminated copy of the W&B form in the FO side in the glareshield, but with the new W&B forms a couple of years ago it's gone. Then they got rid of the QRH and its useful charts, including this one. I've suggested to several people that we put that chart back in there, since it is now buried in the volume 1. Not a good place for a chart that should be consulted before every flight. The end result: nobody does the correction anymore. The anti-QRH squad in the training center evidently thinks that crews pull out their volume 1 on every quick turn. Some very enterprising FOs have copied and laminated the chart themselves for quick reference, but it should be more accessible.

Remember that referencing this chart is only mandatory when the pressure is less than standard or less than 29.92". If the pressure is greater than the chart is only optional, e.g. squeezing on that jumpseater the legal way. I have mine tabbed for easy access. Don't let your FO's be that lazy, make em use it.

pscraig · 12-05-2007, 08:24 PM

I usually hand them the laminated copy I made as an FO if they don't look it up, but my point is the information should be more easily accessible to us.

Endir1001 · 03-10-2008, 11:22 AM

This is all nice for T/O, but DA also effects our landing distances, and no one has ever been able to tell me why we don't have to correct for DA changes in computing landing distances...

Think about it, if 2 kts fast over the fence adds about 5% to landing roll, and you are at a 6000' MSL strip in August with temps in the high 90's, then you TAS is going to be up there as you cross the fence.

I know most field lengths are more than sufficient to account for any increases in landing roll due to DA, but what about an abnormal situation, we have no way of calculating the new roll out. Unless DX has it buried somewhere, but I doubt it.

Yariv · 03-11-2008, 11:53 AM

Quote:

Originally Posted by Endir1001

This is all nice for T/O, but DA also effects our landing distances, and no one has ever been able to tell me why we don't have to correct for DA changes in computing landing distances...

Think about it, if 2 kts fast over the fence adds about 5% to landing roll, and you are at a 6000' MSL strip in August with temps in the high 90's, then you TAS is going to be up there as you cross the fence.

I know most field lengths are more than sufficient to account for any increases in landing roll due to DA, but what about an abnormal situation, we have no way of calculating the new roll out. Unless DX has it buried somewhere, but I doubt it.

you have a good point. you can calculate density altitude on the FMS landing page 2, and use that instead of pressure altitude. even without doing that, the landing distances in the charts are in increments of 1000' pressure altitude, so if you look at the next higher 1000' increment your covering the increase due to high temp. between those two methods you're getting a safe estimate of landing distance without taking an unnecessary penalty.

Yariv · 03-11-2008, 01:46 PM

you can also use the temp/press correction chart (if you can find it) in REVERSE to reduce the ATIS baro setting due to high temp, increasing pressure altitude (the result will be close to density altitude)

j_spicoli · 03-13-2008, 01:41 AM

my preference for density altitude computation is to determine the field's deviation from standard temp and multiply that by 120'. Add or subtract that to/from pressure altitude and you have an accurate density altitude. In other words, each 1 degree C affects density altitude by 120'. Ex. Field elevation 5,000', temp. 30C, altimeter 30.17.

Pressure alt.= 4750'
standard temp at 5,000' = 5C
30 (current temp) - 5 (standard for the field elev)= 25 (degrees above standard)
25 * 120 = 3000' (this is your correction for non-standard temp)
3000' + 4750' = 7750' (density altitude)

From this you can see that simply going to the next 1000' higher on the pressure altitude for the landing distance chart will cover you for just shy of 9 deg. above standard temp.

kidicarus5897 · 03-13-2008, 03:43 PM

did you also know you can convert knots to MPH in the FMS??? Try and find that page. God I am bored

juice · 03-13-2008, 11:28 PM

Quote:

Originally Posted by ramira18

wowowow step 3 there! I didnt think ya had to do that, you just use what atis throws at ya?

you might be leaving people behind that could have gone, had you done it correctly.... or worse... you could be overweight and not know it:

we were at a high altitude airport with an LR, and it was pretty warm out. luckily the pressure was pretty high, so that gave us a 4 degree "drop" for the performance data, and that made a BIG difference in the climb limit, and we could get everyone on (not 50, but all 46 they had booked) had we not taken the credit for the pressure, we would have had to leave people behind (due to missing this step, would have been restricted to about 42).

other way around. lets say that same airport. climb limit is 47000 at 22C. but lets say the pressure is low, and you have to add 4 degrees, now at 26C the climb limit is 44000. you miss that step. you end up weighing 46990 lbs, so you think all is well. I hope an engine doesn't pop at V1.