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Planning in progress for 2nd Annual Tom Allensworth Memorial Flight.  Seeking volunteers to help plan and coordinate and calling all simmers around the world to join in on a fly-in event with your fellow community members.  Please head over to this forum post for more details.


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110 replies to this topic

#31 aentwis

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Posted 03 April 2009 - 04:14 AM

Bryan, you beat me to it... I would also be interested in this information...And once again, Bryan, a BIG thankyou for doing this, and secondly for making it commonly available...Andrew
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Andrew Entwistle

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#32 claviermichel

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Posted 03 April 2009 - 05:00 AM

Hi There,I am getting old but if my memory is still intact the formulas should be:Pa = Alt + 145442.2*(1-(P/29.92126)^0.190261) Pa= Pressure Altitude, a=Altitude, P=pressureDa = ((Pa + 288.5-0.0019812*Pa)/0.0019812)*(1-((288.15-0.0019812*Pa)/(T + 273.15))^0.234969) Da=Density Altitude, Pa=Pressure Altitude, T=TemperatureWould be nice if someone could double check the accuracy of these formulas as I am not home right now to check my old flight school books.Cheers
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Michael A. R. Clavier

#33 CptSpeaking

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Posted 03 April 2009 - 12:22 PM

Okay here's a quick summary...The altitude you read on the guage when it's set to the current altimeter setting is INDICATED altitude. PRESSURE altitude is what the instrument reads when it is set to standard pressure (29.92/1013.2). To make the conversion if the pressure is not standard, you have to do a little math. Remember that 1" of pressure is 1000 feet. If the indicated altitude is 300 and the baro is 30.02, simply subtract 29.92, getting .1. So, .1" is equal to 100 feet. Subtract that from your indicated altitude to get a pressure altitude of 200 ft.Converting PRESSURE altitude to DENSITY altitude requires a bit more math, and honestly, I've only ever done it with a flight computer (E6B or CR3). Basically, you're correcting pressure altitude for non-standard temperature, so if the temp is higher than standard, your density altitude goes up and performance goes down. If the temp is less than ISA, density altitude decreases and performance increases.Important to remember is that density and density altitude are different. In other words, higher density is a lower density altitude. It's all in the terms...Here is a great little Density altitude calculator...http://wahiduddin.ne.../calc_da_em.htmAnd also, to augment my crude explanations, Wiki!!! :)http://en.wikipedia....ensity_altitudeMost performance charts I've seen are in Pressure altitude, not density...thats the only reason why I asked the question, as it makes a significant difference...
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Jonathan Bullock
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#34 Saturn_V

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Posted 03 April 2009 - 01:03 PM

Pa = Alt + 145442.2*(1-(P/29.92126)^0.190261) Pa= Pressure Altitude, a=Altitude, P=pressureDa = ((Pa + 288.5-0.0019812*Pa)/0.0019812)*(1-((288.15-0.0019812*Pa)/(T + 273.15))^0.234969) Da=Density Altitude, Pa=Pressure Altitude, T=Temperature

The altitude you read on the guage when it's set to the current altimeter setting is INDICATED altitude. PRESSURE altitude is what the instrument reads when it is set to standard pressure (29.92/1013.2). To make the conversion if the pressure is not standard, you have to do a little math. Remember that 1" of pressure is 1000 feet. If the indicated altitude is 300 and the baro is 30.02, simply subtract 29.92, getting .1. So, .1" is equal to 100 feet. Subtract that from your indicated altitude to get a pressure altitude of 200 ft.Converting PRESSURE altitude to DENSITY altitude requires a bit more math, and honestly, I've only ever done it with a flight computer (E6B or CR3). Basically, you're correcting pressure altitude for non-standard temperature, so if the temp is higher than standard, your density altitude goes up and performance goes down. If the temp is less than ISA, density altitude decreases and performance increases.Important to remember is that density and density altitude are different. In other words, higher density is a lower density altitude. It's all in the terms...Most performance charts I've seen are in Pressure altitude, not density...thats the only reason why I asked the question, as it makes a significant difference...

Thank you both! I feel like I
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Bryan Schleipen

#35 aentwis

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Posted 03 April 2009 - 01:05 PM

Cheers all of you... :)No doubt I could throw together a little excel file... You put in Pa and T and get out Da...But that little Da calculator will also do the trick... Cheers Jonathan! :)Andrew
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Andrew Entwistle

#36 Houghton11

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Posted 03 April 2009 - 04:31 PM

Just pull out your good 'ol E6-B and everything you need is right there!Patrick
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#37 yahavl

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Posted 03 April 2009 - 08:53 PM

Ok, I would leave mathematics to flight engineers (;You can just use the rule of thumb:118 feet increase in altitude for every 1C increase of temperature.10 feet increase in altitude for every tenth of inch of pressure decrease.So for example: 1 Rwy elv.2000ft 2 Baro 30.023 Oat 18C1=+17C2= -1C3= (18-ISA) +3CAdd them all and you get +19C. Add ISA and you get +34C That
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Yahav Lipinski

#38 Saturn_V

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Posted 04 April 2009 - 03:17 AM

[quote name='Houghton11' post='1565322' date='Apr 3 2009, 09:31 PM']Just pull out your good 'ol E6-B and everything you need is right there!Patrick[/quote]For a moment I really wondered what Looking Glass had to do with it but then I got it :(. Cool looking gadget, where to get one!?![quote name='yahavl' post='1565387' date='Apr 4 2009, 01:53 AM']You can just use the rule of thumb:
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Bryan Schleipen

#39 Cuddly1956

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Posted 04 April 2009 - 08:14 AM

Excellent and hard work there... and loads of patience :( I'm going to try a V1 cut at lower ends of runway lengths to see if it works out ok. Can't see why not, brakes are pretty good lol.I admire your dedication there.John E
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#40 downscc

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Posted 04 April 2009 - 03:30 PM

<<Cool looking gadget, where to get one!?>> You can get a E6-B at any pilot shop, they might allow you to use fancy calculators but they shouldn't IMHO. The best pilot shop is Sporty's: www.sportys.com
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#41 Saturn_V

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Posted 04 April 2009 - 04:26 PM

Well, it looks like I
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#42 justme

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Posted 04 April 2009 - 04:38 PM

Amazing job. Thanks.Matyas Majzik
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Posted 05 April 2009 - 08:35 AM

Awesome! :( /Gustav
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#44 tubavet

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Posted 06 April 2009 - 04:10 PM

Excellent Bryan and thanks for doing this.Phillipps any chance of making a sticky or at least the opening post before all the hard work gets lost?
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#45 downscc

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Posted 06 April 2009 - 04:26 PM

I would love to see this put in an article in the PMDG Ops wiki section. The articles are not hard to create (I've added two).
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Dan Downs, Maj USAF (RET)
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