Density Altitudes?

[CessnaFlyer]

as the name of the thread suggests does FSX simulate Density Altitudes or not? I took a friend of mine plane spotting last week and we hear a guy make a phone call to get weather and it mentioned Density Altitude. and I was just wondering as to whether or not FSX has that?

[shadowfaxcrx]

Yup. Try setting the temperature to 95 degrees F and then set yourself at an airstrip in the Rocky Mountains outside of Denver in the default Baron or Cessna. You'll be lucky to get off the ground before you hit a tree.

 

Note that some planes are more susceptible to performance problems with high DA than others -- a turbocharged piston, or a turboprop/jet won't lose nearly as much power in high DA than a naturally-aspirated piston. You'll still have longer takeoff rolls because the air is thinner and so the plane generates less lift, but at least you won't be wheezing down the runway with an engine producing a fraction of its normal power. So if you pull out the Lear, you won't notice the DA as much as with the pistons. You'll also notice that the Mooney does a lot better, and that's because it has a turbo-charged engine which compensates for the low air pressure.

[pvarn]

This is not much different than ISA (International Standard Altitude.) No matter what you call it, does FSX use this terminology in its in-sim literature or interface or ATC? No.

Does FSX display ISA without a gauge calibrated to that scale? No.

Does FSX without external weather input calculate the correct pressure at any given altitude if the sea level pressure at that location does not change? Yes. FSX performs ISA altitude/density calculations.

Will FSX AI provide you the ISA/DA on request? No.

Will online ATC provide you this information on request? Not likely unless they have an ISA/DA calculator handy.

Will knowing ISA/DA assist you in a GA flight? Doubt it.

Where knowing if the Standard Atmosphere is too high or low for the performance of your aircraft is most likely useful at very high altitudes where engines are operating near their design limits and a low pressure storm is in the area.

However, Knowing ISA is not enough when this calculation is important to you because the same engine design limits you care about when the pressure is very low, the TEMPERATURE limits of the engine also come into play. Most FS simmers are not likely to be forced to take these factors into consideration when planning if a flight might not be possible at particular cruise altitudes under 56K ft. For High altitude aircraft like the Concorde or SR-71, ISA and Temperature are decisive factors in the success of flying the plane.

 

For more reading on this subject:

https://en.wikipedia.org/wiki/Density_altitude

 

-Pv-

[CessnaFlyer]

as to the second post I will try that. and to the first post. I think the way FSX handles weather is unrealistic. but for real flight planning you might want to check the DA. if not like shadowfaxcrx said you may find your self trimming trees with the propeller. lol

[pvarn]

" I think the way FSX handles weather is unrealistic."

This is true when using FSX's dead Jeppeson external dynamic weather. This has been true since dynamic weather was first introduced into FS close to 15 years ago.

 

If, however you consider a high fidelity external weather program like Active Sky 2016, you will get a technical realism unavailable in FSX otherwise. Anyone who really cares about weather rather than what you get is good enough, AS16 is considered a much have program for weather enthusiasts.

-Pv-

[lnuss]

This is not much different than ISA (International Standard Altitude.)

 

Pv, your post is the first time I've seen a density altitude discussion filled with ISA. Density altitude is calculated referenced to ISA, but by definition is not ISA (except rarely, when they match), so is sometimes only a little different from ISA, but often is a LOT different from ISA -- Denver on a 95º day, for example, since ISA for Denver would be 41º at 29.92 and 0%. As a concrete example, the density altitude at Denver (5280 feet) with a 95º F temp, 29.92 in Hg, and dewpoint at 75º, would be 9104 feet. That's not even close to ISA.

 

I've got many, many years of dealing with density altitude in real aircraft (Denver, Albuquerque and more), and even at sea level there can be a significant effect. At 680 feet (many northern Ill. airports are near that elevation), 95º, 29.92, and 88º dewpoint (more likely in Ill.), DA is 3673 feet. That's a LOT different from ISA. Many aircraft over the years have crashed from a pilot ignoring DA. On a hot day in Denver, a Cessna 172 is a two place airplane*. That's a BIG difference. Takeoff distance is affected, engine power is affected, climb rate is affected.

 

Where knowing if the Standard Atmosphere is too high or low

 

The SA is the SA and doesn't change for a given elevation (as your statement implies) -- it should read, "Where knowing if the Density Altitude is too high or low". You'll almost never have the standard atmosphere anywhere, since humidity in ISA is always 0%, and only on rare occasions is the sea level temperature at 59º F (or the 5,000 foot temp is 41ºF). It's just a standard against which many things are calculated, such as aircraft performance.

 

OK, you can get by with a lot in the sim simply because there are no serious consequences, but many of the folks are wondering about how it relates to the real world, and the sim does simulate density altitude and it CAN be important to sim pilots, either because they're working with a short runway (I do that all the time), or a low-powered aircraft at high altitude fields, or both.

 

---

* A 1968 C-172 (because I have its manual) at 59º (ISA) at sea level at its 2300 lb. gross weight requires 865 feet of runway for liftoff, while at 2500 feet and 50º it requires 1040 feet and at 5000 feet and 41º it requires 1255 feet of runway. The book says increase distance by 10% for each 25º F above ISA. Practical experience over the years says that these figures are rarely attained by most pilots, since they were figured by a test pilot under ideal conditions (rarely seen).

[pvarn]

Regardless of what reference you use, SA, ISA, MBAR, MMHG, QNH, KPA, etc. FS calculates pressure differences based on altitude. Does FS use DA? Yes. Pressure changes with altitude above sea level regardless of the relative altitude of the aircraft. If you set the Clear and Calm option in the sim and static weather, FS will follow ISA perfectly up to 56K ft.

 

OP:

"a friend of mine plane spotting last week and we hear a guy make a phone call to get weather and it mentioned Density Altitude. and I was just wondering as to whether or not FSX has that?"

 

Yes, but it's not a term used in the sim as a rule. You can select to use mBAR or QNH in the sim as altitude/pressure display.

 

Is the default weather system(s) in FS "unrealistic?" Yes, but it doesn't have to be that way because MS provided powerful injection tools to manipulate FS's internal weather system to improve on it.

-Pv-

[CessnaFlyer]

" I think the way FSX handles weather is unrealistic."

This is true when using FSX's dead Jeppeson external dynamic weather. This has been true since dynamic weather was first introduced into FS close to 15 years ago.

 

If, however you consider a high fidelity external weather program like Active Sky 2016, you will get a technical realism unavailable in FSX otherwise. Anyone who really cares about weather rather than what you get is good enough, AS16 is considered a much have program for weather enthusiasts.

-Pv-

 

I will look into that. I use REX for my weather engine lol

[pvarn]

"I use REX for my weather engine lol" ... and you still think FSX weather is unrealistic. You definitely need to step up to the best. Add Active Sky Cloud art too so your clouds will change color, texture and choose from 100 cloud types on the fly as you go. The word "unrealistic" will go out the door.

-Pv-