does that mean at higher altitudes the air is more dense? I thought the higher the less dense. Thanks for all the explanations, aviation and weather are both interesting. But does this whole density thing, is the effect really big?

The higher you go the density does decrease relative to sea level. But say for example from sea level to FL350 the only thing that changed was temperature (in this case the temp drops) then the air would be denser. I meant it in this manner. I know it's confusing.

Wait, so how about high bypass engines? The engine company says that 80% of the thrust is from the bypass. Is that only at lower altitudes cause I think bypass is N1

Generally N1 is the fan at the front of the engine. This is of course what produces the air flow through the bypass. There for the 80% figure is really just low altitude. At high altitude, the turbine section produces a majority of the thrust. I must admit I don't know the difference between high bypass and normal bypass (if there actually is a difference). To my knowledge the 80% figure is the maximum figure among all engines. Some produce less but I believe the goal at low altitude is usually about 80% for engine manufactures.

I thought I understood this, but I guess I'm now completely confused. In the '80s I was on a "Twin Otter" at 8000 ft. It was the hottest day that summer, about 90F. The plane was pushed back all the way to the end of the runway, the engines were as loud as I've ever heard them. We rolled much longer than usual, went off the end of the mesa, and dropped about 500 feet. (My stomach remained at 8000 ft, though). Somewhere around 600 ft below the mesa we either got enough power or enough density altitude (density pressure? the stuff under the wings) and headed off towards Albuquerque. In many previous trips, we'd never, umm, fallen that far. The ideal gas law says PV=nRT, so P=nRT/V, so as T (temperature) increases, P (pressure) decreases, right? So the reason the DASH-8 went down....why? the air wasn't dense enough, or the engine/propellers/something else/ couldn't spin fast enough?thanks,danp.s. Had we dropped another 200 feet, we would have conveniently landed in the sewage treatment plant at the base of the canyon.

The most likely reason it dropped (not to scare you) was that the aircraft wasn't going fast enough to fly. Although the aircraft may have been airborne before you went over 'the edge', it may still have been in ground effect. Ground effect is caused by the airflow off the wings interacting with the ground. This results in less drag than usual and slightly more lift and only occurs within about one wingspan of the ground. The risk here is that the aircraft can fly at a lower airspeed than normal providing it is still in ground effect. Once the aircraft went off the end of the mesa, the normal drag and lift forces on the aircraft was present and now it needed to pick up just a little more speed to fly.

In the '80s I was on a "Twin Otter" at 8000 ft.[...]So the reason the DASH-8 went down....why?

Ryan answered that one, in terms of what all happened, but which was it?Twotter (Twin Otter):http://www.vistaliner.com/P3110004.JPGDash 8;http://upload.wikimedia.org/wikipedia/commons/a/a6/Dash_8.jpgTwo very different aircraft.Either way, it wasn't going fast enough, like Ryan said. The reasons? Density altitude played a huge role in that the engines couldn't produce as much power on the thinner, hotter air, which made the roll longer, and slower on the acceleration. I doubt it was simply that you ran out of runway. You probably lifted off, but were in ground effect, as Ryan pointed out. When the ground dropped out, that "cushion" of air was gone. It's also unlikely you dropped hundreds of feet. I'm not accusing you of exaggeration, but without true visual references, the human body interprets the smaller motion in the restricted visual environment as hundreds of feet. Playing roll-off-the-mesa gravity games like that would get someone's operations certificate revoked.

Hi,The way to think of density altitude is like this: Assume STP at sea level. ISA delta temperature is 0. Density Altitude is sea level (I'm assuming sea level is uniform across the planet).Now assume that OAT is +40 °C. Standard Temperature is +15 °C, so ISA is now +25. What is my density altitude now?The rule-of-thumb method is a good ball-park:Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA_temperature))Density altitude in feet = 0 + (120 x ((+40) - (+25)))Density altitude in feet = 0 + (120 x ((+40) - (+25)))Density altitude in feet = 1800 ft.There is a more complex, and accurate, formula to compute this, that takes into account the pressure. Low air pressure means the air is less dense, so the worst-case scenario is a hot, dry, and high airport. When an engine is "flat rated" to +30 °C it means that up to +30 degrees C the engines can output the same power, but above this temperature the engine power drops off. Note that in order to keep outputting the same power that the engines has to work faster/hotter. EGT can become a limiting factor.If your aircraft has a service ceiling of e.g. 14000 ft, this does not mean you can always fly to 14000 ft AMSL. More than a few pilots have been killed in mountainous terrain by thinking the mountain tops are at 12000 ft, and that they can climb to 14000 ft, but they neglected their density altitude. It has worked out that the DA was 14000 ft at e.g. 11000 ft AMSL. They can't clear the mountain tops, and get into major difficulty by trying to climb too high (and beyond the performance limit of the aircraft), then they try and maneuver out of the valley, and spin in, or otherwise crash into the mountain top by trying to fly over it anyway.Best regards,Robin.

I thought I understood this, but I guess I'm now completely confused. In the '80s I was on a "Twin Otter" at 8000 ft. It was the hottest day that summer, about 90F. The plane was pushed back all the way to the end of the runway, the engines were as loud as I've ever heard them. We rolled much longer than usual, went off the end of the mesa, and dropped about 500 feet. (My stomach remained at 8000 ft, though). Somewhere around 600 ft below the mesa we either got enough power or enough density altitude (density pressure? the stuff under the wings) and headed off towards Albuquerque. In many previous trips, we'd never, umm, fallen that far.

Just curious...Which airport was it? Sounds like you're describing Los Alamos...Jim Barrett

Somewhere around 600 ft below the mesa we either got enough power or enough density altitude (density pressure? the stuff under the wings) and headed off towards

Neither - you got enough airspeed to climb. You needed more runway than you had.Best regards,Robin.

KPHX was nearly shut down yesterday here - we got to 118, they'll shut it down at 120.

We actually had KPHX and all other airports of the valley yesterday due to Dust Storm :/Stephan

Hot and humid at haneda, nearly reached 60 degrees Celsius. With that I give up on trying to see the 787

Ryan answered that one, in terms of what all happened, but which was it?Twotter (Twin Otter):http://www.vistaliner.com/P3110004.JPGDash 8;http://upload.wikime...a/a6/Dash_8.jpgTwo very different aircraft.Either way, it wasn't going fast enough, like Ryan said. The reasons? Density altitude played a huge role in that the engines couldn't produce as much power on the thinner, hotter air, which made the roll longer, and slower on the acceleration. I doubt it was simply that you ran out of runway. You probably lifted off, but were in ground effect, as Ryan pointed out. When the ground dropped out, that "cushion" of air was gone. It's also unlikely you dropped hundreds of feet. I'm not accusing you of exaggeration, but without true visual references, the human body interprets the smaller motion in the restricted visual environment as hundreds of feet. Playing roll-off-the-mesa gravity games like that would get someone's operations certificate revoked.

It was likely the DASH-8. The airline ran two different planes, the DASH and the Twin Otter. I'd forgotten that. It was the larger of the two a/c. The "700 ft" was from something the pilots said when we landed in KABQ. The comment was "we dropped most of the way" down the canyon". I do remember (vividly) watching the layers of volcanic ash going by the wrong way, but we did have some forward motion, too. You're probably right that it was not 700 ft. The a/c had dropped many times before (in the summer), but those were more like a small carnival ride drop.Thanks for explaining the ground effect: I knew it existed in helicopters, but not in airplanes. I'm going to try setting it up in FSX and see if I can replicate what happened.Thank you all,dan

you haven't seen hot till you've been in Vegas (LAS)

Just curious...Which airport was it? Sounds like you're describing Los Alamos...Jim Barrett

Yes, exactly. The airport was Los Alamos, 7900+ ft, 90F, 5% humidity. There were two a/c that were "scheduled". One flew twice a day for passengers and cargo, and the other for passengers. Usually the "cargo" flight (the Dash-8) was more stable, while the smaller tended to get bounced around. I had never been on it when it was towed to the far end of runway, though.This was before KAQB was expanded, though.How did you know?thanks all,dan

Yes, exactly. The airport was Los Alamos, 7900+ ft, 90F, 5% humidity. There were two a/c that were "scheduled". One flew twice a day for passengers and cargo, and the other for passengers. Usually the "cargo" flight (the Dash-8) was more stable, while the smaller tended to get bounced around. I had never been on it when it was towed to the far end of runway, though.This was before KAQB was expanded, though.How did you know?thanks all,dan

Back in the 1960's, my Dad was a pilot for Carco Air Service, which was the company that provided the daily shuttle flights between Albuquerque and Los Alamos. Back then, they operated a fleet of Beech Bonanzas, and the larger BE50 Twin Bonanza, as well as a couple of Beech D-18s. Growing up, I heard many stories about the adventures of flying to and from "The Hill". Carco also operated a base in Las Vegas, where they flew daily shuttles up to the Nevada Test Site on behalf of the AEC and EG&G corporation using a fleet of DC-3s. These were the forerunner of the "Janet" 737 shuttles that still operate today.As I recall, Clark Carr, the owner of the company, retired around 1970 and sold the ABQ shuttle operation to Ross Aviation. I believe Ross is still in business in ABQ?Of course, temps up near 90 are very rare at Los Alamos, but there were a couple of occasions back then, where all flights to LAM were cancelled on unusually hot summer days because density altitudes were so high as to make takeoff impossible - at least with the piston-powered Beech aircraft that were in use.My father was also in the A.F. Reserve, and there were times when military flight ops at the Kirtland AFB side of the ABQ aiport had to be curtailed in summer because of density altitude restrictions. Even back then, runway 8-26 at ABQ was over 10,000 feet long, but sometimes that wasn't enough... Jim Barrett

Any other "hot" stories? Some places are ridicules and still don't close, like 140 degrees. Thats some messed up temperature. How hotter do jet engines make the place? Say if it was 95 degrees then how much higher would the temperature get? I would like to try and stick to a temperature in the summer with the PMDG 747p.s, if you change the barometer setting in the pmdg 747 would that effect the way it flies?

Temperature can easily get up to 40C-50C (104F-122F) in Saudi Arabia. It is also a place where you can find heavily overloaded charter 747s departing from Madinah or Jeddah with full pax and going home from the summer Hajj with all of their zamzam water and souvenirs.

Any other "hot" stories? Some places are ridicules and still don't close, like 140 degrees. Thats some messed up temperature. How hotter do jet engines make the place? Say if it was 95 degrees then how much higher would the temperature get? I would like to try and stick to a temperature in the summer with the PMDG 747

I have a picture back when I worked for IDE of a dry spot on the ground, with the area around it still shimmering from the rain that had just occurred (CRJ APU exhaust is directed outward and down towards the ground). While that only gives you an idea of how hot it really is, how much it affects the temperature in the immediate area isn't exactly known. I'd imagine it raises it slightly, but not too significantly.

if you change the barometer setting in the pmdg 747 would that effect the way it flies?

Not to my knowledge. Changing the baro pressure only affects the altimeter. I believe the information being fed into the air data computers from the various probes and sensors takes care of that automatically. Why would they do that? Relieve pilot workload, and send better data to the computers, regardless of what baro readings the ground is recording (we set 29.92/1013 at transition level, which, in non-standard conditions would send bad data to the computers).

it would change the way it flies a bit. If the pressure is lower the plane would have to fly at a faster speed to not stall. They would need a higher rotation speed. I was wondering if that stuff is modeled in FSX

Temperature can easily get up to 40C-50C (104F-122F) in Saudi Arabia. It is also a place where you can find heavily overloaded charter 747s departing from Madinah or Jeddah with full pax and going home from the summer Hajj with all of their zamzam water and souvenirs.

wow, impressive. Same in Japan I guess, heavily overloaded Jumbos taking off with loads of tourists and freight.