I did my initial training in a high altitude area, where it got pretty hot in the summer, so I'm not at all surprised when I hear an ASOS annouce "Caution: Density Altitude (a few thousand above field elev.)".
But recently, I was flying in SE. Kansas when it was extremely cold outside. The ASOS announced "Caution: Density Altitude minus 1800 ft".
I understand that cold winter air can make the air extremely dense, leading to a negative density altitude. But I don't understand the caution part of it.
As far as I'm aware, low density altitudes mean your engine generates more power, and your wings generate more lift. Everything is better with low density altitudes, right?
What is the caution for? What should I be looking out for?
Low density altitudes mean your engine generates more power, and your wings generate more lift. Everything is better with low density altitudes, right?
The potential danger in low density altitudes is that your engine generates more power than it can handle. Even without exceeding the maximum rated RPM and manifold pressure, you can exceed the rated power of an engine under low density altitude conditions.
At low temperatures the air becomes denser. At standard sea level conditions air has a temperature of 15 degrees Celcius and a density of 1.225 kg per cubic meter. When the temperature is -30 degrees Celcius, the density increases to 1.452 kg per cubic meter. The equivalent altitude where this density is found under standard atmosphere conditions would be about -5900 ft. This is called the density altitude.
When air with these properties enters the combustion chamber of your engine and is combusted to the normal exhaust temperature it expands (increases in pressure) far more that air normally would, resulting in excessive power.
This FAA paper on Winter Flying Tips says the following about the risks associated with low density altitudes:
Do not overboost supercharged engines. This is easy to do because at very low density altitude, the engine "thinks" it is operating as much as 8,000 feet below sea level in certain situations. Care should be exercised in operating normally aspirated engines. Power output increases at about 1% for each ten degrees of temperature below that of standard air. At -40 degree F, an engine will develop 10 percent more than rated power even though RPM and MP limits are not exceeded.
Rather than over-performance being cause for the warning, I would suggest that the more likely reason for the "Caution" would be that you don't really want to try to land at -1800 ft if you just flew from somewhere with a DA of 0. Reset your altimeter. ;-)
EDIT: Oops. Ignore me here. This is what happens when my mouth runs before my brain fully engages. Abelenky is right. The DA wouldn't affect your indicated altitude if you set your altimeter on the reported pressure. It would just affect the performance of the aircraft.
On an opposite note, I grew up flying in Louisiana where we were essentially at sea level, but we'd have days in the summer where the density altitude would be well over 5000 ft. It sucked pretty bad to be sit in a non-airconditioned plane in that kind of humidity. And it was just as bad to watch so much runway scoot under you while your Cessna wasn't lifting off.
field elev.)". But recently, I was flying in SE. Kansas when it was extremely cold outside. The ASOS announced "Caution: Density Altitude minus 1800 ft". I understand that cold winter air can make the air extremely dense, leading to a negative density altitude. But I don't understand the caution part of it. As far as I'm aware, low density altitudes mean your engine generates more power, and your wings generate more lift. Everything is better with low density altitudes, right? What is the caution for? What should I be looking out for?
How do turbochargers improve engine performance at high altitudes / density altitudes? Could you also address how this benefit could become a liability at low density altitudes, for example when the density altitude is negative?
Non-precision instrument approaches generally have altitude restrictions which get lower when you get closer to the airport. I always figured these restrictions were AMSL using the current altimeter setting, not compensating for temperature. Some have heard the mnemonic that mountains are higher come wintertime, which basically means that colder weather make your altimeter read higher than you actually are (or, as most pilots prefer to think, you're lower than what your altimeter reads) Have a look at this VOR approach into Newark Most altitude restrictions are a minimum level, so
If you fly low, air is dense so you can get more thrust from your engines, but you get more drag. On the other hand if you fly higher you have less drag but the output of engine decreases as well. So what's the optimum altitude to fly at, and how does one determine it?
for ATC to issue a descend via clearance, though I could be wrong about the second part. Some charts say "VERTICAL NAVIGATION PLANNING INFORMATION" before the expected altitude, so my assumption is that these altitudes are strictly for planning purposes and allow a top of descent to be calculated. My questions are: When you enter the STAR in the the FMS, will it pre-populate the expected altitude, and does this show up in the FMS exactly the same as a mandatory crossing altitude? Are expected altitudes treated as suggestions by pilots or controllers? For example, in the above chart, if you
I read once that the speed of sound decreases as altitude increases. I wonder if that's true and, if so, are the pressure waves made at high altitude lesser than the ones at low altitude because of the difference in air pressure? And, lastly, does that mean that lower speeds at higher altitudes will be subject to the problems one encounters when approaching mach speed?
(background: low-hour, low-experience, private pilot.) I've got an issue regarding talking to air traffic controllers, especially when I think they may have messed up, yet still giving them the respect they deserve as hard working professionals. One common scenario is when I'm ready to go, along with multiple other planes, all coming from various taxiways. I know the tower tries hard to clear us each more-or-less in the order we've called up. But when one plane feels they've been badly skipped, or possibly forgotten, it can get awkward. I've heard a pilot say (very politely
As we all know from our instrument training, the MOCA is: MINIMUM OBSTRUCTION CLEARANCE ALTITUDE (MOCA)- The lowest published altitude in effect between radio fixes on VOR airways, off-airway... of a VOR. Whereas the MEA is: MINIMUM EN ROUTE IFR ALTITUDE (MEA)- The lowest published altitude between radio fixes which assures acceptable navigational signal coverage and meets obstacle clearance requirements between those fixes. The MEA prescribed for a Federal airway or segment thereof, area navigation low or high route, or other direct route applies to the entire width of the airway
heading. He didn't complain, but I'm still not sure if that's what he wanted. A bit later I got a similar call (callsign) request QNE However, I was unfamiliar with that Q-code (as a private pilot... on board (which I incorrectly assumed at that point was what QNE meant), again, using "request". Anyway, I've never heard a controller say "request" before, is it just army version of "say"? I'm pretty...I recently had the opportunity to fly a PAR approach into Büchel Airbase in Germany. It was a ton of fun and I'll definitely try it again when I get the chance. However, as we were getting set up
In an ATIS or AWOS/ASOS recording, is the reported altimeter setting corrected for temperature? I think it must be, because on a hot day, the altimeter should still show correct field elevation. If it wasn’t corrected for temperature, then the reported field elevation could be badly wrong on a really hot or really cold day. (This and other questions describe pressure and density more generally.)