If memory serves me correctly there used to be a local Tuskegee Airman instructor who talked of training someone to fly in 1 hr. Really, it all comes down to understanding how to control airspeed and stay within the flight envelope. You can drive a plane like a car down to the runway if you understand that. It might not be pretty but very doable.
Back to General Aviation…
My dad had his BD-5 about 80% finished when he was transferred from DAG to WJF and the moving company smashed it. He used the insurance settlement to buy a 1970 Cessna 172K.
What are “flight levels” and why is the altimeter adjusted when you get there?
In the video below (queued to the right spot so no need to watch more than 15 seconds or so) the pilot notes that as he passes 18,000’ in altitude he is in the “flight levels” and then he presses a “standard” button on his altimeter which adjusts the altitude it is reporting (not a whole lot it seems but hard to tell in the video).
What is happening here? Why would you adjust an altimeter like that? Shouldn’t they all be reporting the same altitude?
Once you get above 18,000 feet, presumably you’re in it for the long-haul. So at that point you set your altimeter to 29.92 instead of monitoring one local barometric reading after another.
and 29.92 is used because it’s the standard atmospheric pressure at sea level. And you need to use something in place of measurements that don’t exist without a reporting station.
Do altimeters report height above sea level or height above the ground? For example, if you are in a plane, on the ground in Denver does your altimeter read zero or does it read 5,280 feet?
Barometric altimeters (what’s generally used) report height above sea level. Radar altimeters report height above ground but I believe those are mostly used as part of instrument landing systems in the big iron. I’ve never seen one.
Everyone does the same thing so they are still all referencing the same altitude. In fact it becomes more “the same” because below 18000 you could have two planes in the same place referencing two different pressure settings.
The transition altitude and level are different in different countries. Australia you change at 10,000’ climbing and FL110 descending. In New Zealand it’s 12,000’ and FL130. In the UK it’s very low and changes depending on where you are.
They report above the pressure reference you’ve set. If you set QNH then it’s reading above sea level (approximately), if you set QFE then it reads above the airport elevation you got the QFE pressure for. I believe they do this as a matter of course in Russia but you might also want to do it for a low level aerobatic display. The “standard” setting for flight in the flight levels is called QNE.
Of note is that setting a QNH value for a high altitude airport such as Denver doesn’t give you the actual sea level pressure (assuming there was one), it gives a setting that corresponds to seeing the correct altitude when on the ground at Denver.
The altimeter assumes a standard atmosphere, any deviation from the standard temperature will result in altimeter errors. This doesn’t matter for the purposes of avoiding other planes because everyone’s altimeter works the same way and has the same errors. However it does matter when flying an approach in cold temperatures because the altimeter will over read and this becomes important for clearing obstacles. Some approaches will only be valid above a particular temperature or there may be corrections that need to be made.
Thanks for the response. I am a still a little unclear.
I can see a pilot needing to know both height above sea level and height above the ground. If there are mountains in the area they are measured from sea level so you want to know you are high enough to clear them.
But, height above the ground seems an issue too. I have seen videos where the pilots get an audible warning about decision height. They need to know when they are 500’ above the ground (or whatever the rule is) to decide if they have the runway in sight or need to go-around.
Obviously, 500’ above the ground looks different on an altimeter in Chicago than it does in Denver.
So, I am not sure what the altimeter is telling the pilots since both data points can be important to them.
The altimeter reports height above the pressure reference. Normally that means sea level or something close to it.
For height above the ground, if you’re VFR you can see it so you don’t need your altimeter to tell you for the purposes of not hitting it (flying through the mountains? look out the window, don’t hit the mountains). For flying a circuit that’s supposed to be 1000’ above the ground, you just add 1000’ to the airport elevation. Airport is 5035’? Then you fly your circuit at 6100’ (rounded for ease of use).
If you are IFR, instrument flight rules, you might not be able to see the ground so altimetry becomes far more important. On basic aircraft you still fly with reference to the altimeter which is reading above sea level. On approach the decision altitudes and minimum descent altitudes reference sea level. The approach might have a decision altitude of 2100’ at an airport that is at 1800’ so you will be 300’ above the ground when you need to be able to see the runway.
Some aircraft have a radar (aka radio) altimeter, or RADALT. This includes all airliners and some small aircraft. I think it’s mandatory equipment above a certain number of passengers bu don’t know the details on that.
The RADALT gives height above the ground beneath the aircraft (not the runway unless it happens to be over the runway.) The RADALT is used for inputs into the aircraft systems such as traffic collision avoidance (TCAS), terrain proximity warning (EGPWS), and auto-land. Although there is a readout for the pilots and it is useful information, you don’t fly with direct reference to it, you are almost always flying with reference to the pressure altimeter. The callouts you hear in some videos are triggered by the RADALT, things like “FIVE HUNDRED”, “FIFTY, FORTY, THIRTY, TWENTY, TEN”
All excellent info, especially just above by @Richard_Pearse. No surprise there. I’ll back off to the bigger picture of what the overall system trying to accomplish.
When you’re not immediately landing or taking off, what matters is not how far away from the ground you are. “It’s way down there somewhere that I can’t readily run into” is good enough even in a Cessna.
Nor really does it matter how high you are above sea level, whether over ocean or over land. Once again, “It’s way down there somewhere that I can’t readily run into” is plenty good enough.
What does matter, and matters critically, is “How far above or below me are the other airplanes going right past me?” In a busy sky it is essential that everybody within a few minutes flying time of each other be using exactly the same baseline altitude reference so when one plane thinks we’re at 10,000 feet and somebody coming the other way thinks they’re at 11,000 feet, there really is 1,000 of vertical spacing between us. Not 473 feet, or -100 (IOW the 11,000 guy is actually below the 10,000 guy), or worse yet, zero. That gets noisy with the sound of rending aluminum, and we hate it when that happens.
As said earlier, barometric altimeters are simply fancy barometers that measure outside air pressure where you are (both geographically and vertically), compare that to the official standard of how pressure varies by altitude worldwide, and display that pressure reading as a height in feet.
We also all know that air pressure varies from day to day and place to place. We’ve all seen weather maps with highs and lows depicted. The difference between possible extremes here on Earth can be the equivalent of a couple thousand feet. An altimeter that lacked a feature to compensate for this natural variation would be pretty useless for terrain clearance but would still work fine for separating airplanes from other airplanes.
And this is the key realization right here. We have two goals: terrain clearance when we’re close the ground, and inter-airplane clearance when we’re WAG half a mile or more above the rocks. Those are different goals with different solutions. Continuing …
The weather authorities measure and report the local barometric pressure all over the world, and especially at airports. Which readings are then converted to what they represent as an equivalent pressure at sea level. Which is reported as “the altimeter setting”. With the effect that if somebody parked at an airport has set their altimeter to the local altimeter setting, the altimeter will read the same as the airport elevation. And if two planes in the sky near that airport both have that same altimeter setting, they can be confident not only of their altitude above the airport, but also confident that their respective altitude readings will be compatible so if they’re to pass each other with 1,000 vertical separation shown on their altimeters, it really will be 1,000 actual feet (plus/minus minor installation variations I’ll ignore).
This is all well and good for takeoffs and landings and bumbling around overhead some city. The problem comes in for fast airplanes going long distances. You don’t want 2 airplanes near each other using different altimeter settings. That compromises the intended 1,000 foot (or 500!) spacing. And at jet speeds, somebody 100 miles away is “nearby”.
So once you’re some distance above the ground, as Richard explained, everybody just resets their altimeter to 29.92 inHg (or metric 1013.2HP/mB). Now everybody is on the same setting, the vertical separation between plies of airplanes is assured, and we don’t need to reset our altimeters every 5 minutes for hours on end, just to read accurately versus a sea level that’s a mile or 7 beneath us. This also solves the problem of what is the local sea level pressure someplace over the middle of an ocean with no weather stations? The answer is “Frankly my dear, we don’t give a damn. We just use a setting of 29.92 and don’t much care whether we’re actually at 33,000 feet as our altimeter says or are actually just 32,174 feet above the waves. What we care about is that all the nearby airplanes have the same readings we do.”
Of course this means that as you climb, there must be a height where you change from the local setting (QNH in the argot) to the universal setting (QNE in the argot). IOW QN-Here versus QN-Everywhere. And vice versa, during descent you also have a height at which to switch the other way. Which height is standardized by country or region but in any case is chosen to be far enough above the local rocks that everyone will be on QNH while terrain clearance is a significant concern.
And there’s also a vertical buffer zone at that transition height where airplanes generally cannot cruise because of the possibility of somebody on one setting going up or down while somebody on the other setting is cruising. So everybody just slips up or down through the buffer zone quickly and all is well.
With all that background …
So given a barometric altimeter set to local QNH, how do you know how far above the immediate terrain you are? Maps. Or the electronic equivalent.
The design of instrument takeoff, departure, arrival, and approach procedures all specify a minimum altimeter altitude to fly to remain a safe distance above the unmoving rocks along that route segment. As long as you stay on course and abide by those published altitudes and have a correct altimeter setting, all will be well. In the immediate terminal area lots of the route segments are short as you slide down around and between the rocks at varying altitudes. Slithering in towards a runway. Departures work similarly.
When controllers are radar vectoring airplanes around not on published (=surveyed) routes, they have a map on their scope which tells them what the lowest safe altitude is in that immediate (few miles) area. We also now have the same maps, at least for most places, so we can check their work.
Clear as mud?
the barometer based altimeter is giving you elevation above sea level. Aviation maps are based on elevation above sea level. So a mountain that shows 8500 feet means it’s that high above sea level. When you adjust the altimeter to local pressures it gives the altitude of the plane above sea level.
If you zoomed in on SFO (SAN FRANSISCO) AIRPORT on a map you’ll see that it says 13 toward the bottom left of the airport info. That’s 13 feet above sea level. SkyVector.
Meanwhile, out west:
Quoting a bit of my earlier post above for context:
The newish new news is that a couple weeks ago the NTSB came out with a preliminary report on the easy obvious stuff. Which states the left main gear, not the nose gear, failed on landing. See DCA22FA132.aspx (ntsb.gov)
Once that tore off the airplane, the pilots were likely mostly along for the ride. Between hydraulic system damage and significant asymmetric friction versus the runway, they were pretty well doomed to slide off the left side of the runway someplace down-track. Which they duly did. All that remains at that point is to correctly manage the evacuation & post-evac passenger control herding.
The dead jet is now up on jacks at one of the repair / overhaul ramps that occupy a bunch of the non-airline part of MIA. The company name on the forward fuselage has been whitewashed over, but the tail is still their distinctive red, gray, and blue logo.
“Some of the videos have some good detail.”
I suppose its sort of instinctive but I noted some of the passengers took the time to grab luggage before running while in fear of burning to death. I understand that this is not uncommon. All I can say is, “If the plane is on fire, don’t be going through the overhead while standing in my way. I ain’t waiting!”
A hefty chunk of people will go into the overheads and take some or all of their stuff. We can’t stop it. Wish we could, but we can’t. It vastly slows the evacuation.
The good news is that in this case the difference in time didn’t matter since the fire wasn’t that intense, it was quickly attacked by the firefighters, it was only on one side, etc.
But in any mishap akin to this where the crash forces were highly survivable but it then degrades into an inferno after stopping and not everybody makes it outside, my personal opinion is that probably half the folks left trapped inside died for somebody else’s suitcase. Boils my blood. Just as it boils the blood of the folks left inside.
It would be a great safety feature if the pilots could flip a switch and lock all the overheads.
Nope. Then they’d stand there fighting with the bin thinking they’re using the latch wrong. We see this on every ordinary deboarding. That cure would be worse than the current disease.
If we don’t want them to get their goods we need to go back to the 1950s. Men may bring a hat and the contents of their pockets aboard. All else goes in the cargo hold without exception. Women may bring a clutch purse in addition to their stylish pert little pillbox hat. And an irritating Pekinese.
Locking them is immediately followed by “the bins are locked, get the fuck off the plane, we’re not kidding”