78
votes
Accepted
How can an altimetry system error during cruise (500 ft at 35000 ft high) cause an insufficient fuel situation?
If the aircraft is cruising at FL350 it is operating in Reduced Vertical Separation Minima (RVSM) airspace (between FL290 and FL410).
With an altimetry discrepancy between the two altimeters of more ...
- 83.8k
48
votes
Accepted
Why does a radar altimeter show negative values on the ground?
The zero reading is calibrated for main-wheel touchdown in a landing attitude. With the antenna forward of the main gear, it is, at that point, higher than it will be after the nosewheel is lowered to ...
- 52.6k
46
votes
Accepted
How is altitude reached by aircraft flying above the stratosphere measured?
They used inertial altitude on the X-15 for high altitude measurements. This works just like an IRS (Inertial Reference System), which was used on airliners of that era: you can get the position (...
- 57.7k
46
votes
Is there a place where altitude while flying is negative?
There are quite a few airports near sea level since many larger cities are located near the sea. There are even some airports below sea level. Amsterdam Schiphol is somewhat famous for it (at least ...
- 57.7k
34
votes
What should I follow, if two altimeters show different altitudes?
The G5 is not certified as a primary altimeter- its information is advisory only. From a legal standpoint, your primary altimeter is always the certified one- in your case, the steam gauge one.
- 17.6k
27
votes
Accepted
Is there a place where altitude while flying is negative?
Short answer
If you just wanted to know some places below sea level then there are plenty of related collections for places and airports and there is little interest to copy them here. But actually ...
- 77.1k
24
votes
How do radio altimeter systems compensate for objects on the ground?
Objects on the ground are negligible because the radio altimeter is not designed nor used to such high precision.
There are several uses of the radio altimeter. The first one is for timing the flare ...
- 39.8k
20
votes
What kinds of altimeter are used in modern airliners?
Main instruments
In large aircraft, main altimeters are not separate mechanical altimeters. Instead the pitots, static probes and angle of attack probes, which are redundant, have their outputs ...
- 77.1k
19
votes
Accepted
What is the meaning of "altimeter three-zero-one-one"?
That is a QNH altimeter setting in inches of mercury, with the decimal omitted. So in the case given, that is a QNH of 30.11 inHg or about 1020 hPa.
- 52.6k
16
votes
Why is Landing Altitude displayed in amber on B777?
The Boeing 777 is a very automated aircraft. The takeoff altitude, landing altitude, and cabin pressurization altitude are all automatically set when you enter the departure airport and destination ...
- 37.9k
15
votes
Did the Stuka Ju-87 really have a radio altimeter? If so, when?
It looks like the actual date is 1942. The 'Nachtschlachtgruppen' aircraft were given Ju 87 D aircraft with numerous upgrades including the FuG 101 radio altimeter.
This would make sense because to ...
- 1,596
15
votes
What is the purpose of the leaf spring in an altimeter?
The function of the spring is to pull on the evacuated capsule, such that it has a volume. Without the spring, the evacuated capsule would be entirely flat.
If the capsule were flat, a change in ...
- 83.8k
15
votes
Accepted
What does CPLD mean in aviation in general and on this altitude alerter?
The unit in the image is a Cessna AA-801A altitude alerter.
CPLD Green Lamp:
Lights when airplane reaches selected altitude and Integrated Flight Control System altitude hold mode is automatically ...
- 166
14
votes
Why isn't transition altitude standardized in Europe?
You're right that having a common transition altitude is better- efforts are underway in Europe to set a common transition altitude. Multiple studies have been carried out by Eurocontrol and it has ...
- 100k
14
votes
Accepted
What is that green rectangle next to altitude readout on Boeing aircraft?
It indicates that the altitude is below 10,000 ft.
My Boeing 777 FCOM shows the same rectangle in the PFD description, but does not mention any details about it.
However, the Boeing 747 FCOM includes ...
- 57.7k
14
votes
What is the legally accepted tolerance for altimeter errors under EASA for a VFR flight?
The relevant information is contained within ICAO Document 8168 Volume 1 Part 3
A serviceable altimeter indicates the elevation of the point selected, plus the height of the altimeter above this ...
- 35.2k
14
votes
What should I follow, if two altimeters show different altitudes?
Garmin itself says:
With the Auto Calibration feature turned on, the altimeter accuracy can range from +/- 50 to 125 feet (same as GPS elevation).
You can draw your own conclusions from this!
- 259
13
votes
Is there a place where altitude while flying is negative?
There are several regions where you could (at least in theory) fly below sea level. For example, in the Dead Sea depression, that is the lowest stretch of land on the planet, reaching a maximum depth ...
- 11.1k
13
votes
What is the purpose of the leaf spring in an altimeter?
Without the spring, the capsule would tend to collapse at or below sea-level pressure-altitude. The spring opposes the high pressure.
Source
A variation of the principle above it to put the spring on ...
- 77.1k
12
votes
Why is a TFR issued when barometric pressure exceeds 31.00 InHg?
That is because most of the legacy altimeter setting barometers cannot handle such high pressures. Most of them, like the one pictured below has a range from 28 to 31 inches of mercury.
Kollsman ...
- 100k
12
votes
Accepted
How will the altimeter read in air colder than ISA?
Air expands as the temperature increases and it compresses when the temperature decreases.
image source: aviationweather.ws
When it is colder than ISA, the air column is compressed and therefore ...
- 83.8k
12
votes
Accepted
Are altimeters calibrated to ISA geopotential altitude or ISA geometric altitude?
The TSO for Air Data Computers is TSO-C106. It specifies for the minimum performance standard "SAE Aerospace Standard 8002, Air Data Computer - Minimum Performance Standard."
From AS8002:
3.6 ...
- 20.1k
12
votes
Accepted
Why, and how, are barometric altimeters accurate enough for approach and landing?
A difference of 1 hPa results in an error of 27ft in the altimeter. To have the altimeter showing 3000ft when the aircraft is actually at sea level, the altimeter must be off by 111hPa. Standard air-...
- 14.6k
12
votes
Do ATC say "Climb to XXXX on standard" or "Climb to standard XXXX" or something else?
Standard phraseology says that below transition level, the controller may use the phrase "altitude", or omit it completely, but it is still clear - eg/
BigJet 456, Climb and maintain 5000 ...
- 35.2k
11
votes
Why can't you just set the altimeter to field elevation?
Because air pressure is not constant over wide areas. Weather provides us with high and low pressure areas, and you may not know the pressure at your destination when departing. Indeed, it may have ...
- 2,587
11
votes
Accepted
What is the effect of wind on altimeter reading?
A steady wind will not effect the altimeter reading on an airborne aircraft in any way, since the aircraft will be moving in relation to the airmass. This fully removes any effect a steady wind will ...
- 30k
10
votes
Accepted
Where does the altimeter measure from?
There are a few types of altimeters but they all generally do the same thing and can be calibrated to anything if need be. Where they measure from and what they consider 0 are really 2 different ...
- 102k
10
votes
Accepted
When is QFE used?
It is used in aerobatic competition.
It is much safer and easier to read AGL altitude directly from the altimeter than to attempt to do the arithmetic immediately prior to performing a maneuver.
I ...
- 6,989
Only top scored, non community-wiki answers of a minimum length are eligible