77 votes
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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 ...
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48 votes
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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 ...
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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 ...
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  • 44k
44 votes
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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 (...
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27 votes
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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 ...
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25 votes
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How do aircraft altimeters calculate altitudes accurately while airborne?

Altimeters are calibrated to a standard atmosphere model (International Standard Atmosphere, ISA). What the altimeter shows you is the vertical distance between the altitude equivalent to your ...
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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 ...
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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 ...
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19 votes
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Where did the standard pressure 29.92" come from?

It is called "standard pressure" because 29.92 In-Hg (or 1013.25 hPa) is the standard atmospheric pressure at sea level according to both the ISA (International Standard Atmosphere) and the US 1976 ...
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19 votes
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What is the difference between "sensitive" and "non-sensitive" altimeters?

All altimeters are "sensitive" to some extent (an "insensitive" altimeter would be one where the internal pressure-sensing mechanism has failed: the pointer wouldn't ever move). What the FAA calls a "...
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19 votes
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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.
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16 votes

What is the meaning of the crosshatch marking on altimeters?

It looks like the crosshatch (or barber pole) indicator doesn't have a consistent meaning (a similar marking is used on some airspeed indicators, by the way). I found several different examples of ...
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16 votes
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How precise are altimeters?

While Accuracy and Precision are closely related, they're not the same thing. In the context of your question both are important: Accuracy In the US altimeters are broadly required to comply with ...
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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 ...
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  • 31.8k
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 ...
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  • 1,588
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 ...
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15 votes
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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 ...
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14 votes

Where did the standard pressure 29.92" come from?

It is called "standard pressure" because at an altitude of mean sea level (mean of all tides low to high) averaging the air pressure across equator to pole, winter to summer, over land and sea and day ...
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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 ...
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14 votes
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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 ...
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13 votes
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How are airspace violations detected?

The transponder usually uses its own pressure reading, not what is set in the pilot's altimeter. So to prevent cheating as you describe, it is inspected and calibrated every 24 months. Tampering with ...
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  • 4,213
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 ...
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  • 10.4k
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 ...
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  • 67.6k
13 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 ...
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  • 25.3k
12 votes

What is the meaning of the crosshatch marking on altimeters?

It replaces to 10 thousands digit, evidently because X9000 has been found to be easier to read than 09000 or just blank space before the 9000. That crosshatched area would also scroll to display "NEG" ...
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  • 41.7k
12 votes

How does an altimeter deal with the non-linear pressure gradient?

It uses a model called The International Standard Atmosphere (ISA) - so may be inaccurate if you don't have one of those to fly through :-)
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12 votes
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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 ...
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12 votes
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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 ...
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