Questions tagged [altimeter]

The instrument which indicates the aircraft's altitude, usually based on atmospheric pressure, but sometimes on radar or GPS data instead.

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How did the X-15 measure altitude above 86 km? [duplicate]

The 1962 standard atmosphere ended at 86 km (282,000 ft MSL), didn't it? How high was the X-15's altimeter able to work and how did it measure the altitude when going higher than the Standard ...
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4 votes
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Is the ISA still up to date?

The Standard Atmosphere dates back to 1962 when Earth was colder. The 1976 update didn't make changes within 50 km altitude (the tropo- and stratosphere). It assumes 15°C at sea level, decreases until ...
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If you are in a terrain escape maneuver, would you be able to see traffic above you displayed in the cockpit of airliners [closed]

nayed If you are in a terrain escape maneuver, would you be able to see traffic above you in the cockpit of big airliners
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6 votes
3 answers
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How many times does a pilot usually change local pressure while landing or departing?

The local air pressure (QFE, and therefore QNH as well) may change throughout the day. ATC will advise pilots of the new altimeter setting on a regular basis. How often do pilots truly update this ...
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5 votes
2 answers
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What is the legally accepted tolerance for altimeter errors under EASA for a VFR flight?

I have a discussion going on with a flying club member. He states that altimeters may have an error margin of up to +/- 200ft. Example: ATC instructs you to fly 1500 feet, you would still stick to ...
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What is that green rectangle next to altitude readout on Boeing aircraft?

Does anyone know what this green rectangle represents? It's from a Boeing 777 PFD. I can't find answers anywhere.
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Difference between "pressure levels raising on warm days causing lower IA" and "TA being higher than IA on warm days" [duplicate]

IA: Indicated altitude TA: True altitude I have seen this picture before: Concept 1 and I understand this. On cold days, the pressure gradient is shorter which means the indicated altitude at a ...
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Which radar altimeters are susceptible to 5G C-band altimeter interference?

There is apparently a great deal of consternation over the pending expansion of 5G The emergency petition claimed that interference from the C-Band "will cause irreparable harm and jeopardize ...
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What does CPLD mean in aviation in general and on this altitude alerter?

I saw the shortcut CPLD on this altitude alerter from a Cessna 210, and could not work out what it could possibly mean: What does it mean on this altitude alerter, ...
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13 votes
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What is the purpose of the leaf spring in an altimeter?

What is the use of the leaf spring present around the partially evacuated aneroid capsule of sensitive altimeter.
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What is the minimum and maximum altitude reading for the average radar altimeter?

The valid range for latitude is -90 to 90 degrees. The valid range for longitude is -180 to ...
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What kinds of altimeter are used in modern airliners?

What's the mechanism of the altimeters in big jets?
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What are the three different altitudes shown in the Perlan II?

The Perlan II glider has two altitude indicators and an altimeter on the navi screen, each showing different altitudes: How is each of the altimeters different? One indicator certainly shows MSL and ...
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Is a balloon's record altitude measured from the top of canopy, the bottom of the basket, or some other point?

I wonder where exactly on the balloon is its altitude measured. By the time a space balloon reaches its maximum altitude it is extremely large, the higher the larger. The record altitude for a balloon ...
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Is there a place where altitude while flying is negative?

while testing and working with avionics systems I have never seen an altitude near 0 or negative. Is there a place when flying above the Altitude tape shows -ve value ? Maybe while flying between ...
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If a plane was put into a vacuum chamber, what would its altimeter read? [duplicate]

Just curious. What if you put a (general aviation or airliner) plane into a hard vacuum chamber? Would its altimeter simply read the highest possible altitude it could? That's rather impossible for ...
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How can I compute indicated altitude?

I would like to compute what an altimeter would display given a true altitude and atmospheric conditions. Specifically: Given: True altitude, in feet Outside air temperature, in degrees C Local ...
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Station pressure vs. reported altimeter

I must be missing something big. Page 12-5 of the PHAK tells me that a station at 5,000 feet above sea level would convert its 24.92 pressure reading to 29.92. I understand that this helps to track ...
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What does setting an altimeter actually do to the altitude?

I have looked at probably 10 different webpages and re watched the Sportys video on altitudes, multiple times, but I have a very fundamental question, which I am unable to wrap my head around. Is my ...
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37 votes
1 answer
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Why does a radar altimeter show negative values on the ground?

This applies to the Airbus 320 or Boeing 737, among others: Although it is useless on ground, the radar altimeter will repeatedly display a value (including negative) for a specific runway and airport ...
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Is the true altitude always lower than the indicated altitude as long as a correct local altimeter (QNH) is set on the altimeter?

Given the fact that the altimeter setting is corrected for the temperature of the airport, on the condition that a correct local altimeter setting (QNH) has been made to the altimeter, is the true ...
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Is there a regulation that defines when to use QFE or QNH for landing?

Is there an EASA regulation that says to use QFE or QNH when landing?
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Is this vintage altimeter from the Cuatro Vientos?

My father who was a pilot, passed away some years ago and I have a vintage altimeter that he told me someone gave him. They assured him that it belonged to the Cuatro Vientos airplane, but I'm not ...
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4 answers
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Why doesn't temperature affect Pressure Altitude?

In this picture: it states the tops and bottoms of the 3 columns of air have the same pressure. This explains the error in altitude indications given in a non-standard temperature. Pressure only ...
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14 votes
2 answers
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Why can't you just set the altimeter to field elevation?

Why do you have to put the current altimeter setting in the kollsman window? Why can't you just set it to field elevation when you're on the ground? I understand why you would have to do it in the air....
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4 votes
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Which altimeter setting is used to define airspace boundaries?

Let's say that we're flying VFR over the top of Class D airspace at 3,000 feet. The Class D extends up to 2,900 feet - we're above it, but just barely with a 100' buffer. We're also talking to ...
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Calculation of temperature correction for altimeter

I try to calculate the correction of cold temperature for altimeter , i use for this the equation 24 of the Engineering Sciences Data Unit (ESDU) publication but it give me very high number . if i ...
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3 votes
1 answer
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If an aircraft has two altimeters what is the allowed difference between them?

If an Altimeter must be within +/- 75 of known field elevation, and the aircraft has two altimeters, one could be +74 while the other could be -74 and both would be within 75 of known field elevation, ...
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Why are the airport pressure values for "altitude" (inHg) and "sea level" (mb) slightly different on the NOAA web sites?

If one looks, for example, at the NOAA web site for Denver International Airport we see (at the time of posting this question) the Pressure/Altimeter is 29.78 inHg, the Pressure/sea level is 1006.1 mb ...
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Relation between altimeter setting (QNH) formula and the barometric formula

I'm searching for a reliable source for a formulization of the QNH adjustment to the barometric altitude. I found this document from weather.gov that gives a formula for the Altimeter Setting, which ...
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4 votes
1 answer
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Altimeter setting and pressure altitude

Student pilot here. I thought I was smarter than this, I’m struggling with this concept. I got the ASA test prep book and there’s a question asking what happens to the indicated altitude when the ...
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How is altitude reached by aircraft flying above the stratosphere measured?

Aircraft flying above the stratosphere (above 30km) are rare, but there is still the X-15 and the spaceship two. For those aircraft, maximal altitude is measured to within 500 ft. As described in this ...
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Why did the altimeter in an F-16 jump suddenly as it passed through Mach 1?

In this video, Destin — the creator of YouTube channel Smarter Every Day — rides in an F-16 with a USAF Thunderbirds pilot. If you start watching the video at the 11:06 mark, you will see what happens ...
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1 vote
3 answers
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IFR requirement for altimeter question (i.e. +/- 75 feet) [closed]

What if the ATIS/AWOS/ASOS results in a very low or very high altimeter setting (i.e. barometric pressure)? Surely this setting will result in an altimeter setting way outside the envelope of the +/- ...
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3 votes
2 answers
769 views

How is Altitude/QNH defined state when passing transition altitude?

Let's say an airplane is climbing towards transition altitude (TA = 5000 ft), and it has it's altimeter set to QNH = 1030 hPa. ...
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2 votes
2 answers
547 views

What is the main reason why cruise flight in the transition layer is prohibited?

What is the main reason why cruise flight in the transition layer is prohibited? For example if we have a separation of 1000 ft between transition altitude and transition level, why should we not ...
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8 votes
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Who is responsible to check for radar-altimeter disagreement?

I have searched the site but failed to find a similar question. I know that modern civil aircraft are equipped with multiple radar altimeters. Question is: In the unlikely event of those outputting ...
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2 votes
2 answers
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What does the altimeter indicate if it's at a certain elevation

The purpose of the altimeter is to show the pilot the altitude and the pressure above a fixed level. It uses barometric pressure and is 29.92Hg in ISA conditions. What I dont understand is, how can ...
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2 votes
1 answer
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Why does the QNH of an aircraft change or need to be changed?

This is mainly a 3 part question. First, is it the QNH of of the aircraft? of the aerodrome? of the region or area? Or am I understanding it wrong my looking at it like that? If the QNH is set ...
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2 votes
2 answers
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Why is ISA Standard pressure 1013.2 not used for contour charts instead of QFF?

I am confused because QFF being actual mean sea level pressure is subject to change why would we use it for mapping terrain? Does this mean the height of a given terrain changes with the change in QFF?...
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10 votes
6 answers
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What would the altimeter indication be through a cold front?

Cold air is heavier than hot air because of its greater density, so the QNH of an airport inside a cold front would be higher. Flying towards the cold front would be, "low to high, hello sky". But I ...
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6 votes
1 answer
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Altimeter shows 100 foot drop when power is added. Is this a leak in the static system or broken altimeter and how long will the fix take?

My instructor and I were preparing for my Private Pilot Check ride today when we noticed that the analog altimeter would suddenly lose 100 feet when we added power. The only way I can think to ...
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4 votes
1 answer
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How do altimeters cope with varying lapse rate?

How do barometric altimeters measure altitude above atmosphere datum if the laps rate of the pressure is not uniform, not constant? Imagine these lines representing pressure lapse rate: ...
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6 votes
1 answer
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Why are/were three-pointer altimeters so prone to being misread?

The three-pointer dial altimeter (with a shortest pointer indicating tens of kilofeet, a short pointer indicating kilofeet, and a long pointer indicating hectofeet), the most instantly-recognisable ...
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If the 10000ft pointer is reading 5 (50000ft) when you are at 10ft how do you get it back to zero [closed]

Kollsman altimeters 10000ft pointer adjustment
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Vintage army Kollsman altimeter - any info?

I have a vintage Kollsman altimeter A.C. U.S. Army. I can't read the text on the front because of the needle arms are in the way, but might state: Type 5 Serial No A 464. Back plate is very hard to ...
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How do barometric altimetry systems compensate for nonstandard pressure lapse rates?

Barometric altimetry relies on atmospheric pressure dropping at a steady rate with altitude, starting from a measured ground-level pressure. This, in turn, relies on atmospheric density dropping at a ...
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1 vote
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How does the mounting surface flatness affect radio altimeter reading?

It is recommended that the antennas be installed on flat surfaces. For some UAVs, the bottom of the fuselage is very narrow and sharply curved away from the antennas. How does it affect the ...
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3 answers
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Why, and how, are barometric altimeters accurate enough for approach and landing?

The barometric altimeter is used as the primary altitude indicator during approach and landing right up until the aircraft crosses the threshold (after which the radar altimeter takes over and makes ...
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5 votes
3 answers
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What’s the setting range altimeters can handle?

What’s the setting range altimeters can handle? Is there a standard requiring a certain range for aircraft to be certified? Are there many occasions where a QNH is so high or low that airplanes have ...
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