What part of the aircraft is the altitude measured from?
Posted by GoHuskertrading@reddit | aviation | View on Reddit | 31 comments
So, this is essentially a random technical question, and I thought this subreddit would have the right people to answer it.
When the instruments in the cockpit display an altitude, from which part of the aircraft is the altitude measured?
The reason I ask this is that I recently watched a video of two aircraft, an ERJ-175 and a 777, landing simultaneously on parallel runways. They were at the exact same altitude, with both having their landing gear extended. Considering their different ground clearances, engine sizes, and hull frame sizes, even though they appear to be at the same altitude visually, would the instruments still show a different altitude?
I was on a recent trip and included this picture as thought I looked cool and hope you all enjoy.
Usernamenotta@reddit
Aircraft engineer here.
You first have the radar altimeter, which works mostly during landings and it is not working during cruise flight. It uses radio waves reflected by ground to tell you at what HEIGHT the belly of the aircraft is.
GNSS altitude. Altitude determined from GNSS signals, be it directly from constellations or from augmentations. It is still debated whether it's accurate enough for procedures (and it's even more questionable in the era of GNSS interference). It measures the height between the virtual center of mass of the aircraft and the virtual 'contour' of WGS84/ whatever ellipsoid the constellation is using,
On board computer assisted by pressure and temperature sensors. Technically, the basic instrument is the static pressure sensor. This was the sole tool to determine the altitude back in the day, and it still is the sole tool in many General aviation aircraft (or older airframes that are still flying). You would typically have 2 ports (for redundancy) on each side, and the value indicated would be the barometric altitude at which the ports exist.
However, those measurements are not precise enough for systems like autoland, or even RVSM. This is because 1. At high altitudes and speed, the effect of the airflow and temperature over what air is going into the static port is no longer negligible 2. There are multiple sets of pressure ports on the aircraft and the values indicated by them differ.
So the way you deal with this is that you have an on-board computer, which calculates the barometric altitude adjusted for temperature at each static port, and the computer also has a mathematical model of the aircraft, complete with its virtual center of mass (CoM) and the positions of the static probes relative to that CoM. So the computer filters the readings from each port, puts them into a voting algorithm to ensure redundancy and then takes the values he deems fit into account in computing the barometric altitude for the virtual CoM of the aircraft. [note, I do not remember exactly if it's actually the CoM or the geometrical center of the aircraft. Ideally it should be the same, but CoM varies during the flight]
I could go into more detail, but it's almost 1 am here and I'm starting to get dizzy after a full day at work. If you spot a mistake or want more details, leave a comment and I will answer tomorrow evening
A_Tangential_Phase@reddit
This post is so confidently worng... Rad Alt gives distance to closest thing generally under the aircraft. It only actually gives height over flat ground. GPS altitude is measured to the antenna location, not some "virtual center of mass". Pressure altitude is what RVSM is based on. Don't spread misinformation.
Omgninjas@reddit
Fellow Aviation Engineer!!!
This is a great explanation, and I can add some more details for those curious. Though please note I'm a business jet guy so some of this will be different for commercial. They have different rules and what not.
Nothing really to add. Radar altimeters are solely for the distance the aircraft wheels are above the ground directly below the aircraft.
GPS altitude is never a primary source of altitude to the pilot, but there are GPS approaches like LPV that do allow the aircraft to fly pretty low based off the 3D position calculated based off the GPS. LPV approaches generally (not always though) let you get all the way down to 200 feet above the runway before you need a visual confirmation of said runway to complete the approach.
Also the FAA is using ADS-B GPS altitude data to verify RVSM (Reduced Vertical Seperation Minimums) altitude reported by the ADCs (Air Data Computers) for all aircraft. If your GPS Alt and Pressure altitude differ too much the FAA sends you a letter and you have to get it corrected, or be barred from flying into RVSM airspace.
If anyone has any more Avionics questions ask away.
AI-Coming4U@reddit
Thanks for an amazing answer at 1 am! Had no idea it was this complex behind the scenes.
kazak9999@reddit
One of the best geekouts on Reddit Ive ever read. Clear, concise, complete. Thank you!!
Shadowrend01@reddit
There’s 2 ways to do it
The Radar Altimeters are mounted on the belly skin and measure from there. They aren’t 100% accurate and only work up to a certain altitude
The Barometric Altimeter uses air pressure from outside the aircraft compared against air inside a sealed capsule. It’s mounted in an instrument inside
WLFGHST@reddit
The Barometric Altimeter is usually mounted about half way up the fuselage, or on the lower half; to answer OP's question.
No-Level5745@reddit
To be the most accurate, the altitude is not read at the static port location, but rather where the pressure sensor is located (the hydrostatic column of air continues to reflect altitude throughout the static line tubing). It could be higher or lower than the port depending on aircraft attitude and installation
WLFGHST@reddit
Oh dang I actually didn’t know that (or ever even think about that), the more you know!
Negative-Box9890@reddit
Also, Rad Alt is only active from approximately 2300ft AGL.
GregTheIntelectual@reddit
Its worth noting that 5g networks can mess with the radio altimeter readings.
There's airworthiness directives right now that will address the issue with frequency filters or upgrades but for the time being if your aircraft isn't 5g compliant your rad altimeter might give fault readings around cities.
eightstravels@reddit
There was a big scare around that (work in telecom, heard about it very intensively for a few months leading up to Cband launch) but as memory serves they couldn’t actually produce any case studies/real world examples of interference. There was conjecture that it would be possible, but only if the radalt was operating way out of the band it was supposed to (aka was already malfunctioning, albeit possibly in a way that was still giving good readings to the end user and was thus unnoticed)
chaosattractor@reddit
This is a bit misleading; the radalt is the most accurate altitude sensor on the plane within the range it's supposed to be used in, but its accuracy drops off a lot beyond that.
cjhubbs@reddit
Yeah RadAlt is the gold standard up to about 2500’ AGL.
athlaka916@reddit
Radio altimeters are only really used in sub cat 1 approaches I believe
chaosattractor@reddit
You know it's the radio altimeter that's hooked up to the (E)GPWS callouts on final approach (and also just in general for terrain avoidance), yeah?
Specialist_Reality96@reddit
Where ever the staic ports are mounted there are usually a number of them across the aircraft, although typically barometric altitude is not accurate within 10-20 ft and very much depends on if the barametric pressure is set to QNH, QFE or QNE.
Close to the ground a combination of visual cues like VASI and radar altimeter is used, even on a instrument approach if the runway cannot be seen at 50ft a go around is initiated.
CounterSimple3771@reddit
The bottom.
Independent-Reveal86@reddit
What hasn't been mentioned about the radar altimeter is that it is calibrated to read 0' when the aircraft is on the ground, so in that sense it is measuring the distance between the ground and the bottom of the wheels. The B777 and ERJ-175 should both show 0' on the RADALT when on the ground.
escape_your_destiny@reddit
This is not quite true. The radio altimeter are calibrated to show 0 in landing configuration, right as the wheels would touch down. That also includes the pitch angle in landing config.
On the B777 the rad alt antennas are just aft of the nose gear, so with pitch angle they are higher off the ground.
Sitting on the ground, the B777 will usually show -2 or -4 for rad alt, because of main gear strut compression and without the landing pitch angle.
Here is a cockpit shot showing the rad alt reading -4
SnooMaps7370@reddit
this also applies to modern LPV-capable GPS navigators. When i installed my Avidyne unit, it needed to be told the height of the antenna, so that it correctly reads 0' AGL when on the ground.
Independent-Reveal86@reddit
Thanks I wasn't entirely sure how it was calibrated with reference to landing attitude.
Essunset@reddit
Two systems. A Pitot Static system and a Radio Altimeter.
Pitot static uses probes on the front of the engine that are essentially pressure transducers. One probe is recessed into the plane, while the other sticks out from the side and measures air as it goes by. The airplane compares these values to come up with a corrected airspeed and altitude.
The Radio altimeter sends and receives radio waves from antennnas near the tail. These radio waves bounce off the ground and the plane measures the delay in time to calculate its altitude. If I recall, the airplane only uses radalt at about 1500ft. I could be wrong , and I probably am on that
GregTheIntelectual@reddit
1500ft is about right for older systems, but newer radio altimeters like ALT4000+ and ALT1000+ can go up to 2500ft. Supposedly, that's they say on the tin anyways.
Most people upgrading them are just doing so for 5g compliance probably.
Darksirius@reddit
The pitot tubes are usually mounted on the front of the fuselage, on the sides under the front windows.
Same with the AoA sensor.
Essunset@reddit
Correct. I meant front of fuselage, not engines
ZZ9ZA@reddit
Unless you’re talking about the radar altimeter, which only works at low altitudes (and doesn’t actually report altitude anyway) they just aren’t that precise. They work off air pressure. I doubt actual accuracy is between than +/- 20ft or so. The altitude readout on virtually all jets will show a zero all the time in the last digit or two of the altitude readout.
chaosattractor@reddit
Radio altimeters do report altitude, I'm a bit confused about what you mean
Like sure it's corrected so that it reads zero when the aircraft is on the ground (versus reading [height of the landing gear]), but still
Usernamenotta@reddit
Radio altimeter typically reports HEIGHT, not ALTITUDE.
Height is the distance from the ground, which varies in space. Altitude is determined according to the Mean Sea Level. (Or to WGS84/whatever mathematical model GNSS uses)
If you want to land, what you are typically interested is height, so you don't come too hard down.
If you navigate at cruise level, you typically want altitude, so you and everyone around you has the same reference.
chaosattractor@reddit
"HEIGHT, not ALTITUDE" is a senseless phrase because altitude is a term that encompasses many different things and "height above ground level" (also known as...absolute altitude) is one of those things. "Altitude" is not determined according to the mean sea level, elevation (true altitude in aviation terms) is.
At cruise level aircraft use barometric/pressure altitude, not any geometric altitude whether absolute or true. And no, pilots are not only interested in height "so they don't come too hard down", they are interested in it because the vast majority of the world's airfields are not in fact at sea level. Obviously things like the altitude gate for a stabilised approach are defined relative to the airfield's elevation.
agha0013@reddit
There are different systems for different needs.
When landing, or near the ground there is a radar altimeter for very accurate measuring, it provides info to the pilots as well as the ground proximity warning system and the flight computer.
For routine stuff the altimeter a use the static pressure system that has a few ports along the fuselage and provide you with altitude based on air pressure. That system requires you to calibrate it using local air pressure settings when below a certain altitude. Or when above a certain altitude it is set to a standard pressure setting. That way all the places in a certain chunk of airspace are all looking at the same thing.
Forgetting to adjust the altimeter pressure setting can throw your altitude off enough that you could potentially be in conflict with other traffic.
That static system is not accurate enough for zero visibility landings so the radar altimeter steps in.
These days there's probably also a bunch of GPS bases altitude measuring if you have enough satellites in sight