Inertial Altitude (Boeing)
Thread Starter
Join Date: Feb 2004
Location: Australia
Posts: 1,307
Likes: 0
Received 0 Likes
on
0 Posts
Inertial Altitude (Boeing)
I'm trying to better understand this concept and how it is used on later and updated Boeing aircraft.
Traditionally, when a particular autopilot is engaged, the ADC which supplies data to that autopilot is used as a reference and at least one of the pilots' altimeters should match the selected Mode Control Panel altitude.
However, along came "inertial altitude", and now pilots tell me that the autopilot can hold an altitude which agrees with none of the altimeters.
The question is... under what circumstances does this happen?
Is it a short term thing which happens during, say, turbulence?
Is inertial altitude only active in VNAV mode?
Does it only happen when an aircraft reaches a particular altitude and the autopilot locks in a barometric setting, thus allowing a pilot to change his/her baro setting without making the aircraft move up or down?
My engineering training notes are ambiguous at best
Thanks for any insight
Rgds.
NSEU
Traditionally, when a particular autopilot is engaged, the ADC which supplies data to that autopilot is used as a reference and at least one of the pilots' altimeters should match the selected Mode Control Panel altitude.
However, along came "inertial altitude", and now pilots tell me that the autopilot can hold an altitude which agrees with none of the altimeters.
The question is... under what circumstances does this happen?
Is it a short term thing which happens during, say, turbulence?
Is inertial altitude only active in VNAV mode?
Does it only happen when an aircraft reaches a particular altitude and the autopilot locks in a barometric setting, thus allowing a pilot to change his/her baro setting without making the aircraft move up or down?
My engineering training notes are ambiguous at best
Thanks for any insight
Rgds.
NSEU
Join Date: Jul 2008
Location: EDDF
Age: 43
Posts: 119
Likes: 0
Received 0 Likes
on
0 Posts
I think the inertial altitude is the absolute altitude above the earth.
An altitude on a barometric altimeter is actually just a pressure. For example FL200 is the level in the atmosphere with 465 milibar of air pressure. The true distance from that flightlevel to the sea level may be far off 200.000ft. It all depends on the atmosphere. On hot days, with high pressure on the ground, you will fly actually higher than the altimeter indicates, on cold days in an atmospheric depression you will flight lower. But so will everybody else, so you don't hit each other. (But watch out for mountains, they don't play along
)
The inertial altitude is the absolute altitude. So if you fly 200.000 inertial altitude, that is unrelated to the pressure around you, so your altimeter will probably not indicate 200.000 ft.
The big question is: how do you accurately measure inertial altitude?
ATCast
An altitude on a barometric altimeter is actually just a pressure. For example FL200 is the level in the atmosphere with 465 milibar of air pressure. The true distance from that flightlevel to the sea level may be far off 200.000ft. It all depends on the atmosphere. On hot days, with high pressure on the ground, you will fly actually higher than the altimeter indicates, on cold days in an atmospheric depression you will flight lower. But so will everybody else, so you don't hit each other. (But watch out for mountains, they don't play along
)The inertial altitude is the absolute altitude. So if you fly 200.000 inertial altitude, that is unrelated to the pressure around you, so your altimeter will probably not indicate 200.000 ft.
The big question is: how do you accurately measure inertial altitude?
ATCast
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes
on
0 Posts
AFAIK, not on the 747-400. The altimeter rules the autopilot. I have never heard of 'Inertial Altitude' on that plane. There was no way the autopilot would maintain any altitude that did not come from an altimeter. GPS can give an accurate altitude, but it serves no function on an aeroplane. Any inertial derived altitude from measuring vertical acceleration is possible, but not really usefully functional in an aeroplane.
Join Date: Mar 2007
Location: USofA
Posts: 1,235
Likes: 0
Received 0 Likes
on
0 Posts
Rainboe, you might be a little quick there and I don't pretend to know the answer exactly but both the 737NG, with IAN approach capability, 777 and now the 787 have vertical RNP values for approach. I had asked earlier how these were derived but in spite of several attempts to answer this, I'm still a little confused.
Join Date: May 2000
Location: Seattle
Posts: 3,196
Likes: 0
Received 0 Likes
on
0 Posts
Though inertial altitude is liekly calculated continuously in the IRS, I have never seen any reference to the use of "inertial altitude" in any 744 documentation.
It is possible inertial altitude is used as a feed for the IVSI, but I am not certain. In a previous airplane, inertial altitude was used to "smooth" calculations of instantaneous altitude and V/S (to overcome pitot system lag), but were not accurate enough as a sole source.
ADC is still the most accurate and consistent source of altitude information for RNP purposes, though GPS and/or inertial altitude might be used in some airplanes as crosscheck sources.
It is possible inertial altitude is used as a feed for the IVSI, but I am not certain. In a previous airplane, inertial altitude was used to "smooth" calculations of instantaneous altitude and V/S (to overcome pitot system lag), but were not accurate enough as a sole source.
ADC is still the most accurate and consistent source of altitude information for RNP purposes, though GPS and/or inertial altitude might be used in some airplanes as crosscheck sources.
Thread Starter
Join Date: Feb 2004
Location: Australia
Posts: 1,307
Likes: 0
Received 0 Likes
on
0 Posts
There are certainly references to Inertial altitude in the Boeing Maintanence Manual and the Wiring Schematics for both the 767 and 744 (Chapters 22 and 34 for various functions).
That it exists, is fact. I was trying to ascertain how this data is used.
Inertial altitude is used by the FCC's, FMC's and EGPWS.
Here's a reference to Inertial altitude in my 767 training notes (which are more detailed than my 744 notes)...
"The altitude used [by the FCC] is inertial altitude from the IRS with a baro correction added, [which is] based on the difference between 29.92 altitude and baro altitude from the ADC. The baro correction is latched at mode engage, so that subsequent baro correction changes do not affect operation."
To me, this is rather vague. Is the FCC inertial altitude still using 29.92 data from the ADC at all times (plus/minus a memorized height difference) as a long term correction? (Pure inertial altitude will surely be susceptible to long term errors. The IVSI, for example, is fed by the IRU's, but has ADC data for long term correction (The ADC data is processed in the IRU, then sent to IVSI).
With the Left A/P engaged, will the Captain's altimeter (set to STD), say, in cruise, ever vary from the MCP altitude, apart from excursions during turbulence? Or will it only vary if the Captain changes his baro setting?
Hope this makes sense.
Rgds.
NSEU
That it exists, is fact. I was trying to ascertain how this data is used.
Inertial altitude is used by the FCC's, FMC's and EGPWS.
Here's a reference to Inertial altitude in my 767 training notes (which are more detailed than my 744 notes)...
"The altitude used [by the FCC] is inertial altitude from the IRS with a baro correction added, [which is] based on the difference between 29.92 altitude and baro altitude from the ADC. The baro correction is latched at mode engage, so that subsequent baro correction changes do not affect operation."
To me, this is rather vague. Is the FCC inertial altitude still using 29.92 data from the ADC at all times (plus/minus a memorized height difference) as a long term correction? (Pure inertial altitude will surely be susceptible to long term errors. The IVSI, for example, is fed by the IRU's, but has ADC data for long term correction (The ADC data is processed in the IRU, then sent to IVSI).
With the Left A/P engaged, will the Captain's altimeter (set to STD), say, in cruise, ever vary from the MCP altitude, apart from excursions during turbulence? Or will it only vary if the Captain changes his baro setting?
Hope this makes sense.
Rgds.
NSEU
inertial height
Inertial systems suffer from drift but the error in the horizontal components are limited by schuler tuning. The vertical channel is unlimited and will exponentially drift. I read the extract to say that the IRS was corrected by the altimeter readings (at some undeclared periodicy) with the static offset (reading to datum) of the latter determined as a one-off on mode engage (whatever that is).
Join Date: Nov 2007
Location: United Kingdom
Age: 71
Posts: 713
Likes: 0
Received 0 Likes
on
0 Posts
Inertial Altitude can only be referenced to where the INS/IRS was originally 'switched ON'. The Earth isn't a perfect sphere; hence, if you were to compare, for example, two different Earth locations; one at Nairobi and the other at say London, there would be a difference in height of some 5000 feet.
Space vehicles, such as the 'Space Shuttle' will obviously use that extra INS/IRS 'dimension' i.e Inertial Altitude for the calculation of orbits etc. I'm certainly not a rocket scientist, but unless a correction factor can be entered into the height calculation then it couldn't be used for accurate 'height/altitude keeping', because many (all) other aircraft must reference their height/altitude systems to a common reference i.e. Atmospheric Pressure. With 29.92 (1013.2) set, all aircraft are then singing from the same Hymn sheet and won't (shouldn't) bump into each other!
As 'Intruder' suggests, Inertial Altitude data might well be utilised in the 'smoothing' of the IVSI data.
Inertial Altitude is, certainly, a very interesting concept; however, for the reasons previously stated (the Earth not being a perfect sphere), it's use is perhaps only accurate for extra-terrestrial navigation purposes i.e. a trip to the Moon.
TCF
Space vehicles, such as the 'Space Shuttle' will obviously use that extra INS/IRS 'dimension' i.e Inertial Altitude for the calculation of orbits etc. I'm certainly not a rocket scientist, but unless a correction factor can be entered into the height calculation then it couldn't be used for accurate 'height/altitude keeping', because many (all) other aircraft must reference their height/altitude systems to a common reference i.e. Atmospheric Pressure. With 29.92 (1013.2) set, all aircraft are then singing from the same Hymn sheet and won't (shouldn't) bump into each other!
As 'Intruder' suggests, Inertial Altitude data might well be utilised in the 'smoothing' of the IVSI data.
Inertial Altitude is, certainly, a very interesting concept; however, for the reasons previously stated (the Earth not being a perfect sphere), it's use is perhaps only accurate for extra-terrestrial navigation purposes i.e. a trip to the Moon.
TCF
Join Date: Mar 2007
Location: USofA
Posts: 1,235
Likes: 0
Received 0 Likes
on
0 Posts
This is a facinationg subject and I remain somewhat clueless so can some -one explain what Vertical RNP is measuring whenit is utilized during an Integrated Navigation Approach. This is a term coined by Boeing for the IAN capabilty that is available, if not standard in the B737NG and B787 aircraft. Basically there is a 400' wide decent path created that looks like a glideslope and whether it be one mile or twenty-five miles from the MAP, it remains only 400' wide, unlike a ILS glideslope that expands in depth as it moves back from the MAP or runway TDZ. Sort of like a tube that descends to the MAP. This is different than say a VNAV approach and you utilize a glideslope just as you would in an ILS approach. During this approach a visual glidesople along a visual RNP and ANP pointers are presented for both lateral nav and vertical nav.
So the question remains in my mind just what are we looking at when we see vertical RNP values. Might menton that the vertical RNP/ANP scales do not appear until we wre excactly 1,024 feet above the MAP.
So the question remains in my mind just what are we looking at when we see vertical RNP values. Might menton that the vertical RNP/ANP scales do not appear until we wre excactly 1,024 feet above the MAP.
Join Date: Oct 2009
Location: UK
Posts: 1,270
Likes: 0
Received 0 Likes
on
0 Posts
Hi NSEU,
"The altitude used [by the FCC] is inertial altitude from the IRS with a baro correction added, [which is] based on the difference between 29.92 altitude and baro altitude from the ADC. The baro correction is latched at mode engage, so that subsequent baro correction changes do not affect operation."
I think this mode is only available when in "ALT Cruise" (or the equivalent). If you are in "ALT" and change the pressure setting on your Altimeter, the aircraft will follow the Altimeter. Once in "ALT Cruise" the aircraft will not follow an altimeter subscale setting change, but remains at an altitude referenced to the subscale setting when "ALT Cruise" engaged. I think there is some vertical IRS information used in the calculation.
"The altitude used [by the FCC] is inertial altitude from the IRS with a baro correction added, [which is] based on the difference between 29.92 altitude and baro altitude from the ADC. The baro correction is latched at mode engage, so that subsequent baro correction changes do not affect operation."
Join Date: Sep 2009
Location: Europe
Age: 78
Posts: 48
Likes: 0
Received 0 Likes
on
0 Posts
Hope the following two exerpts explain principles of enhancing altitude measurement and their inflight application.
1. Integrated approach navigation system, method, and computer program product
Systems, computer program products, and methods for displaying navigation performance based flight path deviation information during the final approach segment to a runway and during landing of non-precision flight modes are provided. Improved graphical depictions of navigation performance based flight path deviation information provide pilots and flight crew members with clear, concise displays of the dynamic relationship between ANP and RNP, mode and aspect of flight and related procedures, intersecting flight paths, and current actual flight path deviation from a predefined flight path during the final approach segment to a runway and during landing. For example, an enhanced IAN display may include NPS-type deviation scales to show RNP/ANP relationships and predetermined RNP markers to alert the pilots and flight crew members that the FMC has transitioned from an NPS display for RNAV (LNAV/VNAV) flight procedures to an enhanced IAN display for a non-precision (non-xLS) approach and/or landing.
2. COMPENSATION OF PRESSURE SENSOR ERRORS IN BARO-INERTIAL ALTITUDE DATA
A system and method for improving the accuracy of altitude determinations in an inertial navigation system. The system utilizes pressure measurements which are taken by a barometric altimeter and converted into an estimated pressure altitude using any known pressure-to-altitude conversion. The estimated pressure altitude is then converted into a pressure correction value using a correction value generating formula that is a function of altitude. The pressure correction value is then multiplied by a pressure offset value for the barometric altimeter to generate a pressure offset error for the barometric altimeter. This pressure offset error is used in the present invention to modify the altitude estimation in order to generate an altitude determination having an improved accuracy. The present invention further determines an amount of observation noise in the barometric altimeter that is a function of pressure noise and altitude, where the altitude estimation is further modified to account for the observation noise.
1. Integrated approach navigation system, method, and computer program product
Systems, computer program products, and methods for displaying navigation performance based flight path deviation information during the final approach segment to a runway and during landing of non-precision flight modes are provided. Improved graphical depictions of navigation performance based flight path deviation information provide pilots and flight crew members with clear, concise displays of the dynamic relationship between ANP and RNP, mode and aspect of flight and related procedures, intersecting flight paths, and current actual flight path deviation from a predefined flight path during the final approach segment to a runway and during landing. For example, an enhanced IAN display may include NPS-type deviation scales to show RNP/ANP relationships and predetermined RNP markers to alert the pilots and flight crew members that the FMC has transitioned from an NPS display for RNAV (LNAV/VNAV) flight procedures to an enhanced IAN display for a non-precision (non-xLS) approach and/or landing.
2. COMPENSATION OF PRESSURE SENSOR ERRORS IN BARO-INERTIAL ALTITUDE DATA
A system and method for improving the accuracy of altitude determinations in an inertial navigation system. The system utilizes pressure measurements which are taken by a barometric altimeter and converted into an estimated pressure altitude using any known pressure-to-altitude conversion. The estimated pressure altitude is then converted into a pressure correction value using a correction value generating formula that is a function of altitude. The pressure correction value is then multiplied by a pressure offset value for the barometric altimeter to generate a pressure offset error for the barometric altimeter. This pressure offset error is used in the present invention to modify the altitude estimation in order to generate an altitude determination having an improved accuracy. The present invention further determines an amount of observation noise in the barometric altimeter that is a function of pressure noise and altitude, where the altitude estimation is further modified to account for the observation noise.
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes
on
0 Posts
It's all very well having 'enhancing altitude measurement' for those aeroplanes fitted with inertial, but you need to have every aeroplane in the sky fitted with it, or none, surely? Seems little point in having enhanced altitude measurement for some only!
Why do opposite direction aeroplanes 1000' above always show on the TCAS as +900'? Are we always flying 100' above our true pressure altitude?
Why do opposite direction aeroplanes 1000' above always show on the TCAS as +900'? Are we always flying 100' above our true pressure altitude?
Join Date: Mar 2001
Location: I wouldn't know.
Posts: 4,499
Likes: 0
Received 0 Likes
on
0 Posts
This is a facinationg subject and I remain somewhat clueless so can some -one explain what Vertical RNP is measuring whenit is utilized during an Integrated Navigation Approach. This is a term coined by Boeing for the IAN capabilty that is available, if not standard in the B737NG and B787 aircraft. Basically there is a 400' wide decent path created that looks like a glideslope and whether it be one mile or twenty-five miles from the MAP, it remains only 400' wide, unlike a ILS glideslope that expands in depth as it moves back from the MAP or runway TDZ. Sort of like a tube that descends to the MAP. This is different than say a VNAV approach and you utilize a glideslope just as you would in an ILS approach. During this approach a visual glidesople along a visual RNP and ANP pointers are presented for both lateral nav and vertical nav.
So the question remains in my mind just what are we looking at when we see vertical RNP values. Might menton that the vertical RNP/ANP scales do not appear until we wre excactly 1,024 feet above the MAP.
So the question remains in my mind just what are we looking at when we see vertical RNP values. Might menton that the vertical RNP/ANP scales do not appear until we wre excactly 1,024 feet above the MAP.
The vertical path and the vertical RNP/ANP scales are calculated using baro altimetry and it is therefore very important to set the current QNH in the descent forecast page as the glidepath will be calculated using that value. Even 1 hPa difference will mean around 30ft difference above the runway which can become a problem in marginal weather or an annoyance in good weather when you deviate visually to the correct glidepath and get a "GLIDESLOPE GLIDESLOPE" callout from your EGPWS.
Join Date: Mar 2001
Location: I wouldn't know.
Posts: 4,499
Likes: 0
Received 0 Likes
on
0 Posts
Deviation scales are proportional to RNP. Similar to ILS localizer and glide slope deviation scales, the IAN deviation scales become more sensitive as the airplane approaches the runway.
We do have that 400' number as well, however that is the vertical RNP of all VNAV flight operations until the start of the approach, ie. the whole descent path from top of descent until approach intercept. For lateral RNP it depends on airspace, flight regime and selected approach, but usually is 2NM during cruise, 1NM on departue/arrival procedures and 0.5NM on transitions to approach, non precision approaches are 0.15 or 0.10 depending on type of approach.
Thread Starter
Join Date: Feb 2004
Location: Australia
Posts: 1,307
Likes: 0
Received 0 Likes
on
0 Posts
Sounds like the periodic update of altitude is correct. Here's the latest from a 744 pilot (hope he doesn't mind me quoting him)...
"In VNAV I selected 1013 on the altimeter to tie up with standard then lowered the QNH to make the altimeter under read, by about 100ft, the aircraft slowly climbed back up to the bug. I put the aircraft in ALT HOLD and wound off some more QHN with the same result. As it was climbing up to reach the magenta bug I selected VS 0 and the aircraft leveled below the bug, once it was level changing QNH had no affect the aircraft remained level. I didnt try FLCH for question 2 as it was already hunting for the bug in alt hold, FLCH would have just done exactly the same and regained the selected altitude."
Thanks for all the comments.
Cheers.
NSEU
"In VNAV I selected 1013 on the altimeter to tie up with standard then lowered the QNH to make the altimeter under read, by about 100ft, the aircraft slowly climbed back up to the bug. I put the aircraft in ALT HOLD and wound off some more QHN with the same result. As it was climbing up to reach the magenta bug I selected VS 0 and the aircraft leveled below the bug, once it was level changing QNH had no affect the aircraft remained level. I didnt try FLCH for question 2 as it was already hunting for the bug in alt hold, FLCH would have just done exactly the same and regained the selected altitude."
Thanks for all the comments.
Cheers.
NSEU
