All About Altitude, not attitude
UnderTow
  Posted: Sep 25 2006, 03:41 PM


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I'm just about sick of going over and over and over this. But I must remember not everyone has read everything I have read. And we are all very different people with different lives, habits, and abilities.

So let's start with the basics from the source.

For Kollsman Specifications go here:
Kollsmans Inc Instruments

You will find several documents, let's look at the Servoed Altimeter, since this is the one most likely used by commercial cargo fleet.

A41322 PDF
Accuracy: +- 20 Feet

I will continue my break down later. Work calls...
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Russell Pickering
Posted: Sep 25 2006, 06:17 PM


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UT,

Thank you!

This is an area I obviously have tons to learn. One of the things about looking into all of this is that it requires multiple specialties. The learning curve can be harsh.

As part of this thread maybe you could post what the .csv file says about altitude, pressure and heading for the first and last minute of the flight. That would help very much since I am developing some graphics (if they can be recovered from my hard drive).

I am looking forward to your developing this thread.

Russell
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johndoeX
Posted: Sep 25 2006, 07:20 PM


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QUOTE (UnderTow @ Sep 25 2006, 11:41 AM)


You will find several documents, let's look at the Servoed Altimeter, since this is the one most likely used by commercial cargo fleet.

A41322 PDF
Accuracy: +- 20 Feet


This is the type of altimeter found in transport category aircraft.


Highly accurate.
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johndoeX
Posted: Sep 25 2006, 07:23 PM


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QUOTE (Russell Pickering @ Sep 25 2006, 02:17 PM)
UT,

Thank you!

This is an area I obviously have tons to learn. One of the things about looking into all of this is that it requires multiple specialties. The learning curve can be harsh.

As part of this thread maybe you could post what the .csv file says about altitude, pressure and heading for the first and last minute of the flight. That would help very much since I am developing some graphics (if they can be recovered from my hard drive).

I am looking forward to your developing this thread.

Russell

Russell.. dont you have the .csv file and the animation?

If not.. the csv file can be downloaded here
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Russell Pickering
Posted: Sep 26 2006, 03:43 AM


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off-topic

This post has been edited by UnderTow on Sep 27 2006, 01:55 AM
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UnderTow
Posted: Sep 26 2006, 03:49 AM


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off-topic

This post has been edited by UnderTow on Sep 27 2006, 01:55 AM
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Russell Pickering
Posted: Sep 26 2006, 07:43 AM


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off-topic

This post has been edited by UnderTow on Sep 27 2006, 01:56 AM
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Terry Zarzoff
Posted: Sep 26 2006, 10:43 AM


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You do realise that the altimeter in the cockpit, per se, plays no part in the DFDR altitude data?

Yes, there is data about what QNH was set in the Kollsman window - but the DFDR was taking its pressure altitude data from the Air Data Computer.

What is important is the sensor system that was providing information to the ADC, and the ADC itself.

Some background reading for you:

http://www.ssec.honeywell.com/pressure/datasheets/AN104.pdf



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UnderTow
Posted: Sep 26 2006, 01:26 PM


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Actually that is somewhat incorrect. The ADC, Alitmeter Gauge, FDR, (and other sytems) are linked together and cross check each other with reliabilty signals every second.

And from your document
QUOTE
The SAE defines a minimum performance standard for primary air data computer accuracy (3), as measured at room temperature. At 50,000 ft, the allowable tolerance is +/-125 ft, which converts to 0.010 psi (0.020 inHg). For a 17 psi full scale transducer, this is 0.059% of full scale. At 5,000 ft, the allowable tolerance is +/- 25 ft; equivalent accuracy is 0.012 psi (0.024”Hg), or 0.068% of 17 psi.

(3)“Air Data Computer - Minimum Performance Standard,” SAE Aerospace Standard AS8002, Rev A, Sept. 1996.


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UnderTow
Posted: Sep 26 2006, 02:08 PM


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One factor discussed in ARINC Standards is Accuracy

This is the total accuracy of the input data before recording as described in Section 4.1.1 of ARINC 573 and ARINC 717. Some factors included in the accuracy figure are:
-- Sensor tolerances.
-- FDAU tolerances.
-- Non-linearities of the analog to digital conversion process.

For example:
Parameter Range || Accuracy (±)
x < 10000 ft || 20 ft
10000 < x < 30000 ft || 25 ft
x > 30000 ft || 30 ft


Another factor discussed by ARINC is Transport Delay.
QUOTE
This refers to a range of possible acquisition channel delay times, which are defined as the time, in milliseconds, between the input sampling at the sensor and the output for recording.


Example:
QUOTE
For example, in the case of an ARINC 429 parameter that is broadcast 5 times a second and acquired and recorded 4 times a second, the delays consist of the fixed delays introduced by the electronics of the ARINC 429 source, the acquisition unit and the recorder, and also a variable delay of 0 to 200ms introduced by the broadcast nature of ARINC 429 bus. A DC-analog parameter recorded in the same system, however, might only have a much smaller and fixed delay introduced by the acquisition unit and the recorder.


----
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johndoeX
Posted: Sep 26 2006, 03:35 PM


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Redundancy is the backbone of every aircraft.

Nice job UT... thumbsup.gif
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Terry Zarzoff
Posted: Sep 27 2006, 10:31 AM


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QUOTE (UnderTow @ Sep 26 2006, 11:26 PM)
Actually that is somewhat incorrect. The ADC, Alitmeter Gauge, FDR, (and other sytems) are linked together and cross check each other with reliabilty signals every second.

And from your document
QUOTE
The SAE defines a minimum performance standard for primary air data computer accuracy (3), as measured at room temperature. At 50,000 ft, the allowable tolerance is +/-125 ft, which converts to 0.010 psi (0.020 inHg). For a 17 psi full scale transducer, this is 0.059% of full scale. At 5,000 ft, the allowable tolerance is +/- 25 ft; equivalent accuracy is 0.012 psi (0.024”Hg), or 0.068% of 17 psi.

(3)“Air Data Computer - Minimum Performance Standard,” SAE Aerospace Standard AS8002, Rev A, Sept. 1996.

A beg to differ, but you are somewhat incorrect.

The ADC feeds the data to the altimeter and to the DFDR. In case of ADC failure, there are backup modes.

The altimeters can both fail entirely and it will not affect the DFDR Pressure Altitude data unless the ADC or the DFDAU fail / are in error.

Excerp from the B757 manual:

QUOTE
AIR DATA SYSTEM

General

The air data system consists of a pitot static system (see Pitot Static Schematic), one temperature and two angle of attack probes, two air data computers and electric flight instruments. Standby airspeed and altimeter indicators are also provided.

Sensed air data is provided to the air data computers. Processed pressure and temperature information from each computer provide input signals to certain flight instruments (Electric Mach/Airspeed Indicator, Electric Altimeter) and other using systems (IRS, AFDS, FMC, FDR, EEC, TMC, GPWS, ATC and others). The Captain's instruments use the L ADC and the F/O's instruments use the R ADC. The opposite ADC is available as an alternate source.

Unreliable air data signals cause warning flags to appear on the air data instruments. When a malfunction occurs in instruments with failure monitors warning flags appear.

Two test switches on the accessory panel are provided for ground air data computer testing. They are inoperative inflight. For further information on this test, see Supplementary Normal Procedures.

Mach/Airspeed Indicator

Each Mach/Airspeed Indicator displays airspeed, mach and Vmo from the selected air data computer. The command airspeed bug on each Mach/ Airspeed Indicator can be automatically positioned from the FMC CDU or manually from the MCP IAS/MACH Selector.

Standby Airspeed Indicator

A Standby Airspeed Indicator is installed on the center panel. The instrument is connected directly to the R AUX pitot and the alternate static ports.

Altimeter

Each pilot is provided with a Primary (electric) Altimeter that receives altitude signals from the selected air data computer. An Altimeter Altitude light and an Altitude Alert Light are provided for altitude alerting. The Automatic Flight chapter describes altitude alerting in greater detail.

Standby Altimeter

The Standby Altimeter (pneumatic) provides standby altitude information. Input for the indicator is from the alternate static ports.


The other quote is correct. That is the accuracy required by the ADC according to that standard. By that, you would have to say that the DFDR pressure altitude data is within +/- 25 feet.... if the system is within its certified operating range.

As was pointed out on another thread, you have to find out if the sensors and ADC were accurate when operating some 100 knots faster than Vmo. Until you do, the DFDR pressure altitude data may be in error. The airspeed sensors are rated for that speed but the altitude data may not be.
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UnderTow
Posted: Sep 27 2006, 01:40 PM


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Perhaps we are saying the same thing.

user posted image
user posted image
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johndoeX
Posted: Sep 27 2006, 06:29 PM


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QUOTE (Terry Zarzoff @ Sep 27 2006, 06:31 AM)
you have to find out if the sensors and ADC were accurate when operating some 100 knots faster than Vmo. Until you do, the DFDR pressure altitude data may be in error. The airspeed sensors are rated for that speed but the altitude data may not be.

Vmo is aircraft specific.. not instrument specific.

These ADC's and DAU's are used in all types of aircraft with all types of Vmo's.

You dont start getting into instrument error on these Air Data Computers until you hit transonic/supersonic ranges. And from what i understand, they just make a slight "tweak" to the ADC if it is going into a supersonic aircraft.

Hanging onto "errors" in order to not disturb govt theory is reaching in its purest definition of the word.

I have already covered lag issues and/or instrument error. If you think in any way, shape or form that aircraft was low enough to hit the light poles.. the descent rate would have been too great to pull out of that dive instantaneously to have been level across the lawn.. (it is at that point right now without errors)... as shown in the DOD video.


Again.. what good is an FDR that cannot record valid parameters outside the aircraft envelope?
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George Hayduke
Posted: Oct 6 2006, 01:22 PM


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QUOTE (johndoeX @ Sep 25 2006, 07:20 PM)
QUOTE (UnderTow @ Sep 25 2006, 11:41 AM)


You will find several documents, let's look at the Servoed Altimeter, since this is the one most likely used by commercial cargo fleet.

A41322 PDF
Accuracy: +- 20 Feet


This is the type of altimeter found in transport category aircraft.


Highly accurate.

Like, for example, a C130, like the one spotted above the Pentagon and the one spotted in Penn. air space near where everybody has been lead to believe F93 crashed?
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