Tolerance of Standby Altimeters in Jet aircraft. (Airbus)
 

HiGuys,

Recently, had a mate who shared his experience with me from his flight. His Primary altimeters had a discrepancy of 80ft and his standby altimeter on the ISIS (Integrated Standby Instrument System)’s PFD had a discrepancy of 300ft with his altimeter.

From my understanding, in RVSM airspace, only the discrepancy between the 2 primary altimeters is taken into consideration, with a tolerance of 200ft. Nothing mentioned about the standby altimeter as it’s not rated for RVSM.

My questions are:-

1) Any of you guys are aware if there is any tolerance for standby altimeters?
2) How are the AAD/TVE/ASE determined by the ATC?
AAD is the difference between the transponder Mode C altitude and the assigned altitude / flight level. ASE is the difference between the pressure altitude displayed to the TVE is the vertical geometric difference between the actual pressure altitude.
3) What is YOUR tolerance for a discrepancy in standby altimeters reading?

Thanks for your time and comments.

The tolerance for Airbus altimeters is +/-75 ft of known airport altitude and +/- 25 ft in between PFD 1 and 2. If I don't get wrong the ISIS altimeters are both servo-assisted altimeters, so the maximum tolerance should be [+/- 30 ft at MSL].

Hmmm, on the ground, it's 75ft between the primary altimeter and field elevation. Nothing mention about standby altimeters though. The standby altimeters got their reading from their seperate probes ..... not too sure where the 30ft AMSL come from though....

Care to highlight ?

Thanks anyway

The FCOM (1.34.25 p.5) doesn't give any tolerance value for standby altimeters, so I'm quite keen to believe that the standard tolerance value (+/-30 ft at MSL) for servo-assisted altimeters apply. Studied this number on the ATPL-Instrumentation Oxford Manual, anyway it might change with airline SOP's ...

On FCOM 3.04.34 p.6 ,there's some light on the matter...

ciao

Hi rc,

Care to share with us the extract ?

cheers

As altitude increases the minimum tolerance for altitude gross error reporting increases. Normally at RVSM altitudes somewhere around 300ft "D" is reportable as a gross hight keeping error between the capt/fo displays.

The standby as you understand is not RVSM but should be reported in the log in this case.

The 80ft between the rh and the lh side does not seem like a big deal and should be trusted as the altitude you are operating at.


1- if not in fcom doccument and will be addressed by amm limits
2- reported by primary ADC's depentant on aircraft
3- The stby is a STBY, like I said above if you lose your primary x2 you are in bad form, nobody cares about RVSM in this case, it is there only to give you a reference to terra firma!!!

My standby reads 40 feet lower than the primaries on the ground and about 250ft lower at FL400. Is it because it's not servo assisted? is it basically a pressure altimeter like those found in an old C172?

FCOM 3.04.34 p6:

FLIGHT INSTRUMENT TOLERANCES
The values given below apply to aircraft in symmetrical flight (no sideslip), in clean configuration, and in straight and level flight.
ALTITUDE TOLERANCES
- PFD 1 or 2 at ground check : ± 25 feet (8 m) - standby altimeter at ground check : ± 300 feet (91 m)
Note: On ground, as the standby altimeter’s vibrator is off, the standby altimeter’s tolerance value is high. In flight, the vibrator is on and the value is lower.

Best I can do on full page copy: :ok:

SUPPLEMENTARY TECHNIQUES 3.04.34 P 6
NAVIGATION SEQ 001 REV 32

FLIGHT INSTRUMENT TOLERANCES (non ISIS fit)

The values given below apply to aircraft in symmetrical ight (no sideslip), in clean
conguration, and in straight and level ight.

ALTITUDE TOLERANCES
- PFD 1 or 2 at ground check : ± 25 feet (8 m)
- standby altimeter at ground check : ± 300 feet (91 m)
Note: On ground, as the standby altimeter’s vibrator is off, the standby altimeter’s
tolerance value is high. In ight, the vibrator is on and the value is lower.

MAX IMUM DIFFERENCES BETWEEN ALTITUDE IN DICATIONS

FL/SPEED
ALTITUDE (ft) COMPARISON BETWEEN
(1) ADR 1 and ADR 2 (on PFD)
(2) ADR 3 and ADR 1 or ADR 3 and ADR 2 (on PFD)
(3) STBY ALTI and any ADR 1 or 2 or 3

(1) (2) (3)

GND CHECK 20 (6 m) 20 (6 m) *
FL50/250 kt 50 (15 m) 80 (24 m) 130 (40 m)
FL100/250 kt 55 (17 m) 80 (24 m) 185 (56 m)
FL200/300 kt 90 (27 m) 145 (44 m) 295 (90 m)
FL300/.78 130 (40 m) 355 (108 m) 390 (119 m)
FL390/.78 130 (40 m) 365 (111 m) 445 (136 m)

* On ground, the check is meaningless because the standby altimeter’s vibrator is off.



ISIS fit:

ALTITUDE TOLERANCES
- PFD 1 or 2 at ground check : plus or minus 25 feet (8 meters)

MAX IMUM DIFFERENCES BETWEEN ALTITUDE IN DICATIONS

FL/SPEED
ALTITUDE (ft) COMPARISON BETWEEN
(1) ADR 1 and ADR 2 (on PFD)
(2) ADR 3 and ADR 1, or ADR 3 and ADR 2(on PFD)
(3) ISIS and any ADR 1, or 2, or 3


GND CHECK 20 (6 m) 20 (6 m) 100 (30 m)
FL50/250 kt 50 (15 m) 65 (20 m) 130 (40 m)
FL100/250 kt 55 (17 m) 80 (24 m) 185 (56 m)
FL200/300 kt 90 (27 m) 135 (41 m) 295 (90 m)
FL300/.78 130 (40 m) 195 (59 m) 390 (119 m)
FL390/.78 130 (40 m) 195 (59 m) 445 (136 m)

I'd be surprised if 300' split were acceptable at any FL.been too may years since on the buses.
servo assisted or direct reading its all baro alt one way or another.Just tweaked and presented nicely on the primaries.

Its straight out of the FCOM, what more can you say.

thanks guy !! Appreciate it.

Cheers

PFD 1 OR 2 at grnd check :+/- 25ft
 

Hi guys can share your opinion on the Altitude tolerance PFD 1 or 2 at gnd check +/- 25ft (8m) means?

On the A330 the static pressure ports are located such that the local pressure in flight differs slightly from the true static pressure. This pressure difference introduces an error in the measured altitude, which is corrected by the so called Position Error Correction (PEC). The PEC is a function of altitude and Mach and is presumably calculated in the ADIRU's but maybe not in the ISIS. AT FL350 and Mach 0.8 the PEC is about 300 ft.

Guys, I am resurrecting this old thread.
I found the below:

"At sea level, on a typical servo altimeter, the tolerance in feet from indicated is:

+/- 60 feet
+/- 75 feet
+/- 30 feet <= correct answer
+/- 70 feet

Servo assisted altimeters are accurate to 1mb, ±30ft at sea level and ±100ft at 40 000ft.

However, required accuracy is +/- 60ft."

I am not English native and I do not fully understand the difference between the 30ft mentioned, and the 60ft.

Can somebody explain this to me ?

Thank you.

I have struggled for years to find an authoritative source for an operational test of altimeter accuracy in EASA operations. I can't find anything in EASA docs but if I have missed it I would be grateful for a reference. I have not looked at TSOs as they are pay-to-use, maybe there? If so it would be a calibration limit.

FAA TSOs appear to require a calibration accuracy of 20 ft with 29.92 set and within 1000 ft of sea level. FAR 91.411 appendix E appears to confirm that. The FAA Instrument Flying Handbook, however, suggests an operational limit (rather than a calibration limit) of 75 ft.

There is an underlying ICAO requirement for an operational test in PANS-OPS Part III Section 1 Chapter 3.2 which is:

A serviceable altimeter indicates the elevation of the point selected, plus the height of the altimeter above this point, within a tolerance of:
a) ±20 m or 60 ft for altimeters with a test range of 0 to 9 000 m (0 to 30 000 ft); and
b) ±25 m or 80 ft for altimeters with a test range of 0 to 15 000 m (0 to 50 000 ft).

I suspect that the question you quote originates from the UK CAA in pre-EASA days. Another thread here

Hi, I actually had a doubt to the same query, let me know, if you find an answer to this question