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Differential Altimeter Indications (photo)  
User currently offlineINNflight From Switzerland, joined Apr 2004, 3766 posts, RR: 60
Posted (4 years 2 months 2 weeks 1 day 20 hours ago) and read 4584 times:

G'day everyone,

came across this photo today


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Photo © Rafael Finter



...and noticed upon taking a closer look that the primary altimeter just right of the PFD is showing the aircraft at FL350, while the back-up altimeter to the lower right shows FL355.

What are the causes for this? Is it because the back-up altimeter uses an alternate static source, or was it simply not reset to 1013,25 at transition altitude?

Thanks,


Jet Visuals
10 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (4 years 2 months 2 weeks 1 day 16 hours ago) and read 4497 times:

Quoting INNflight (Thread starter):
What are the causes for this? Is it because the back-up altimeter uses an alternate static source, or was it simply not reset to 1013,25 at transition altitude?

You shouldn't get that much error from an alternate static source...forgetting to apply the correct baro value is the most likely cause.

Tom.


User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 2, posted (4 years 2 months 2 weeks 1 day 15 hours ago) and read 4479 times:

Quoting tdscanuck (Reply 1):
You shouldn't get that much error from an alternate static source

The tolerance of an standby altimeter rises with altitude and is not a fixed margin in % like you would have with electronic equipment.
If I recall correctly from my head it's +/-20ft at 0 to 1000ft which increases to +/-500ft at flight lvl 500.
So in this case it is still off a bit but not a dramatic amount, probably around 150ft or so.



The truth lives in one’s mind, it doesn’t really exist
User currently offlineN243NW From United States of America, joined Jul 2003, 1630 posts, RR: 20
Reply 3, posted (4 years 2 months 1 week 5 days 10 hours ago) and read 4082 times:

Wow, good catch. I never would have noticed that looking at the photo.

Quoting tdscanuck (Reply 1):
You shouldn't get that much error from an alternate static source...forgetting to apply the correct baro value is the most likely cause.

I think it was a simple forgetting to change to standard pressure upon reaching FL180. 500ft difference in altimeter readings is under no circumstances normal and is grounds for a maintenance squawk. The preflight procedure for our fleet is to set the altimeter to the current barometric pressure as specified in the ATIS/AWOS and verify that the indication is within 75 feet of airport elevation.



B-52s don't take off. They scare the ground away.
User currently offlinePGNCS From United States of America, joined Apr 2007, 2821 posts, RR: 45
Reply 4, posted (4 years 2 months 1 week 5 days 2 hours ago) and read 3961 times:

Quoting N243NW (Reply 3):
Quoting tdscanuck (Reply 1):
You shouldn't get that much error from an alternate static source...forgetting to apply the correct baro value is the most likely cause.

I think it was a simple forgetting to change to standard pressure upon reaching FL180. 500ft difference in altimeter readings is under no circumstances normal and is grounds for a maintenance squawk. The preflight procedure for our fleet is to set the altimeter to the current barometric pressure as specified in the ATIS/AWOS and verify that the indication is within 75 feet of airport elevation.

Our tolerances are even tighter than yours for all altimeters (including the standby) on the ground. I am curious what kind of aircraft are in your fleet; I don't know what you are flying or how high the aircraft go, but it does make a difference. Our manuals also point out that the standby altimeter is the only uncorrected altimeter in the cockpit, and cannot be used for altimetry in RVSM airspace. There are no inflight tolerances listed in our operating manual for standby altimeter accuracy, and differences of 200 or 300 feet are very common at higher altitudes. I don't recall ever seeing a 500 foot discrepancy (and I have a lot of hours in the 767), but don't know what the maintenance limitations (if any) there are for the 767.

Understand that while 500 feet is a lot at altitude, the uncorrected standby and ADC-corrected primary altimeters converge as you get closer to the ground, which is when accuracy is far more important.

I have no idea if the standby is set to 29.92, but am certainly not in a position to presume that I know whether or not the aircraft needs to be written up or not.


User currently offlineHAWK21M From India, joined Jan 2001, 31679 posts, RR: 56
Reply 5, posted (4 years 2 months 1 week 4 days 18 hours ago) and read 3846 times:

Tried zooming to Maximum but the resolution did not permit clarity on the Baro readings.
I would suspect considering the RVSM Airspace tolerances...Its likely the Baro correction is not set accurately,as the Difference seems too high,although both sources of pickup will be different.
regds
MEL.



Think of the brighter side!
User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 6, posted (4 years 2 months 1 week 4 days 17 hours ago) and read 3829 times:

Quoting N243NW (Reply 3):
I think it was a simple forgetting to change to standard pressure upon reaching FL180.

Didn't disagree with that.

Quoting N243NW (Reply 3):
500ft difference in altimeter readings

I was talking about tolerances not readings.

Quoting N243NW (Reply 3):
The preflight procedure for our fleet is to set the altimeter to the current barometric pressure as specified in the ATIS/AWOS and verify that the indication is within 75 feet of airport elevation.

This should lead to a mx squawk as this is outside the manufacturers tolerance for all brands I am aware off, well in the shop at least.
Unless you are on a field with a very high elevation of course.

Afaik most of them are tested from -250(I live in Holland   ) upto 50.000 ft with as said before a tolerance(depending manufacturer and type) of 20ft upto an altitude of 1000ft which increases to 500ft at an altitude of 50.000ft.

Quoting PGNCS (Reply 4):
Our manuals also point out that the standby altimeter is the only uncorrected altimeter in the cockpit, and cannot be used for altimetry in RVSM airspace

Exactly and that's why it's not such a big deal, below fl 280 the accuracy will improve by some 50% compared to fl 500.

Quoting PGNCS (Reply 4):
There are no inflight tolerances listed in our operating manual for standby altimeter accuracy

That surprises me a bit as they are in every component maintenance manual.

Quoting PGNCS (Reply 4):
and differences of 200 or 300 feet are very common at higher altitudes.

Which is exactly in line with normal tolerances of most of them and lead me to the statement that it's off by about 150ft.



The truth lives in one’s mind, it doesn’t really exist
User currently offline747classic From Netherlands, joined Aug 2009, 2109 posts, RR: 14
Reply 7, posted (4 years 2 months 1 week 4 days 16 hours ago) and read 3816 times:

747 altimeter limits :






IMO, 500 ft difference between pneumatic and electrical altimeter readings with correct baro setting is outside limits and has to be noted in the Aircraft Maintenance Log.
On an outstation you could have a serious delay. (full altimeter system check, plus changing of defective parts.



Operating a twin over the ocean, you're always one engine failure from a total emergency.
User currently offlinelonghauler From Canada, joined Mar 2004, 4927 posts, RR: 43
Reply 8, posted (4 years 2 months 1 week 4 days 16 hours ago) and read 3807 times:

You never know, it might be intentional, dependant on the airlines SOPs or pilot techniques.

If close to top of descent, the destination field QNH might be set, just as a reminder for when going through transition level, simply as a pilot technique. Or, perhaps even destination QFE is set per airline SOPs.

But I agree with the above, a 500 foot difference is pretty high at any altitude.



Never gonna grow up, never gonna slow down .... Barefoot Blue Jean Night
User currently offlineN243NW From United States of America, joined Jul 2003, 1630 posts, RR: 20
Reply 9, posted (4 years 2 months 1 week 4 days 11 hours ago) and read 3754 times:

Quoting PGNCS (Reply 4):
I am curious what kind of aircraft are in your fleet

We're just a rinky-dink Part 141 training fleet of Diamonds and Pipers...I probably shouldn't have mentioned our SOP, since it's comparing apples to oranges.

Quoting Aviopic (Reply 6):
Didn't disagree with that.

And I should have specified that I didn't disagree with your initial analysis either...my apologies.



B-52s don't take off. They scare the ground away.
User currently offlinePGNCS From United States of America, joined Apr 2007, 2821 posts, RR: 45
Reply 10, posted (4 years 2 months 1 week 4 days 8 hours ago) and read 3661 times:

Quoting Aviopic (Reply 6):
Quoting PGNCS (Reply 4):
There are no inflight tolerances listed in our operating manual for standby altimeter accuracy

That surprises me a bit as they are in every component maintenance manual.

In the last twenty or so years the limitations sections in our pilot manual set has become much shorter for better or worse (I used to be required to know that the DC-9-30 airfoil anti-ice heated the slats to 242 degrees C plus or minus 5 degrees....) I am sure that the MX guys know the numbers (or can find them quickly enough,) but I guess it isn't considered enough of a problem to make the pilots aware of as long as it passes the tests for accuracy on the ground in the preflight.

Quoting Aviopic (Reply 6):
Quoting PGNCS (Reply 4):
and differences of 200 or 300 feet are very common at higher altitudes.

Which is exactly in line with normal tolerances of most of them and lead me to the statement that it's off by about 150ft.

I suspect that you are right, but I really don't know the maximum allowable tolerances.

Quoting 747classic (Reply 7):
IMO, 500 ft difference between pneumatic and electrical altimeter readings with correct baro setting is outside limits and has to be noted in the Aircraft Maintenance Log.

Assuming the values are identical in the 747 and 767. I don't know what the book says for the 767, and I agree that 500' is more than I would expect to see. I personally am guessing there is some pilot technique involved here. A lot of pilots preset the landing field baro setting in the stby altimeter before top of descent, normally after getting the arrival ATIS; I tend to believe that there is a likelihood there's something we don't know about involved in this photo.

Quoting longhauler (Reply 8):
You never know, it might be intentional, dependant on the airlines SOPs or pilot techniques.

If close to top of descent, the destination field QNH might be set, just as a reminder for when going through transition level, simply as a pilot technique. Or, perhaps even destination QFE is set per airline SOPs.

That was actually my first thought, but since it was completely speculative, I didn't bring it up. I agree with longhauler in all his points.


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