Altimeters

oldtimer · Post by **oldtimer** » Sat Oct 16, 2004 10:35 am

I am updating some instructional material, especially on the CFIT problem and there is a discussion on the problem of different presentations on aircraft altimeters. The drum/single pointer vs the three pointer. Usually the drum/single pointer is an encoding altimeter installed on the left side of high performance airplanes and the 3 pointer is installed in the right side. That is the side the voice activated autopilot usually occupies. Modern jets and other 2 crew airplanes have a drum/single pointer on both sides. Has anyone heard of problems missreading altitude because of this anomoly? I have an paper from the Flight Safety Foundation with accident report reprints but nothing Canadian or with the typical small twin or high performance single.

GARRETT · Post by **GARRETT** » Sat Oct 16, 2004 1:46 pm

This was a sad one. A mistake any of us could have made quite easily.

http://www.tsb.gc.ca/en/reports/air/199 ... 5p0004.asp

bandaid · Post by **bandaid** » Sat Oct 16, 2004 2:46 pm

Lost a good friend and troup mate on that accident.

ahramin · Post by **ahramin** » Sat Oct 16, 2004 4:50 pm

I believe that accident was because the altimeter was set to the wrong pressure, not because it was missread.

Oldtimer, i am not sure what anomaly you are talking about. Missreading the three pointer one, or the single pointer one?

oldtimer · Post by **oldtimer** » Sat Oct 16, 2004 8:16 pm

The Flight Safety Foundation and other safety organizations have done a few studies on the different presentations and it has been proven by far that the three pointer altimeter, even the ones with the hash marks that appear as you get lower, will be misread quite often. I got a paper from FSI but I have not had time to fully digest the article but in it there is a description of two accidents, both 727's, where the co-pilot was flying, misread the altimeter and crashed into the water. Yet almost every small airplane has that combination. A drum/single pointer on the left and a 3 pointer on the right and that is supposed to be wrong. I notice our Lears have drum/single pointer altimeters on both sides. Most airplanes of vintage and "small" are either flown single pilot in the USA or by a type rated pilot in the left seat and a non type rated pilot in the right. In other words all the brains are in the left seat. This is why it is difficult to develope SOP's because the right seater is sometimes hampered by lack of equipment and maybe this is why many operators switch seats with the F/O rather than have the F/O fly from the right seat. In the Masset accident, I do not know if they ever determined which pilot was flying and if missreading the three pointer altimeter in addition to not setting the proper altimeter setting was a factor. I will dig further but in the mean time if anybody out there is flying two crew with a 3 pointer altimeter, it has the highest chance of being missread and maybe if we mention it, someone just may store this little tidbit in the back of your mind and at an appropriate time it will pop up and save a catastrophe..

switchflicker · Post by **switchflicker** » Sun Oct 17, 2004 10:52 am

I think the Merlin at Thompson MB quite a number of years ago which hit the Hotel beacon with both pilots fataly injured was also partly due to the pilots in seats they were not accustomed to and the altimeter mix of which Doc speaks

bluesideup · Post by **bluesideup** » Sun Oct 17, 2004 1:44 pm

Oldtimer,

I have flown aircraft with both configurations that you speak of. And though I personaly have not had any seriouse problems with miss-reading either on my part or with F/O's, but just want to comment that the drum style is alot easier to read. And if given a choice would take the drum style over the three pointer any day. As a part of good CRM this is one of the area's that should be "crossed checked" by both pilots whenever an altitude change is given. When approaching level off altitudes the calls should confirm the correct altitude(1000 to go, 500 to go, 100 to go)and if flying on autopilot should be referenced to the altitude alerter. The correct QNH should be crosschecked too.

This topic reminds me of an incident that is a bit off topic but what about cold weather corrections. Does any one out there actually input these corrections? I certinaly do now after an incident that I hinted at earlier. Talk about a wake up call!!!

B S U