Which one do I use

[DLconnection]

Given: when shooting an ILS Cat I, the DH is usually 200 ft AGL. When shooting these things in a crew environment you might call out 1000 above, 500 above, 100 above, minimums. My question is, which altimeter would you go by if u are flying an airplane equiped with a radar altimeter?
I was flying an ILS the other night, calling out altitudes based on what the actual altimeter was saying. The captain said that I had missed my 1000 ft call. The altimeter said differently. So with a conflict between the radar altimeter and the actual altimeter in the airplane, which one should I trust and go by?

 

[UPS Capt]

CAT I approach...you use baro altimeter for callouts. Some pilots get lazy or don't know any better and use the radar altimeter for all callouts during an ILS including the "mins" callout. The correct way is to you the baro for callouts on a CAT I ILS approach.

CAT II/III are different for "mins" callouts only... you use the radar altimeter for these, unless, of course, on those rare occassions when the RA is unauthorized, then you use the inner marker or baro with higher mins (CAT II).

 

[BobbyB]

You need to use the barometric altimeter. The 200 agl is good info to have the the DH, but agl could be misleading near any airport with significant terrain. For example, if there is a hill several miles from the runway that is 500' high, and you should be at 1500' msl, your radar altimeter could read 1000' when, at a sea level airport, you are truly 1300' above the DH (200agl and msl). The radar altimeter is a great backup down low.

 

[NJA Capt]

My question is, which altimeter would you go by if u are flying an airplane equiped with a radar altimeter?
I was ...calling out altitudes based on what the actual altimeter was saying. The captain said that I had missed my 1000 ft call. The altimeter said differently. So with a conflict between the radar altimeter and the actual altimeter in the airplane, which one should I trust and go by?

Very good question.
Always, Always, Always use altimeter based callouts. (ie...height above runway elevation if VFR and height above DH/MDA if IFR). NEVER radar altimeter! Radar altimeter values are only accurate when over a level surface. Such as the runway and the 1000' Glide slope clearway prior to the runway. An exaggerated example would be on approach to Telluride. When you are 200' above touchdown zone elevation, you are 3000' above terrain. Terrain on final approach can vary greatly giving false basis for callouts.

Did you question the Captain when you got on the ground to clarify? Did he expect you to use the RA or did he maybe correct you in error?

If you were flying the approach, wasn't HE suppose to be making the callouts?

 

[DLconnection]

I was the NFP. He did say that I was "technically" correct in using the barometric altimeter. For some reason he was use to using the radar altimeter. He said that he usually uses the radar altimeter for some reason. Thats why I needed clarification as to whether or not I was doing the right thing.

 

[80/20]

A valid question in this discussion is; why do we have each specific call out?
Airlines have different procedures. Sometimes based on individual preferences, other times it is called for by various types of operations.

Here are some specific considerations suggested by FAA when designing procedures:
CAT I and non-precision approaches:
Minima should be based on barometric altitude; call outs such as "DH" and "100 feet to go" should be based on barometric altimeter.
Radio altimeter (RA) should be used for minima on ILS CAT II & III and to check auto pilot systems, crew incapacitation and barometric altimeter errors.

Example of RA call-outs from Boeing's philosophy:
"500 feet RA" Objective: (1) Check for pilot incapacitation (2) Clarify if ILS Autoland or manual landing (3) Check systems for correct indications which is triggered by radio altitude. The exact points will wary depending on terrain and thereby also aid in terrain awareness.
"1000 feet RA" Objective: (1) Check correct annunciations (2) Confirm that you have remembered to set local altimeter settings (3) Detect errors.
Many major airlines operate outside the US where 29.92 is used below FL180 down to various levels depending on local procedures/ATIS. Many international operators have added the call out "Radio Altitude Alive" to remind the crew to check that they have remembered to change from 29.92 to local setting.
Barometric altimeter is still the primary source of altitude information during the entire approach (RA primary for CAT I&II minima).

Cold weather operations should involve adjustments of barometric minima - radio altitude callouts have helped crew to identify that they are lower than indicated. (Assuming that they understand how the RA works and have good terrain awareness).

Again what's right in one airline might not always be right in another - but the basic rule should be to follow the company's SOP.