From: rdd@rascal.ics.utexas.edu (Robert Dorsett)
Newsgroups: sci.aeronautics
Subject: A320 vertical-speed indicator
Date: 29 Jul 1992 21:14:40 -0500
Organization: UTexas Mail-to-News Gateway
NNTP-Posting-Host: cs.utexas.edu

[The first attempt at this ended up in the bit-bucket; here's another go.]


A number of people in the recent discussion on the Strasbourg crash
offered comments ranging from "why weren't they monitoring vertical speed?" 
to "it should have been obvious from their vertical-speed indicator that 
they were dropping like a rock!"  I replied essentially saying the pilots 
didn't have good trend information, but the comments continued.  At the risk 
of re-opening that rather unpleasant discussion, here's a short description 
of the VSI:

Schematically, it's a trapezoid about 7 millimeters wide, located to the
right of the altitude tape, and about as tall as that tape on the "high"
side, about 2/3 as tall on the "small" size.  Sort of like: 
      ____
     |    |  |\
     |    |  | |
     |    |   9 
     |    |  | | 
     |    |  - |
     |    |  | |
     |    |  | |
     |    |  | | 
     |    |  | |
     |___ |  |/

The object on the left is the altimeter and the object on the right is the
VSI.  This is intended to show placement only; the altimeter's a lot more
involved.

The display's major component is a short "needle" emanating from the midpoint 
of the "short" side to unlabelled graduations on the "long" side.  There is 
no room for any labels, but the documentation indicates the graduations are 
at 500' increments, with "0 fpm" at the mid-point of the long side of the
trapezoid.  Sandwiched in the mid-upper or mid-lower parts of the instrument, 
as appropriate, are two-digit vertical-speed indications (e.g., 4 = 400 fpm, 
40 = 4000 fpm, needle off the scale).  In the example, if I could draw
an pretty ASCII needle, it'd be showing a climb 900 fpm UP.

If the vertical speed is > 2000 fpm, the needle points to the appropriate
end of the scale.   

The *color* of the needle is green.  It turns amber on the following schedule:
	* V/S > 6000 fpm.
	* (V/S > 2000 fpm) && (1000' < Z < 2500')
	  -or- (V/S > 1200 fpm) && (Z < 1000').
Z == altitude, agl.  If the V/S is < 200 fpm, the needle is *removed*.  As 
are digital indications (yes, Airbus documentation is much easier to read
if one's a programmer :-)).  

If there is a failure, the scale is removed, and replaced by a red "V/S"
flag.


IMHO, this display is all but unusable, and places a greater importance 
on the design of the altimeter.  WHICH, being a tape, is by definition sub-
ject to trend-interpretation errors.  Hence, I can at least *understand*
orders-of-magnitude descent-errors, which may have happened at Strasbourg.


The 747-400 (and apparently the 777) uses something a little bit different.  
Like the A320, they use "all in one" primary flight displays (as compared
to the A310/757/767 which have a "half & half" approach, of electronic
attitude displays and electromechanical, conventional airspeed, altitude,
and vertical-speed indicators).  

On the 747-400, the VSI display is also done on a gray background, but with
greater contrast.  It is about twice as wide.  Small numbers are clearly 
indicated on the left side of the scale, which is logarithmic (ranging from 0 
to 6000 fpm, with most available vertical space representing the [-1000,1000]
fpm interval.  The precise numerical V/S value, to 10's, is displayed above 
the display (example: 1800 = 1800 fpm up, -1800 = 1800 fpm down).

Note that the 747-400 has significantly more real estate to play with (11" 
diagonal vs. 7.25" diagonal).  Despite the A330/A340 being newer airplanes, 
they use the A320 design, for flight-deck commonality reasons; this design 
was probably "matured" around 1984. 


Note that I'm not convinced the 747-400's approach is any more usable, but to
my eye, it's more intuitive.  I appreciated the comment of the gentleman
who noted the desirability of VSI displays with needles that pointed 
straight "down" to cue the pilot that he's headed toward the ground at high 
speed. :-)



---
Robert Dorsett
rdd@cactus.org
...cs.utexas.edu!cactus.org!rdd