PFD explanation

[HalalPork]

Hey Folks,

long time ago one captain explained to me the Airbus PFD it was a different kind of explanation. He asked me questions like; why does the pfd show us 50 knots plus and minus of our current speed and why does it show us 500 feet plus and minus of our current altitude and so on... Does anyone had heard this kind of stuff before and can explain it to me properly? As faras I remember it had something to do with the time of the take off run and so on. I really regret that i didn't made any notes that time. Now the poor guy passed away and i can't even ask him anymore. He said that nothing is made by coincedence but rather somebody planned to do it like this. Ok thats logic. Is there maybe any lecture about that kind of stuff?

Thanks and regards

[rudderrudderrat]

Hi HalalPork,

I don't know of any lecture on it - but it is the best way to display both rapidly changing data and very precise steady state data.
The digital display part is great for accuracy, but is difficult to read when the values are changing rapidly.
The analogue part shows about 100 kts of IAS over the tape, which makes it easy to read a trend rate and direction.
Similarly, the Altimeter shows about 1,000 feet of analogue tape behind the digital display.

[Mad (Flt) Scientist]

AC25-11A (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2025-11A/$FILE/AC%2025-11A.pdf) discusses various aspects of electronic flight deck displays and has this to say about scales, which is not definitive in terms of the ranges chosen but indicates the basic requirements the choice of range is intended to satisfy:

31.(c)(4)(b) Scales, Dials, and Tapes. Scales, dials, and tapes with fixed and/or moving pointers have been shown to effectively improve flightcrew interpretation of numeric data.

1 The displayed range should be sufficient to perform the intended function. If the entire operational range is not shown at any given time, the transition to the other portions of the range should not be distracting or confusing.

2 Scale resolution should be sufficient to perform the intended task. Scales may be used without an associated numeric readout if alone they provide sufficient accuracy for the intended function. When numeric readouts are used in conjunction with scales, they should be located close enough to the scale to ensure proper association, yet not detract from the interpretation of the graphic or the readout.

3 Delimiters, such as tick marks, should allow rapid interpretation without adding unnecessary clutter. Markings and labels should be positioned such that their meaning is clear yet they do not hinder interpretation. Pointers and indexes should not obscure the scales or delimiters such that they can no longer be interpreted. Pointers and indexes should be positioned with sufficient accuracy for their intended function. Accuracy includes effects due to data resolution, latency, graphical positioning, etc.

Appendix 1 gives more details, and includes the statement (in section 2.1):
The minimum visible airspeed scale length found acceptable for moving scales has been 80 knots; since this minimum is dependent on other scale attributes and airplane operational speed range, variations from this should be verified for acceptability.