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On every other PFD (I know), the exact airspeed is displayed as big number on the PFD. To clarify what I mean, here are two pictures:

Boeing PFD

Source, added square, Boeing PFD

D10A EFIS

Source, added square, D10A EFIS

Now, when we take a look at the Airbus PFD, we can see that there is no exact speed display:

Airbus PFD

Source, added square, Airbus PFD

This is one of the biggest differences between the Airbus PFD and other PFDs. What led Airbus to this unusual decision? Is it maybe because Airbus pilots don't need to watch airspeed as close as other pilots, because of the numerous flight envelope protection systems and the fact that the autothrottle controls the speed until 20 feet above ground?

Is there just no need to know the exact airspeed and Airbus wanted to remove a source of distraction?

Is it possible to turn an exact speed display on if the pilot wants or needs it?

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    $\begingroup$ 14 up-votes, an open bounty, and no answer. I'd put it this way...... no body knows why Airbus did that...... $\endgroup$ – kevin Jul 6 '17 at 17:00
  • $\begingroup$ @kevin To be different from Boeing? No seriously... with all the odd peculiarities in Airbus design (additive control inputs, non-mechanically connected flight controls, two completely different feeling controls - left and right stick, strange PTU noises that terrify passengers, and now a non-standard PFD layout), I'm starting to think they more and more simply wanted to be not like Boeing. $\endgroup$ – SnakeDoc Jul 6 '17 at 17:33
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    $\begingroup$ I think it's a pretty good decision to put the exact speed away. Because they must have thought something when they did it and it's definitely a distraction less. But I don't know why they did it, that's so interesting. $\endgroup$ – Noah Krasser Jul 6 '17 at 17:44
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    $\begingroup$ @Terry I am talking about the indicated airspeed, just the exact value that is displayed on the speed strip. On the airbus PFD one can only read the approximate value. $\endgroup$ – Noah Krasser Jul 6 '17 at 18:23
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    $\begingroup$ @JonathanWalters That certainly seems reasonable as a quick reference. Back in the dark ages of the 747-100/200 we compared actual performance to planned performance by keeping track of the time of arrival over each flight planned point to the projected time and, more importantly, to the actual amount of fuel left compared to what was supposed to be left. This was all done on paper flight plans, a copy of which both the two pilots and the flight engineer had. One copy went after each flight to company headquarters for possible review by management and/or the FAA. $\endgroup$ – Terry Jul 7 '17 at 15:54
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Where it came from (history)

The Airbus-style airspeed tape on the A320 onwards comes from the A310 (Flight, 1987).

The present scale convention is similar to that used on the A310, and pilots have obviously readily adapted to it, but it does show that it may take a little time to shake off an old, ingrained response.

What the author is referring to is the speed-deviation indicator, an indicator like that on the 767 shown below that shows how fast/slow the plane is compared to the target speed.

enter image description here
(commons.wikimedia.org)

The A310 had this design when Thomson-CSF (later became Thales Group) took over the forward-facing crew cockpit (FFCC).

Thomson-CSF has been developing its A310 CRT symbology in close cooperation with Airbus and the customer airlines, and many details have changed since the company's selection as A310 supplier in September 1979 (Flight, 1980).

(Emphasis mine.)

Based on the above, Thomson-CSF reached the final design through usability engineering.

enter image description here
The A300 had a circular attitude indicator. The A310 change was to crop the sides to allow for an airspeed indicator.

The electronic attitude director indicator (EADI) which forms the basis of the PFD is no longer round, allowing more airspeed information to be presented on the sliding scale on the left.

(...)

The aircraft's indicated airspeed is shown by a yellow circle, the arrow through it indicating the speed trend. The airspeed symbol and trend remain stationary while the scale moves.

My opinion: This is the closest thing to the old "clockwork" cockpits, where an instrument's hand position was enough to glean the required information. Here the pilot would keep the yellow line (or circle on the A310) away from the too-slow and too-fast regions.

Not just Airbus

Below you can see the different versions on the 737 Classic, the one on the right featuring the "rolling digit cursor."

enter image description here


Something I found during my research I think is worth sharing about Airbus' philosophy that I haven't heard before:

The coach driver gives a command and the horses take care of the road, the A320 pilot makes a control input and the aircraft takes care of the flightpath (Flight, 1986).

I think the Airbus airspeed tape follows that, showing only what's relevant at a glance, as Radu094 wrote in their answer.

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  • $\begingroup$ Very good answer! $\endgroup$ – Noah Krasser Jul 12 '17 at 19:34
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    $\begingroup$ @NoahKrasser - it was fun researching its history, thank you :) $\endgroup$ – ymb1 Jul 13 '17 at 21:11
  • $\begingroup$ Well the addition of the 737 PFDs evolution is very insightful, but it raises more questions than it answers: we first had the "fast-slow" indicator, then the airspeed strip and finally the strip with the numeric value. Now the question mutates to Why Airbus PFDs haven't evolved yet? $\endgroup$ – Stelios Adamantidis Jul 14 '17 at 7:07
  • $\begingroup$ On the other hand, if somebody hadn't asked for it, Boeing wouldn't have any good reason to add it. And as we can see from the question, it's not only Boeing that shows the digits. :) $\endgroup$ – Stelios Adamantidis Jul 14 '17 at 13:01
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I always thought the yellow speed line (vs digital in a box) is a way of indicating the speed (and speed trend) are aproximations and subject to change ( ie. indicating a speed of 134.4345 kts implies a very high precision, which the ADR does not have)

In reality (after taking a long time to get used to it) I started to preffer the Airbus method: the yellow line allows my eyes to focus on the entire speed tape, including the speed trend vector and the upcomming speed limitations on the bar, giving me a better overal picture of the speed. The old (Boeing?) display would capture my eyes on the exact number, to the detriment of the other indications on the speedtape. This is particulary true on turbulent approaches when speed fluctuates a lot.

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    $\begingroup$ Yes, you are so right. I love EFIS, but I think for speed display, the old steam gauges are better. Whenever I am in using an EFIS, I always automatically look at the speed standby gauge. $\endgroup$ – Noah Krasser Jul 6 '17 at 18:14
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Disclaimer: this is more a beautified comment than a real answer as it doesn't really answer the question. Feel free to ignore, or even downvote if you feel punitive.


Although a nice question (and indeed a good observation) this is a dangerous one: it involves guesswork. As @kevin already mentioned, who knows why they made that choice and why?

This involves UI and UX and it's a subjective(?) matter by nature. Some History: Many years ago, Air Inter Flight 148 crash happened involving a A320. One of the reasons that contributed to the crash was the UI/UX: Flight Path Angle autopilot mode display was very similar to Vertical Speed mode. Only one dot difference. See this FAA article for the details but the image (taken from inside the article) depicts the concept:

VS and FPA

That particular design choice was proven terrible after all. But why did Airbus make it, especially if it was dangerous? We don't know and probably never will, and my guess is they didn't think "let's mesh things up and see what happens".

So now back to your question, maybe they concluded that adding it was cluttering the airspeed strip. Maybe they "asked 100 pilots" looking for the most popular answer to the question: "Do you want to see the numeric value on the airspeed strip?" Maybe it was overlapping with the acceleration-deceleration arrows and other indications on the right side of the airspeed strip, drawing the pilots' attentions away of the bigger picture as Radu94 already mentioned in their answer.

But here is the problem, all these are maybe. Hypothesis.

If you feel bold enough, I would suggest to contact Airbus and directly ask them. You might be lucky enough and get an answer (and be kind enough and share it here :) ). I got one from EASA regarding ATC simulator regulations. Although that might not be the same, since if they have put really big effort and manpower in researching and designing this, they might not want to share what they will consider company secrets.

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    $\begingroup$ Also, maybe they kept it more like a traditional "steam" gauge which doesn't show the exact number either. ;-) $\endgroup$ – Lnafziger Jul 12 '17 at 18:59
  • $\begingroup$ @Lnafziger Indeed, good thought! I was thinking of adding it in my answer but then the list just grows bigger without any certain answer but only speculations. :) $\endgroup$ – Stelios Adamantidis Jul 16 '17 at 15:17
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I am UX designer with background in information architecture and thought i would contribute

I don't have a definitive answer but just an informed guess.

In a sense all of the answers raise valid points.Considering the context; in environments where user perform complex task the design of the interface has among other objectives the need to reduce cognitive load. As the brain has limited amount of processing power, too much information delivered in too short time particularly where a lot of other elements are competing for attention can increase cognitive load to a point where it could be overwhelming. I think, variations when displaying airspeed format could be a symptom of that.

Interface designers can reduce this cognitive load by reducing extraneous visual elements such as in the case of the airspeed display for airbus.

This is where it gets a bit tricky because based on my understanding airbus and Boeing have different philosophies in terms of handing control to the user/pilot which include displaying system status or what we refer to in UX heuristics as:

"visibility of system status"

The system should always keep users informed about what is going on, through appropriate feedback within reasonable time.

The heuristics also include guidance which promotes :

Recognition rather than recall

Minimize the user's memory load by making objects, actions, and options visible. The user should not have to remember information from one part of the dialogue to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.

So, overall i think we have two competing heuristics in this particular case and choice is largely based and each airline approach.

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  • $\begingroup$ Welcome to Aviation.se, thank you, hope to see you around! $\endgroup$ – Harper - Reinstate Monica Oct 22 at 16:46
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In a sense, the Airbus does display the exact speed on the PFD. It just requires the pilot to interpolate between the markings.

If you need to the know the FMGC Managed target speed/Mach, it's available from the PERF page on the MCDU while a Selected speed/Mach is shown in the speed window on the FCU.

In almost all case we select a speed in multiple of 5s while the FMGC managed speeds usually (but not always) flies a window of speeds around a desired value depending on the current mode and configuration.

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A breakdown in the Usability Testing process?

In software engineering, user facing applications are often put through a second stage of testing called usability testing. This differs from functional testing, which is designed to root out bugs. With usability testing, application flow, ease of use and intuitiveness of the UI are tested. Companies that can afford to do so will often bring in panels of real users to find out how they perceive an applications usability. As a software engineer and someone who manages software engineers, I've sat in on a number of these testing sessions over the years, getting first hand feedback on the products my groups have produced.

I would have to assume, for such high dollar item, that Airbus would put the PFD through a fairly rigorous UT phase, certainly with real pilots. But no process is perfect, and it's just possible this was missed. This would also mean it was missed in the requirements gathering phase of the project as well, which is really a surprise given how standard this type of display has become.

Of course it is also possible that the usability testing concluded that the exact airspeed should not be put on the PFD.

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  • $\begingroup$ remember the A320 was designed in the 70s/80s when Human-Machine interfaces and UI design was not so evolved. I wonder if any usability testing/design ever went into the display units beyond applying CS-25 conventions $\endgroup$ – Radu094 Jul 13 '17 at 9:50
  • $\begingroup$ Ah yes, the CS-25 conventions...just kidding, had to look that one up ;-) Agree though, the testing conventions I speak of certainly would predate that. $\endgroup$ – bclarkreston Jul 13 '17 at 13:11

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