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I just watched the Mayday Air Crash Investigation about Aeroperú Flight 603 and two questions that weren't answered are:

  1. Who was responsible What caused the ATC to relay false altitude information to the pilots
  2. Has anything changed since then to prevent that from happening again?
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  • $\begingroup$ I'm trying desperately to find a suitable answer to your point 1. Can I ask you to clarify. Do you know that they relayed false information, or are you actually asking "Was ATC in any way responsible for this accident?". That I can answer from the official reports. $\endgroup$
    – Jamiec ♦
    Sep 20, 2017 at 10:43
  • $\begingroup$ @Jamiec The altitude information ATC relayed back was what the plane itself reported to ATC. Neither the pilots nor the controller appeared to realize this. So I wonder if it was bad training or bad user interface design. $\endgroup$
    – snips-n-snails
    Sep 20, 2017 at 14:16
  • $\begingroup$ Yes I got that. Have you read the english traslation of the ATC transcript by any chance? The controller was just reading back what they saw on their screen along the lines of "Yes, we have you at 4000ft". Anyway I think you have an answer to the "cause" which was the source of the controllers info. See @stelios' answer. $\endgroup$
    – Jamiec ♦
    Sep 20, 2017 at 14:25
  • $\begingroup$ @Jamiec My question is why didn't the controller realize that the altitude was what the plane reported and not from radar trigonometry as ratchet freak answered below. Maybe he wasn't properly trained to know this, or maybe the UI on his own screen needed a redesign to make it clear where the data comes from. ("The Design of Everyday Things" by Don Norman has more examples of user interface failures that may be of interest.) $\endgroup$
    – snips-n-snails
    Sep 20, 2017 at 15:05
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    $\begingroup$ @jamiec Well, the pilots had told ATC their altimeters had failed. Presumably if the UI was clear that the data was coming from the plane, ATC wouldn't have believed it (or at least been suspicious). $\endgroup$
    – mbrig
    Sep 20, 2017 at 15:52

3 Answers 3

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who was responsible for ATC relaying false altitude information to the pilots

Not an easy question mostly because it asks (even if not directly) to put the blame on someone. And although there is not an explicit rule for not doing this, I prefer avoiding it in respect to the dead and the ATCOs doing their job as good as they can.

I will focus on the reason behind the "false reports" and not the responsibilities. The RADAR system the ATCOs use relies on aircraft systems for two things: one is to identify the flight (Mode A) and the other is to get its altitude in hundreds of feet (Mode C).

Now if you recall what happened, the static ports were duct taped for some routine cleaning of the fuselage and were not removed. This affected the airplane's altitude indicator and consequently the altitude transmitted by Mode C.

So the ATCO was reporting back to the plane the same erroneous altitude the pilots were seeing in the cockpit.

has anything changed since then to prevent that from happening again?

I don't know if there was any action taken after the accident but I think that there wasn't anyone needed: there is regulation (need to make some research to find it) which specifies that all "foreign" parts attached to a plane for protecting its vital parts should have a prominent red "REMOVE BEFORE FLIGHT" ribbon.

The duct tape was of silver non reflective color (if I recall correctly) and had no ribbons. When the crew inspected the plane at night with flashlights they couldn't spot them.

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    $\begingroup$ Independent of plane systems- ATC should be able give accurate information. It would a mystery why they didn't think wouldn't be important. $\endgroup$
    – user2617804
    Sep 21, 2017 at 9:47
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    $\begingroup$ @user2617804 No actually ATC gives only the information they have. And not all information is accurate since they rely on 3rd parties in acquiring them. There are vertical scanning radars that can provide altitude information but are rare for civil use and their accuracy reduces with range. And they don't really help preventing accidents $\endgroup$
    – Stelios Adamantidis
    Sep 21, 2017 at 10:01
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What caused the ATC to pass incorrect information to the flight crew was that they had no reason to mistrust their own instruments. The altitude information displayed to the controller was transmitted from the aircraft itself, and at the time the controller was no more aware of the issue on board the aircraft than the flight crew were.

The NTSB, although not the investigating authority did produce a short report which noted that:

Section 12-25-01 of the Boeing 757 Maintenance Manual contains the instruction for cleaning and polishing the airplane. When preparing the airplane for cleaning and polishing, maintenance personnel are instructed to prepare the airplane by taping moisture-resistant paper over the static ports to prevent the entry of any contaminant

The safety advice which came from this was:

Immediately review and amend, as necessary, all airplane maintenance manuals to require operators to use only standardized, highly conspicuous covers with warning flags attached in any situation in which static ports would need to be covered.

Ultimately the crash was caused by failure of the maintenance crew to remove the static port tape, failure of the flight crew to spot this defect on inspection. (The following quotes are from here).

It can be deduced from the investigation carried out that the maintenance staff did not remove the protective adhesive tape from the static ports. This tape was not detected during the various phases of the aircraft's release to the line mechanic, its transfer to the passenger boarding apron and, lastly, the inspection by the crew responsible for the flight (the walk-around or pre-flight check), which was carried out by the pilot-in-command

Together with failure of the flight crew to respond appropriately to the ground proximity warning system.

The pilot-in-command [...] made a personal error by not complying with the procedure for GPWS alarms and not noticing the readings of the radio altimeters in order to discard everything which he believed to be fictitious

And furthermore

The co-pilot [...] made a personal error by not being more insistent, assertive and convincing in alerting the pilot-in-command much more emphatically to the ground proximity alarms.

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I believe that for point 1 that both ATC and pilot assumed the height information on the ATC radar display came from radar trigonometry and not the reply of the transponder. Neither was at fault here and this is more likely due to bad training on both sides.

If either had realized where the altitude information came from then the mode C transponder would have been deactivated and if the radar station tracking them was equipped with to do altitude triangulation then the ATC could have provided better altitude information.

Incidentally with GPS being more common nowadays it can serve as a backup altitude indicator.

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