It also reminds me of a time years ago when a team I was on had to extend a Memcache client library, as it couldn't differentiate between the following two conditions:
- "I tried to retrieve a key from cache but was unsuccessful (due to connection error, etc)"
- "I tried to retrieve a key from cache but determined conclusively that key does not exist in the cache"
If it was important for a caching layer on our silly social game, the same thing is clearly important in aviation.
https://www.hl7.org/implement/standards/product_brief.cfm?pr...
in the real world it turns out not to be useful to require that people have names just in order to keep data about them...nor does a name particularly distinguish one person from another.
But the display which displays raw AoA readings to the pilots is an optional extra, only 20% of 737 maxes ship with it enabled. Normally pilots (or at least commercial pilots) are trained to fly without referencing the AoA by staying well within the safety margins.
So, if the AoA isn't shown anywhere in the default configuration, there is no way for the pilots to crosscheck them and see the fault. Which is why you need the AoA disagree warning message.
Unfortunately, Boeing messed up and the AoA disagree warning was only enabled on the planes with the AoA display option enabled.
But the article buried the lede!
There is much bigger problem reported now in this article:
The warning wouldn't have helped even when flying with those companies that paid extra for it!
Note:
"“We were told that if the A.O.A. vane, like on Lion Air, was in a massive difference, we would receive an alert on the ground and therefore not even take off,” said Dennis Tajer, a spokesman for the union representing American Airlines pilots. “That gave us additional confidence in continuing to fly that aircraft.”
But in the last several weeks, Boeing has been saying something different. Mr. Tajer said the company recently told American pilots that the system would not alert pilots about any sensor disagreement until the aircraft is 400 feet above the ground."
Note what happens then: at the moment you are above the ground, and the MCAS activates, we already know that if you "just follow the procedure" the forces against pilot corrections could be too big to do anything. That's what can be concluded from the analysis of the Ethiopian crash.
That means the plane from the "20%" with "paid option" would still eventually crash as soon as AoA vanes fail, just there would be one more of many alerts in the cockpit for some short time before everybody dies.
The sense of "security" Boeing gave the American pilots flying for the companies who paid for that additional feature was completely false. They would have been doomed too as soon as a single AoA vane that the MCAS depended on failed: The warning worked only at the moment it was completely useless.
The U.S. MAX planes were grounded the last, yet they were in the same danger as the other planes.
This is about adding to the narrative of Boeing's "general mismanagement" of the aircraft. Not about adding to the particular 737-800 crash investigation story (directly).
So the warning message could have potentially prevented both crashes and anything like it (assuming it activated during the takeoff roll, before V1 and the pilots were trained to abort take off when it happened)
Though you are right, if the AoA sensor had failed halfway though a flight, they still would have had problems.
Are you sure? The AvHerald narrative for the Ethiopian Airlines flight:
"The takeoff run was entirely normal, both AoA sensors were in agreement. Shortly after becoming airborne the left AoA sensor however began to deviate and reached a position of about 85 degrees nose up, which obviously triggered the left hand stick shaker motor (and provides a very noisy and distracting cockpit environment), which in turn, different to the 737 NG behaviour (where the AoA does NOT correct the pitot data), also caused the IAS and ALT data to differ from the right hand system, in particular the left IAS became 12 knots higher than the right hand IAS (the preliminary report does not mention that the IAS DISAGREE warning activated however, which would invoke the unreliable airspeed procedures). The captain was pilot flying and focussed to keep the aircraft on track (via the flight director) due to terrain around while at the same time trying to understand why the stick shaker activated. Several required callouts by the first officer did not occur (e.g. speed, ...), as result the crew lost speed as well as thrust control completely out of sight."
As I said somewhere below, I don't think the AoA disagree indication would have made much of a difference here.
"We were told that if the A.O.A. vane, like on Lion Air, was in a massive difference, we would receive an alert on the ground" ...
"But in the last several weeks, Boeing has been saying something different. Mr. Tajer said the company recently told American pilots that the system would not alert pilots about any sensor disagreement until the aircraft is 400 feet above the ground."
[0] https://en.wikipedia.org/wiki/Ground_effect_(aerodynamics)
No. A wing will stall at a consistent critical angle of attack. Ground effect works by increasing the lift available for a given angle of attack, not by allowing a larger angle of attack to develop.
Parent comment was asking about the need for minimum airspeed to set the AoA sensors into a reliable state such that they could be checked at the gate, though, which is unrelated to ground effect.
Granted, btw, that AoA readings on the ground may be unreliable - the question remains whether AoA disagree could be detected reliably before takeoff.
An abort just prior to V1 is a no-joke exercise from a safety standpoint. MCAS operation is suppressed with any flaps deployed, so taking an airplane with a failed AoA sensor into the air and troubleshooting/returning to the departure airport if needed or simply flying with an INOP MCAS may very well be safer than a high-speed abort.
Conversely, if your airline has not ordered the optional AoA indicators, and you'd really want the "AoA disagreement" warning light/message, you don't get it.
https://www.quora.com/Is-the-moon-there-when-nobody-looks-If...
I think this is the wish of very a many physicist as they drink their morning tea and contemplate the day.
Electricity suddenly stops working.
Which is why there's a threshold that much be crossed (IIRC ten degrees) before the disagree warning appears.
If you're slipping without flaps I guess it could still be a problem if the disagreement is outside the epsilon spec. I'm not sure if [normal] slip could ever cause that much disagreement.
(I've had the controls of a 172 and a Tiger Moth briefly, but have never _really_ flown a "real plane". I have flat spinned many many rc planes, mostly recovered fine, but also to their occasional doom...)
They're absolutely different. If you're a pilot and you expect to see an error message when the two alpha vanes disagree, you'll be lulled into a false sense of security when that text never appears even in the case of a failure.
A pilot needs to be able to trust all of the instruments in the plane.