An air France captain view and comments on Airbus systems following the AF 447 crash
PNC Contact, hôtesse de l'air, Steward, formations PNC et informations hôtesse de l'air. ? AF447 ? Le rapport de contre-expertise d?un pilote
The full version is in french have translated some exceprts below.
How can a stalled a/c enter a deep stall and then not recover ?
Looks like Airbus series have been designed as "Stall Proof" a/c, algorithms in their systems and flight controls will not be able to deal with stalls and their recoveries
Few minutes after the captain of AF 447 left the flight deck at 02:00 AM, alarms start to ring in the cockpit.
Autopilot disconnects; ATHR (autothrottle) disconnects as well.
These alarms ring continuously for 34''
A/C starts to be unstable due to switching to 'ALT LAW' mode as a consequence of erroneous speed readings due to clogged pitot tubes which at first glance the crew had no way of knowing.
First issue confusing the crew in their analysis of the situation is that the 'FWC' malfunction management system, able to warn the crew on erroneous altitude readings (NAV ALT discrepancy) is unable to do so for erroneous speed indications. the ECAM screen ( Displaying malfunctions and necessary actions to treat them) will be of little help.
Only a NAV IAS DISCREPANCY feature would have helped, but it is not available on the ECAM.
This particular issue provided it is positively identified, can only be adressed via a paper checklist, and as the BEA ( Bureau Enquetes Accidents) clearly states pp. 181 and 203 in the final report: ''ECAM does not provide any indication likely to enable the crew to spot a speed indication malfunction''
The BEA unlike Airbus raises this issue p. 182 and makes a series of recommendations.
Another point which will have a bearing as events unfold is that as a consequence of pitot tube clogging There is a loss of altitude, however marginal this loss ( 350 to 350 ft ) this point is crucial in the understanding of the sequence of events as it explains the crew first move is to pitch up in order to regain the lost altitude (BEA p.179)
Airbus features provide efficient aids through FD (Flight Director) and ''Speed Trends'' (an arrow indicating a speed prediction within the next 10 seconds), very useful during the deceleration and acceleration phases of flight.
FD's are bars helping the pilot to follow his trajectory.
In the time lapse between the onset of the trouble and the stall these aids have been confusing the crew, the Speed Trend indicating an acceleration tendency (!) after the stall warning bell went on for a mere 2'' before going silent again . (BEA p.100, p.186) At the same time these tendency bars went on and off erratically further impairing the pilot s judgement (BEA p.204)
All these events have been unfolding in a degraded environment, at night, in the vicinity of a storm, in a cockpit beset by alarms and bells...with multiple malfunctions to handle and an unstable manual pilotage in ALT Law mode, compounded with erroneous indications of overspeed and tendency bars ordering a pitch up.....(BEA p.183)
As the stall warning went on briefly (BEA p.195) for a mere 2'', whereby the ATHR bell rang continously for 34'', in such a saturated an stressful environment it could easily have gone unnoticed
45'' after the alarms rang, there have been alternate actions to pitch up and down by the crew
And here we need to bring up the peculiarity of Airbuses joysticks and TRIM.
Joysticks only function is to send inputs to the computer management system which either accepts them or rejects them (Hence the motto that airbuses cannot stall)
Whenever the computer acts on a control surface, the joysticks don't move and have no pressure feel, preventing the crew from receiving any sensorial feedback. Left and right joysticks are non interconnected preventing any feedback among the crew members themselves.
And the TRIM: On the Airbus series the trim is automatic, silent, and never used by the crew in normal operations.
The trim on the elevator works as follows: The pilot commands an action which moves the rear part of the elevator ( 1/3 of the surface) the computers then manage the 'trimming action' moving the PHR or ''Fixed part'' forward section of the elevator (2/3 of the surface)
The elevator of AF 447 within 45' from the triggering of the first alarms moves from straight and level to full
pitch up when the stall warning briefly goes on. (BEA pp.64. 66)
This elevator design would also prevent recovery from the stall as a pitch down push action on the yoke to reduce angle of attack would be acting on the rear portion of the elevator only (1/3 of its surface) , the remaining computer controlled 2/3 surface remaining unaffected unless the system senses the stall which may not be in the algorithms as by nature Airbus series are stall proof.
On p.193 of the accident report it is stated that it is not necessary to exert a pitch up action to compensate for a speed decay to maintain lift.
Therefore on the Airbus 330 with its uncoordinated joysticks with moving surfaces and auto trim! The crew have little or no sensory feedback to determine a speed decay.
One would wonder how a pilot would keep pulling on the stick while the stall warning is ringing.
This brings us to the peculiarity of the stall warning on the Airbus.
As noted above the stall warning rings for a mere 2'' in a stressful environment where the ATHR bell goes off consistently may have been overlooked by the crew, considering at this moment the Speed Trend bars show a speed increase tendency (!)
THE STALL WARNING ON THE AIRBUS DOES NOT COMPLY WITH CERTIFICATION REQUIREMENTS and prevented the crew from identifying the stall.
On pp. 143 however the BEA states that it '''complies with the certification requirements in in force at the time of type certificate approval'' though it is also stated (p.47) that if CAS speed values are below 60 Kts stall warning is inoperative (!!!!!!!)
This feature is destined to prevent triggering the warning while the a/c is on the ground.
Airbus considering that whenever the speed is below that value the a/c is on the ground.
Or is it ?
In contrast Boeing considers that an a/c is sitting on the ground when the front gear leg is compressed under its weight.
With reference to the ''Certification Specifications for Large Airplanes'' CS25
CS 25.207.c reads that the stall warning bell should be ringing continuously whenever pitch angle is above stall speed.
On AF447 during the deep stall speed was below 60 kts, with the Stall Warning inop per design.
What further aggravates matters is that it goes on again whenever speed increases to that value, ringing during a nose down action to reduce angle of attack.(BEA pp. 196, 197)
The PF made a corrective pitch down action (BEA p.25) when the a/c was in a deep stall at 40 deg pitch up fully stalled. He was confronted to a Stall Warning deactivating when he was pulling and activating again when he was pushing.
So here we have a situation where the ECAM does not say what is happening, pilot s aids are malfunctioning, the trim helping the a/c stall and a Stall Warning that works the other way around, how could any crew operate in such degraded environment ?
In this cockpit built by engineers, pilots did what they could.
Airbus a/c are by no means dangerous, they are ergonomically flawed, they are as easy to operate on a daily routine basis, as they are tricky and dangerous in critical situations.
There have been a series of stall incidents and accidents, and we can only hope that the Air Asia crash will not be a Rio bis