Unreliable Airspeed – EuroCockpit – SmartCockpit Links
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Air France Flight 447: Unsuccessful Searches And Pitot Maintenance (UPDATED) Black Boxes NOT Located?
By Joelle Barthe – Flight Operations Engineer
Published on SafetyFirst #5December 2007
Unreliable speed is one of the difficul situations that a pilot has to face. Once the failure has been identified, a procedure, based on pitch angles and thrst settings, will assist the pilot in safely flying the aircraft.
But the main difficulty is to rapidly detect an unreliable speed situation. Reaction time is crucial, since the aircraft may stall and overspeed conditions could cause aircraft damage.
The effects of pitot probes obstruction on ground
It intended to make ground and flight crew more sensitive to the consequences of obstructed probes, and to prevent take-off with unriliable speed.
But once airborne, how can the crew handle an unreliable speed situation?
This article is based on A320/A330/A340 design. Cockpit effects, identification and troublshooting, remains similar for wide body aircraft and A380, with some specificities covered in the operational documentation.
2 Effects and consequences in the cockpit
Water, ice, dust, ashes, etc. may pratially or totally block pitot probes and static ports. Equally,tubes misconncected to the Air Data Modules (ADM), plastic covers not removed from probes, insect nest, radome damage, may lead to enrroneous pressure measurements.
The consequences of this erroous pressure information, once used by the ADRs, and/or the standby instruments, are the computation and the display of unreliable speed and/or altitude for all users.
Erroneous speed or altitude indications can be suspected, among others, in the following cases:
- Speed discrepancy (between ADR 1, 2, 3 and standby indication),
- The flutuction of the Indicated Air Speed or of the Pressure Altitude.
- Abnormal correlation between basic flight parameters (IAS, attitude, pitch, thrst, climb rate),
- abnormal AP/FD/ATHR behaviour,
- STALL and OVERSPEED warnings or FLAP RELIEF on ECAM that are in contradiction with ar least one of the indicated airspeeds,
- Inconsistency between radio altitude and pressure altitude,
- Impossibility of extending the landing gear by the normal landing gear system.
Nevertheless, it should be emphasized that identifying an unreliable speed indication is not wlways obvious: no single rule can be givien to conclusively identify all possible erroneous indications and the display of contradictory information may confuse the flight crew. Pilots should therefore be aware of unreliable speed symptoms and consequences.
Depending on the effected probe, i. e. pitot probe os static port, differente indications in the cockpit will become unreliable. Therefore the crew should be aware that some of the usual cues to fly could be unreliable as indicated:
3 Identification and Handling of Unreliable Speed situations
Airbus has developed procedures and guidelines to help crews identify and handle an unreliable speed situation.
The Volume 3 of the FCOM and QRH provide the UNRELIABLE SPEED INDIC / ADR CHECK PROC procedure.
In addition, Airbus has developed training material in the Flight Crew Training Manual ( FCTM, available for A320/A330/A340/A380). The FCTM provides information about the causes and consequences of unreliable ADR computations. It also provides information on how to apply the UNRELIABLE SPEED INDIC / ADR CHECK PROC of the QRH.
An interative trainin tool, the e-Briefing, is also available on https://w3.airbus.com/ in the Flight Operations community, under ther heading “Safety and Operational materials”.
4 – Procedures
As soon as a doubt about airspeed indication arises, or a relevant ECAM alert is triggered (relative to ADRs failure or discrepancy for instance), the UNRELIABLE SPEED INDICATION/ADR CHECK PROC procedure should be applied by the crew, following this sequence:
1) If the safe conduct of the flight is affected, APPLY THE MEMORY ITEMS, i. e. fly a pitch with TOGA or CLB thrust,
2) If the safe conduct of the flight is not affected, or once the memory items have been applied, LEVEL OFF, if necessaru, and start TROUBLESHOOTING,
3) If the affected ADR can be identified, fly with the remaining ADR.
4) If the affected ADR cannot be identified or all airspeed indications remain unreliable, FLY WITH PITCH/THRUST REFERENCES.
4.1 Memory Items
If the safe conduct of the flight is affected, the flight crew applies the memory items: theses allow “safe flight conditions” to be rapidly established in all flight phases (take-off, clim, cruise) and aircraft configurations (weight and slats/flaps). The memory items apply more particularly when a failure apprears just after take-off.
Once the target pitch attitude and thrust values have been stabilized at or above minimum safe atltitude, or when the safe conduct of the flight is nor affected, the flight crew enter the 2nd part of the QRH procedure: level off the aircraft and perform troubleshooting.
4.2 Troubleshooting and isolation
The table provided in the QRH gives the pitch (º) and thrust (%N1) to be applied to level off the aircraft according to its weight, altitude and configuration, along with flying technique advices.
In situations where most primary flight data are erroneous, some indications may stil remain correct and should consequentely be used to help the crew stabilize the flight path. This is the case for the Flight Path Vector (FPV), reliable if the static ports are not blocked, and for the GPS altitude displayed on the MCDU, when GPS is installed.
When the flight path is stabilized, the flight crew will start the troubleshooting, keeping in mind that sometimes two or even all three ADRs might provide identical but erroneous data (e.g. due to icing conditions, flight in volcanic ashes, etc).Therefore, do not instinctivelu reject an ADR that is suspected to be affected.
If the troubleshooting procedure enables the crew to identify the affected ADRs, then a normal situation can be. resumed.
But if the affected ADR cannot be identified, or all ADRs are affected, then the flight crew will fly without speed reference, using the pitch and thrust tables.
4.3 Flying using pitch/thrust tables
First, the crew has to switch OFF two ADRs and keep one ADR ON, to keep the Stall Warning Protection.
Then, the crew will [bold] fly the aircraft without speed references, using pitch (º) and thrust (%N1) settings.
To fly the aircraft using pitch and thrust settings, the crew will find in the QRH the tables relative to each phase of flight: Climb, Cruise, Descent and Approach, talking into account the aircraft weight, configuration and altitude. With theses tables, the crew will be able to safety land the aircraft.
5 Back UP Speed Scale (BUSS)
In order to dedrease the crew workload in case of unreliable speed, Airbus has developed the Back-UP Speed Scale (BUSS) that replaces the pitch and thrust table. The BUSS is optional on A320/A330/A340. It is basic on A380, being part of the ADR Monitoring functions.
This indication is based on angle of atack (AOA) sensor information, and is therefore not affected by erroneous pressure measumements.
The BUSS comes with a new ADIUR standar (among other new system standards), where the AOA information is provided through the IRs and nor through the ADRs. This enables selecting all ADRs off without loosing the STALL WARNING PROTECTION.
The AOA information provides a guidance area in place of the speed scale. When the crew selects all ADRs OFF, then:
- The Back-Up Speed Scale replaces the PFD speed scale on both PFDs,
- GPS Altitude replaces the Altitude Scale on both PFDs.
The Back-Up Speed Scale then enables to fly at a safe speed, i. e. above stall speeds, by adjusting thrust and pitch.
The BUSS will be displayed once all ADRs are switched OFF. Therefore, on aircraft that have the BUSS, when the flight crew cannot identify the faulty ADR(s) when performing the troubleshooting, or when all ADRs are affected, the flight crew will switch OFF ADRs, and will fly the green area of the BUSS.
However, if the safe conduct of the flight is affected, the memory items must still be applied before troubleshooting.
As the BUSS is associated to the ADR monitoring funcitions, some unreliable speed situations can be automatically detected (e. g. new ECAM warning “NAV ADR 1+2+3 FAULT”), and some ECAM procedures will lead to the BUSS activation by requesting to switch OFF all ADRs.
An unreliable speed situatio may be difficult to identify, due to the multiple scenarios that can lead to it. Therefore, training is a key element: indeed the flight crew’s ability to rapid detected the abnormal situation, and to correctely handle it, is cricial.
In case of any doubt, the pilot should apply the pitch/thrust memory items, and then refer to the QRH to safely fly the aircraft, and to positively determine the faulty source(s) before eliminating it (them).
In addition, to further assit the pilot in detecting the failure and safely fly the aircraft, Airbus has developed the BUSS, which provides a safe flying range indication.
Finaly, to reduze the probally of experiencing unreliable speed situations, on-ground actions, such as comprehensive maintenance and through pre-flight exterior inspection, should be stressed.
Although the debate is not authorized until the BEA will decide to talk about the probes issue, and in response to many questions we receive from around the world, we believe it would be useful to clarify a few points.
The operator and manufacturer state that they have «recently discovered» a malfunction with the Pitot probes. They claim they have been working very hard towards finding a solution and state that it is only recently that solutions have been found. We hold a document that may hold a strong indication of the contrary. The problem was acknowledged way before the accident. This document has been provided to us from a well known website that our readers are familiar with. We have often turned towards this web site to seek answers to our questions : type http://forfaiture.freeservers.com in your web browser.
This “Technical Note” comes from Air France and seems to be an authentic document. We audited the origins, dates, and metadata contained in the original file. We have confirmation that this document was valid, validated, and still current as at 31 May 2009. No one had heard from the plane and no one could have imagined that the plane was never going to land in Roissy and yet this document had been printed out, that same day, in order to advise the tech team on the ramp to get ready to change some probes on the aircraft’s arrival.
This paper demonstrates several things:
- ‘We’ were already aware of what the problem was the morning of the accident (the famous erroneous airspeed). There are passed records that display the exact ACRAS messages sent by the AF477. The same causes had already produced the same effects. It is likely that the AF447 experienced the same problems as all the other aircraft that encountered a Pitot defect, the only difference is that those other aircraft had managed to free themselves from that situation.
- The BEA’s communications through the media have raises questions. We believe that the families of the victims have been left in confusion, dragging in despair all that has been said in the press. In the end, we feel that an explanation of the origin of the accident is probably dramatically simpler.
- At the date of this document (file made in June 2008, note dated August 2008), it is written that there had already been 6 cases that had produced the same alarms. The Pitot probes were designated undoubtedly as the cause of these problems (the french version of this note is slightly different than its english translation) : ”At the time of creation of the NT, a case on THT and six cases on A340 AFR have been reported. ” (THT is Air Tahiti Nui, who’s aircraft are maintained by Air France). On june the the 19th, M. Gourgeon (CEO of Air France) stated on RTL (a french radio) that there had never been an incident “before August 2008″. How are we to qualify this statement?
- It is also written that Airbus was aware of the problem at the date stated in the above: “Investigations conducted on Airbus family aircraft showed that most of airspeed discrepancy events were due to Pitot water ingress and to probe draining holes obstructed by external particles. Another hypothesis is in study on a possible saturation of pitots by crystallized ice in high flight level.”
- It is written that the probe type BA have solved the problems: “The new pitot probe corrects the problems with enhanced water trap and relocated drain holes.”
- It is written that “the operational impact” is “NIL” (nil (nihil) = not applicable): how can we imagine that there is no operational impact after the loss of so many vital systems and informations? Who are the pilots or services involved in flight safety that validated this unbearable assertion?
- It is even considered to be a “corrective action”, but only under certain circumstances, that of which the breakdown occurs first on the plane … permitting the installation of “a new standard pitot PN: C16195BA”
Reading the documents from summer 2008 show the awareness of the problem is official and a technical note was therefore created. Unfortunately, this also show the solution to the problem was official, it had been written but never taken care of. By August 2008, 6 different problems had already existed in Air France. Why did Air Caraïbes decided to change its probes in September 2008, a month after it’s first warning, while others with the same problems simply did nothing?
We, like the BEA, have almost approached a conclusion. What we need now is to understand why our colleagues flying the AF477 were unable to get the aircraft out of the mess. After consulting the pilots that have been faced with these same Pitot issues, we realize that a large amount of immediate judgement is needed to overcome the multiple alarms, in a very little time. The cockpit crew must execute long check-lists and procedures, complex and even somewhow contradictory… By night, in manual flight, without any speed reference, with the probable loss of attitude, and partial stand-by instruments… Only a donkey could state in public that a simple hold of «pitch and power» could be enough to solve the problem.
There have been 35 worldwide reported cases of this Pitot probes issue, including 9 this year. The AF447 flight could have become the 36th case on the list, and became the first case that never made it back home.
The day that entering in a cloud will be held as the main reason for 35 different cases of serious A330 / A340 incidents will be the day that we will possibly imagine that the AF477 represented the 36th case. On that day, we will ask Mr Feldzer his opinion on the clouds.
One last important thing that we must do : in order to let you compare the events experienced by the flight AF447 with those described in the technical note mentioned above, we would like to expose the actual list of the contents of the famous ACARS messages sent by the plane. In order to keep our sources secret, we have transformed the appearance of the original file, but the content has remains exactly the same.
There are of course the alarms and warnings mentioned in the Air france technical note.
This Pitot probes issue is the strongest to date.
And it is the most carefully avoided by officials.
No doubt it’s a coincidence …
NOTE: the order of faults and warnings represented here is not consistent with the chronology as experienced on the flight AF447. We are rebuilding this “probable chronology” with specialists, the representation attached is a view sorted as it would be done by the «Airman» software.
publié le 2009-06-24 13:35 par EuroCockpit.
This dissertation grew from a review I did of positions presented in Tim Vasquez’s weather analysis of conditions he expected would have been encountered by AF447 enroute from its last known position at INTOL on Airway UM873.
My original calculations, that alerted me to a potential error in the positions, were not correct either! I had transposed a ground speed. On top of that the only fact we had was the last report from AF447 at INTOL and the manner in which the TASIL position had been derived was from a questionable news report stating TASIL was expected within 50 minutes…. Barry Carlson
SmartCockpit Airbus 330 Related Links (pdf files)
Airbus 330 – Instructor – Unreliable Speed Indication
Airbus 330 – Systems – Maintenance System
Airbus 330 – Flight Deck & Systems Briefing For Pilots
Airbus 330 – Misc – Flight Crew Training Manual