Tuesday, 4 December 2012


A subject rarely covered in recurrent simulator training, often due to time constraints but one which has proved to be more dangerous than any engine failure scenarios, is one where the crew of a jet aircraft is faced with either a total or partial loss of Air Data Computer information.
The aircraft itself is to be found perfectly capable of flight, however, often fatal crashes result due to the crew receiving incorrect airspeed and altitude information and then acting inappropriately.
When I was undergoing my basic training, I commenced a ‘solo’ take-off in a little two seat Piper Tomahawk from Oxford airport. As I accelerated down the runway I noticed that my IAS was not increasing, now I’d never rejected a take-off, so continued down the runway, until the little aircraft became airborne of its own accord. I was climbing away at an above normal pitch attitude and with full power and 0kts IAS. Now I knew I must be faster than 0kts as I was neither a helicopter nor dead! I reverted to P.A.T….Power…Attitude and Trim, I flew an untidy circuit at what must have been a higher than normal approach speed as the houses below seemed to be getting bigger much quicker than normal! I touched down, bounced and then stabilised and taxied to the ramp to meet my equally bemused and relieved instructor. Investigation by the engineers found that the hose behind the pitot tube had become detached. I learnt a very valuable lesson, very, very early in my career.
It is time for all of us to review and learn from the mistakes of others and use current technical information and training guidelines to assist us should we ever be faced with both a widely confusing and terrifying situation, especially at night and/or in IMC. I was lucky it was a beautiful clear summer’s day.
These days’ modern glass cockpit jet aircraft have numerous backup systems alongside an independent standby system to relay correct airspeed and altitude information to the pilots. However, with the addition to numerous warning systems such as Stall Warning, Overspeed and Airspeed Low; the combination of this information can conflict in our perceptions of our situation. This confusion degrades the crew’s management and performance, again often with fatal consequences.

With the recent very public accident involving Air France 447, the aircraft involved, an Airbus 330 crashed as a result of some of the previous mentioned reasons. It now seems that finally the aviation industry is waking up to the realisation that something needs to be done. Americans call this a Monday morning quarterback response, in the UK we say ‘closing the stable door after the horse has bolted’. The result is the same; this wake-up call has come too late for countless hundreds of passengers and crew. Correct training should assist in our own behaviour and responses when faced with a similar situation, eliminating as much as possible the chances of reading accident reports similar to those described further on.

The findings of the Air France flight 447 crash report stated, amongst others, the following:

1.       There was no explicit task-sharing between the two co-pilots. (CRM FAILURE)

2.       There was an inconsistency between the measured speeds, likely as a result of the obstruction of the Pitot probes in an ice crystal environment. (INSTRUMENT FAILURE)

3.       Even though they identified and announced the loss of the speed indications, neither of the two co-pilots called the procedure "Unreliable IAS". (S.O.P. FAILURE)

4.       The co-pilots had received no high altitude training for the "Unreliable IAS" procedure and manual aircraft handling. (TRAINING FAILURE)

5.       No standard callouts regarding the differences in pitch attitude and vertical speed were made. (S.O.P. FAILURE)

6.       Neither of the pilots made any reference to the stall warning and neither of the pilots formally identified the stall situation. (EXPERIENCE/TRAINING FAILURE)


As we all know, during CRM training we discuss the ‘Swiss cheese model’ where it is generally not one action which causes an accident, but multiple actions which when added together, lead to disaster. The above six points relating to Air France show five different ‘actions/failures’ which led to the deaths of 228 passengers and crew, on an aircraft which was perfectly capable of flight.

Again and I cannot reiterate this enough. We all need to learn from these mistakes, through refresher training and ensure that through knowledge and practice; can respond correctly and safely should we ever be unfortunate enough to encounter a similar situation.



Finally both Boeing and Airbus have recognised the need for further training in recognising and recovering from the situation where the pilots have to, sorry, need to ‘return back to basics’; so being able to produce a safe and successful outcome when faced with problems affecting pitot and static failures.

For example, with respect to the soon to be introduced Airbus A350, Airbus Company’s training department has decreed the following at a recent conference.

“Airbus is going to train pilots for its A350XWB differently.

The first three days in the A350 simulator will be about letting the pilots find out that it is "just another aeroplane". Without using any of the sophisticated flight guidance systems they will be able to find out how it flies and what that feels like. These pilots may not have done that for years on the aircraft they fly now, so they might find out a few things about themselves as well as the A350.

Airbus' flying training manager David Owens stated at the Royal Aeronautical Society's annual Flight Crew Training Conference in London that pilots will not be allowed to switch on the automatic systems until they have learned how to fly the aeroplane.”

Although Airbus’s Owens didn't spell it out, it seems the industry is beginning to learn that never letting the pilots treat the aeroplane like a flying machine means they never find out what it can do. And more importantly, what it can't; so again requiring us to go back to basics.

Boeing has been working with the FAA and the aviation industry to develop training aids that improve a pilot's ability to respond to challenging situations. Boeing has provided an updated training aid as part of their continuing effort to reduce loss-of-control airplane accidents. The upset recovery training aid focuses on helping flight crews recover from unusual flight attitudes that can result from unusual weather or other "upset" conditions as a result of loss of reliable cockpit information. This enhanced training also increases the pilot's ability to recognize and avoid situations that can lead to airplane upsets, and potentially fatal consequences.




Going ‘back to basics’ starts with understanding how the pitot static system, when functioning incorrectly can affect our displayed information.



For example when the static ports are blocked, disaster can and has occurred. Partial recognition of this can be explained as follows.

Should the static ports be blocked or covered, then during the take-off roll, both the altimeter and the airspeed indicator operate correctly. After lift-off, assuming the trapped static pressure is that of the field elevation, the altimeter indication remains at the field elevation. With respect to airspeed, the sensed dynamic pressure fails to increase as rapidly as it should during climb because of the trapped static pressure. Therefore, if the airplane actually climbs at a constant speed, the airspeed indication decays, reaching the lower end indication. If the captain relies on the airspeed indicator for proper information, the typical response will be to reduce the pitch attitude to maintain the erroneous airspeed, possibly causing the airplane to exceed its airspeed limitations. Complicating this situation is the fact that the overspeed warning does not operate if connected to the same erroneous airspeed source. 

The above scenario led to the following accident. On October 2nd 1996 shortly after takeoff just past midnight, the Boeing 757 airliner crew discovered that their basic flight instruments were behaving erratically and reported receiving contradictory serial emergency messages from the onboard computer, such as rudder ratio, mach speed trim, overspeed, underspeed and flying too low. The crew declared an emergency and requested an immediate return to the airport.
Later investigation into the accident revealed that duct tape was accidentally left over some or all of the static ports (on the underside of the fuselage) after the aircraft was cleaned, eventually leading to the crash. One of the company’s employees had left the tape on by mistake.

As a result of the blocked static ports, the basic flight instruments relayed false airspeed, altitude and vertical speed data. Because the failure was not in any of the instruments but rather in a common supporting system, thereby defeating redundancy, the altimeter also relayed the false altitude information to the Air Traffic Controller, who was attempting to provide the pilots with basic flight data. This led to extreme confusion in the cockpit as the pilots were provided with some data (altitude) which seemed to correlate correctly with instrument data (altimeter) while the other data provided by ATC (approximate airspeed) did not agree.
Although the pilots were quite cognizant of the possibility that all of the flight instruments were providing inaccurate data, the correlation between the altitude data given by ATC and that on the altimeter likely further compounded the confusion. Also contributing to their difficulty were the numerous cockpit alarms that the computer system generated, which conflicted both with each other and with the instruments. This lack of situational awareness can be seen in the CVR transcript. The fact that the flight took place at night and over water, thus not giving the pilots any visual references, was also a major factor. All onboard died in the ensuing crash. 


Blocked or malfunctioning pitot tubes cause their own system malfunctions and subsequent erroneous information indications. If the pitot probe is plugged, its sense line likely contains air trapped at a pressure equal to that of the field elevation static pressure. During the take-off roll, therefore, the sensed dynamic pressure remains zero and the airspeed instrument remains pegged at its lower stop. If the flight crew does not reject the take-off, the pitot pressure remains plugged at field-elevation pressure as the airplane climbs, but the static pressure begins to drop. The altimeter operates almost correctly during the climb. However, the resulting sensed dynamic pressure causes the airspeed indicator to come alive seconds after lift-off. Regardless of the actual climb speed of the airplane, the faulty airspeed indication continues to increase as altitude increases, until the airspeed catches up to the correct value. The indicated airspeed continues to increase through the correct value as the airplane climbs. The VMO speed can appear to be exceeded. Additionally, an overspeed warning can be triggered. If the pilot flying trusts the faulty airspeed indicator because of the temptation early in the climb to believe that some movement means the indicator has begun to operate normally, the pilot flying is in grave danger of increasing pitch, reducing thrust, or both to reduce the erroneous indicated airspeed. This could cause the airplane to exceed its stall angle of attack, though the stall warning system, which is driven by angle of attack, should continue to function normally.

The above can explain partially what happened to Birgenair flight 301 which was a Turkish registered Boeing 757. During takeoff roll at 11:42 p.m. the captain, one of Birgenairs' most senior pilots, found that his air speed indicator (ASI) was not working properly, but chose not to abort takeoff. The co-pilot's ASI was functional.

While the plane was climbing through 4,700 feet (1,400 m), the captain's airspeed indicator read 350 knots. The autopilot, which was taking its air speed information from the same equipment that was providing faulty readings to the captain's ASI, increased the pitch-up attitude and reduced power to lower the plane's airspeed. Co-pilot’s ASI read 200 knots, and decreasing, yet the airplane started to give multiple contradictory warnings that it was flying too fast, including rudder ratio mach airspeed and overspeed lights and sounds.

The autopilot reached the limits of its programming and disengaged. After checking their circuit breakers for the source of the warnings, the crew then reduced thrust to lower the speed. This immediately triggered the 757's stick-shaker stall alert, warning the confused pilots that the aircraft was flying dangerously slow, seconds after it was warning them that the speed was too high. The co-pilot and relief pilot both seemed to recognize the approaching stall and tried to tell the captain, but did not intervene directly, possibly out of deference to the captain's age and experience. The captain then tried to recover from the stall by increasing the plane's thrust to full, but the plane was still in a nose up attitude, preventing the engines from receiving adequate airflow to match the increase in thrust. The left engine stalled and flamed out, which caused the right engine, still at full power, to throw the airplane into a spin. Moments later, the plane inverted. At 11:47 p.m., the Ground Proximity Warning System sounded an audio warning, and eight seconds later the plane crashed into the Atlantic Ocean. All 13 crew members and 176 passengers were killed.


So three crashes have been highlighted, to three aircraft all of which could fly with a little sitting on your hands analyzing the situation and reverting to basics, Pitch and Power. Have some basic numbers ingrained in your brain with which you can ensure that the aircraft will remain in a stable flight condition and also within the aircraft flight envelope; these figures can be refined once the aircraft is confirmed as being under control.

These scenarios requires good Crew Resource Management skills, use your colleagues on the flight deck to assist in identifying what ‘might’ or has failed, then together come up with the correct checklist. Remember aviate first. The QRH states that when you are faced with ‘flight with unreliable airspeed’, then both altitude and vertical speed indications could be incorrect to.

Which leads us to the QRH checklist:



Airspeed Unreliable

IAS DISAGREE or Airspeed Unreliable

Condition: One or more of these occur:


•The captain's and first officer's airspeed indications disagree by 5 knots or more.

•The airspeed or Mach indications are suspected to be unreliable (Items which may indicate Airspeed or Mach Unreliable are listed in the Additional Information section).


Objective: To maintain control using manual pitch and thrust.


1. Check pitch attitude and thrust.

2. If pitch attitude or thrust are not normal for the phase of flight:


Autopilot disengage switch . . . . . . . . . . .Push

Autothrottle disconnect switch . . . . . . . . .Push

F/D switches (both) . . . . . . . . . . . . . . . . OFF


Establish normal pitch attitude and thrust setting for the phase of flight.


Note: Normal pitch attitude and thrust setting are available in the Flight With Unreliable Airspeed table in the Performance Inflight chapter.


3. Altitude, vertical speed, and reference EPR indicator may be unreliable.

4. Crosscheck PFD airspeed displays and standby airspeed indicator. A PFD airspeed that is more than 10 knots different than the standby airspeed should be considered unreliable. 


Then the Boeing QRH checklist directs you to trouble shooting. But the aircraft is now being controlled. 

The easiest way to maintain control would be if at or above your minimum safe altitude, Level Off. The aircraft is obviously easier to stabilise when flying straight and level.

Disengage the Autopilot Flight Director System and Autothrottle.












So, know your aircraft systems, know your checklist Recall items, know basic aircraft performance numbers know how to practice good CRM……all of which will lead you to ‘knowing’ how to protect your aircraft and stay safe. Even when in the dead of the night when your body is feeling naturally tired, after a moment of reflection, you KNOW what to do.