Airspeed!
Aviation is a young human activity. The speed at which we progressed from open
cockpit bi-planes to our current jetliners does make one marvel at human
ingenuity. Sometimes though I wonder if
we are moving so fast that we are forgetting hard lessons learned along the
way. Specifically for this paper I wish
to reemphasize the need of pilots to understand the relationship of aircraft
control to Angle of Attack and how AoA is affected by pitch attitude and thrust. Airspeed is an indirect indicator of AoA
quality while in normal pitch attitude ranges.
As merely an indirect indicator, there should be no reason for loss of
aircraft control and all souls due to pitot-static system failure.
Birgenair Flight 301
(February 6, 1996)
Birgenair Flight 301 was a Boeing 757 who crashed shortly
after a night take-off. This accident
was caused by blockage of the number one pitot tube. The captain’s lack of understanding of how
airspeed indication is affected by such a blockage caused numerous decision errors
and resulted in total loss of control.
He noticed his airspeed indication was still zero when the first officer
called “80 knots” on take-off. Failing
to abort the take-off he then noticed, as they climbed, his ASI was registering
again and made the assumption that all was normal and he engaged the auto-pilot
to his side of the flight control system.
One of the old lessons hard learned in the days of “steam-gauges”
instrumentation was that a typical pitot tube block issue would cause the
airspeed indication to increase with altitude increases and to decrease with
altitude loss. This poor lost captain
simply engaged his auto-pilot to an airspeed indicator which would keep
increasing with the climb until the aircraft reached a dangerously high angle
of attack. This crew noticed the pitch
attitude / airspeed discrepancy and discussed it at length, but never saw the
core problem. It is notable that even while the captain was aware of their increasing peril he was reluctant to disengage the auto-flight system. It seems that he trusted the automation more than himself even while that same automation was obviously malfunctioning. Adding to the confusion
were numerous warnings and alert messages being generated by the ever widening
airspeed indication error. They crashed into the ocean shortly
after entering the wing stall regime.
Air France Flight 447
(June 1, 2009)
Air France flight 447 was an Airbus 300 which encountered
clouds while cruising in level flight.
This cloud encounter caused some pitot tube ice crystal blockage in one
or more of the pitot tubes. The First Officer, the pilot
flying, as the Birgenair captain before him, was confronted with many
warnings and alert messages which induced confusion and an unnecessary urge for
action, especially since the airspeed error caused the autopilot to disengage. Unfortunately, his action was to grab the side control stick and sharply pitch up causing a dangerously high
angle of attack. This hapless pilot simply had no idea what he was trying to do. Rather than simply holding the proper cruise pitch attitude he pulled up into a deep wing stall, which was held the entire three minutes it took to fall to the
ocean below. I have read many articles on
this accident wherein writers and industry experts cite the “difficulty” in
sorting through all the conflicting signals, warnings, and messages so as to
understand this problem and they all make me sick. How hard is it to see that the pitch attitude
is high as hell, thrust is high as hell…aircraft is falling?! How hard is that? This is aviation 101.
Northwest Airlines Flight 6231, a Boeing 727, crashed shortly after departing the John F. Kennedy airport on December 1st, 1974. The National Transportation Safety Board determined that the crew failed to turn on the pitot tube heaters which became blocked by ice crystals causing both airspeed indicators to show erroneous indications. Almost twenty-two years prior to Birginair 301, this crew misunderstood the "increasing" airspeed indications as they climbed and pitched up into a stall.
The Age of EFIS
Our current generation of airline cockpits almost
universally has the Electronic Flight Instrument Systems. (EFIS) These
systems integrate and share the raw instrument data so as to provide aircraft
status information not only to the visual “instruments” but to other aircraft
systems such as engine control computers (Full Authority Digital Engine
Control - FADECs), aircraft pressurization systems, autopilots and ground proximity systems, to name
just a few. They also have Flight Directors, which are
designed to simplify a pilot’s instrument scan in determining pitch and bank
requirements. Rather than interpreting several basic instruments and mentally processing
the data so as to determine the required pitch and bank for performance goals,
the pilot simply places the aircraft into the attitude displayed by the Flight
Director. The advantage is a simpler scan
and a lower mental effort level. Better yet, engage the autopilot to the Flight
Director and the airplane does it all!
This is the normal operating mode which the Federal Aviation Agency and virtually
every airline have stressed. Full
automation 99% of the time with the pilot hand flying and using basic
instruments kept to a barest minimum.
Here is how my airline phrased this concept in our Standard Operating
Procedures manual:
Where immediate, decisive, and
correct control of aircraft path is required, the lowest level of automation
(hand flying without flight director guidance) may be necessary. Such instances
would include escape or avoidance maneuvers (except aircraft with flight
director windshear guidance) and recovery from upset or unusual attitudes. With
the exception of visual approaches and deliberate decisions to maintain flying
proficiency, this is essentially a non-normal operation for flight guidance on FMS-generation
aircraft. It should be considered a transitory mode used when the pilot
perceives the aircraft is not responding to urgent aircraft demands. The pilot
can establish a higher level of automation as soon as conditions permit.
Emphasis mine
However,
here lies the problem. The full
automation condition is the lowest level of pilot mental engagement and now is
prescribed to be the condition wherein our pilots will spend the vast majority
of our time. When the chips fall badly and
decisive, correct control inputs are required the pilot is expected to suddenly
ramp up his mental and stick & rudder skills to levels of which he has not
practiced for a very long time. Like our
poor Birgenair, Northwest and Air France pilots they will find themselves confused and way
out of their comfort zones. They will
wish to escape back into the warm and fuzzy world of automation. I have personally encouraged my First
Officers to revert to the basic instrument scan and hand flying for “deliberate
decisions to maintain flying proficiency” only to find them very uncomfortable
and greatly relieved when they reengaged the autopilot. Others have hand flown the airplane, albeit with
the aid of the Flight Director, for longer than usual following take-off, knowing my
reputation of encouraging such behavior.
When they feel they have gone long enough, they engage the autopilot and
sit back with a satisfied sigh as if they just completed a long and fruitful
workout. The sad part is those “long”
workouts average about seven minutes on take-off and forty-five seconds on landing.
Back in the “steam gauge” days a pitot tube blockage would
only effect the airspeed indicator to which it was routed. However in our new days of automation and
systems integration such a blockage causes many warnings in systems areas not
typically associated with airspeed. I
believe it this plethora of warnings which are distracting these pilots from
the basic task of maintaining aircraft control.
Rather than flying the pitch and thrust first and sorting out the rest,
they react to remove the warnings first and forget about basic aircraft
operation.
Trans World Express
Flight 7604 (December 11, 2000)
Trans World Express Flight 7604 was en route to Greenville,
SC on a night flight from St. Louis, MO in an Embraer-145 aircraft. While climbing to a newly assigned altitude
the crew alert system suddenly lit up with numerous messages and warnings. There were messages announcing failure of all
four FADECs. There were several
red messages, many amber messages and some blue ones as well. There were so many messages on the panel that
there was a number presented on the bottom of the screen to let the crew know
that they could scroll down to read the extras.
Add to that bells and red lights and you get the idea of how
disorienting such an event can be. The
First Officer immediately noticed the FADEC failure messages and said something
about losing both engines. The captain
however noted that regardless of all the warnings, the airplane was still
flying. While maintaining a constant
pitch attitude and thrust setting it only took him a few seconds to notice that
the airspeed indicator on his side was increasing with the climb. With the
conclusion that the number one pitot tube was blocked with ice he obtained a
clearance from the traffic controller to level off at their present altitude so
as to stop the increasing error and to prevent additional warnings from an
imminent false over-speed indication.
The First Officer excitedly asked which of the Quick Reference emergency checklists
he wanted run, since there were so many to choose from. The answer to that was not to run any of them
because the airplane was flying perfectly and the QRH wouldn’t solve the
problem of a blocked pitot tube. They
obtained a descent clearance and the captain noted with satisfaction that the
erroneous airspeed indication began decreasing with lower altitudes thus
confirming his reasoning. Soon the ice
blockage cleared and all the warning messages disappeared as well. This flight landed uneventfully.
It Really Is Bi-plane
Simple
The new electronic instrumentation and flight control
systems are a marvel and they do greatly aid pilots in situational awareness
and aircraft control. However, the
downside has been a loss of the basic skills and knowledge which the technology
has bridged over. When the technology
fails rather than landing on firm foundational footing, we are finding
ourselves falling into a crevasse. Do
not let the startle factor override your basic aircraft control instincts. Pitch and thrust make the airplane fly, not
“airspeed” or quiet alert systems.
Continue to review the basic lessons learned over those past remarkable
one hundred years. Practice basic flying skills, everyday! Fly safely.
Captain William C. Howe
Trans States Airlines
August 1, 2013
