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Plane Crashes and Pitot Tubes
Dave Jackson
How a Plugged Pitot Tube Can Cause Plane Crashes
Most of us are aware of the devastation a bird strike can have on an aircrafts structure or engines. But what about an insect? Or a few ice crystals? A surprisingly trivial substance can bring down even the largest aircraft.
All avionics systems rely on devices called "pitot tubes." These measure the force (impact) of oncoming air. The "pitot-static" system compares this impact pressure to the neutral (static) air pressure. The information is fed to three of the most critical instruments: Airspeed, Altitude and Rate of Climb. A blocked static port affects all three instruments, while the pitot tube itself will only involve airspeed.
The simplicity of the pitot tubes makes them vulnerable to blockages. The elaborite system of avionics, gauges and computers can be deceived by any number of blockages. Dead insects, mud wasp nests, and ice are the most common.
The most hideous aspect of this malfunction is the way a pilot, or even worse, computer, can be fooled. The flight instruments can provide wrong or conflicting information with a potentially tragic outcome. A pitot system failure is considered to be the worst possible malfunction of an aircrafts avionics.
Even a small amount of moisture can wreak havoc as in the loss of a $1.4 billion stealth bomber. This crash, the most expensive in the history of military aviation, was the result of not switching on the pitot heat system prior to air data calibration.
An X-31A crashed in 1995 after a pitot tube was blocked by ice. Many military aircraft are too unstable to control without automated avionics. In some situations the plane is not recoverable.
Austral Lineas Aereas Flight 2553 was lost, along with 74 lives, when a pitot tube froze solid. The DC-9's instruments fooled the pilots into believing the plane was stalling. Pilots are trained to trust the instruments and computers, especially in darkness and poor weather. Spatial disorientation prevented the pilots from making proper decisions. Structural failure was the result of adding full power and lowering the flaps at high speed. Flight 2553 impacted the terrain at 1,200 kph.
In 1996, Birgennair Flight 301 crashed shortly after takeoff. Mud wasps built a nest in one of the Boeing 757's pitot tubes. The captains airspeed indicator showed 350 knots and triggered the autopilot to pull up and reduce power. The actual airspeed was much slower. The pilots heard contradictory overspeed and stall warnings. Although the co-pilots seperate pitot system was working the pilots could not know which to believe. The stall warning was very real but was ignored to withing 8 seconds of the ground. 183 people were killed. Ground crews could have saved Flight 301 by installing pitot tube covers while it sat idle.
That same year Aero Peru Flight 603 was lost because ground crews failed to remove a piece of duct tape from the static ports on another 757. The computers issued overspeed, underspeed and "too low, terrain" warnings. Again, the pilots had no way of knowing which warnings were correct. They asked Air Traffic Control for the correct airspeed and altitude. With this information they confidently flew the aircraft into the ground. All 70 on board were killed.
This crash illustrates just how deadly a failed pitot-static system can be. The simple devices are the source of the entire system. The failure of even one of the two or three can circumvent the whole concept of redundancy. Besides feeding nformation to the gauges and computers, the faulty readings were also relayed to the air traffic controllers. The aircrafts computer fooled the controllers into confirming horrifically wrong speed and altitude.
It's hard to believe how many aircraft and lives have been lost to such trivial events. It's unsettling to realize they wil not be the last.
WHAT IS AN AIRCRAFT BLACKBOX AND WHY IT IS SO IMPORTANT?
Cockpit Voice Recorders (CVR) and Flight Data Recorders (FDR) and What They Do
-David Sutton
On May 31, 2009 Air France flight 447 disappeared someplace over the Atlantic Ocean with 228 people aboard and with that a search began, not just for survivors, but the important Black boxes. An aircraft's Black Box is in fact orange and, depending on the plane, there may be two of them. The box, or boxes, record two streams of data. There is the Cockpit Voice Recorders (CVR) and Flight Data Recorders (FDR). The boxes have been part of civil aviation since the 1950's. For many years the data was recorded on 1/4 inch magnetic tape but newer boxes are all digital and can record far more numerous data streams and record them for longer periods.
Today's recorders are made to withstand brutal sudden impacts, intense fire temperatures, and the crushing pressures of deep ocean depths. each recorder has a Underwater Locator Beacon (ULB) that transmits a signal at 37.5 KHz so the box can be found if it is submersed in water. The ULB is strong enough to be located under 14,000 feet of water. The Federal Aviation Authority (FAA) requires most commercial aircraft (those flying paying passengers) and many private and corporate aircraft to have functioning CVRs and FDRs on board.
The recorders are used to reconstruct the flight of a plane before a crash. In the event of a crash one of the top priorities after saving lives is the recovery of the CVR and the FDR. If and when found the recorders are sent directly to the National Transportation Safety Board (NTSB) head office in Washington, D.C. where advanced computer and audio tools are used to extract the data from the boxes so that the Investigator-in-Charge and the Safety Board can make a determination as to why the plane went down.
Cockpit Voice Recorders (CVR) and Flight Data Recorders (FDR) and What They Do
The Cockpit Voice Recorder records not just voices but all the sounds made within the cockpit. By listening to conversations between people on the flight deck and conversations between the flight crew and cabin crew, ground crew, airport tower, and Air Traffic Control, investigators can determine what the cockpit crew was doing and thinking up to the time of the crash. Because all cockpit sounds, not just conversations, are recorded investigators can find other clues to what happened before a crash. Things like a change in engine sound, a change in overall noise, audible equipment warnings, and odd or unexpected noises can all provide vital clues that may identify why a plane went down.
Because of the personal nature of the information recorded by the CVR an order of Congress prevents the release of any CVR recording to anyone but investigators. A team made up of representatives of the NTSB, the FAA, the operator of the aircraft, the manufacturer of the airplane, the manufacturer of the engines, and the pilots union all listen to the tapes and jointly agree to an official transcript that will be released for general use.
The latest Flight Data Recorders are required by law to record 88 key parameters of flight such as altitude, attitude, and speed. Some FDRs record more the 1000 aspects of the plane on the ground and in flight.
Used together the FDR and the CVR can help investigators find the cause of a crash, and more importantly, make recommendations that may prevent a future disaster.
Sources
NTSB - CVR & FDR
http://www.ntsb.gov/aviation/CVR_FDR.htm
Did Air France Flight 447 break up midair?
http://www.csmonitor.com/2009/0612/p02s07-usgn.html
