WHAT IF......?
We all know that aircraft takes altitude and speed data from pitot static sensor located at the side of the fuselage. These two sensor are among the most critical sensor present in an aircraft that would render aircraft unservisable if blocked or otherwise damaged.That’s why these are called as eye and ear of an aircraft.
But WHAT IF….. they actually got blocked and your aircraft takes off with it. And to make it even worse the auto-pilot will take input from the blocked sensor.You probably be thinking I’m crazy right, but trust me it actually happened on BIRGENAIR FLIGHT 301, strange isn’t it….
LETS EXPLORE SOME BASICS…
Now lets see how these pressure sensitive equipment namely AIRSPEED indicator, ALTITUDE indicator and VERTICAL SPEED indicator actually works.
AIRSPEED INDICATOR (ASI)
It works on a pressure difference coming from pitot tube and static port installed on a side of a fuselage. as the difference varies with altitude and speed.
Static pressure enters the ASI case, while total pressure flexes the diaphragm, which is connected to the ASI pointer via mechanical linkage .When the aircraft is moving forward, air entering the pitot tube is at a greater pressure than the static line, which flexes the diaphragm, moving the pointer.
ALTITUDE INDICATOR
It works on a principle that as we go up, the air gets thinner as air density reduces and pressure decreases. It contains evacuated bellows surrounded by static pressure coming from static port located at the side of the fuselage. As the aircraft goes higher and static pressure decreases the evacuated bellows (which is calibrated to show 0 at sea level) expand in the same ratio and through linkage show altitude of aircraft at that particular level
.
VERTICAL SPEED INDICATOR(VSI)
A variometer is essentially a pressure altimeter with a leak which tends to make it read the altitude of a moment earlier. It uses full static pressure for the bellows but a restricted one for the space surrounding the bellows. whenever aircraft climbs or descends, the bellows will have full static pressure but the pressure in the surrounding will be restricted and this difference will cause bellows to move and through linkage mechanism so does the pointer.
LET’s GET BACK TO STORY….
Now imagine an aircraft that takes off with pitot probe blocked. What will happen?.. lets explore.
Since static ports are open and functioning altitude and vertical speed indicator will function correctly(as they need only static pressure) but airspeed indicator should stop working due to absence of pitot pressure. BUT here’s what the twist is, according to report on Bergainair flight 301 the airspeed indicator suddenly came to life as the aircraft begins to climb, but it shows readings very high as compared to what normal readings should be. But the question is HOW?, How could something give output without any input. SO HERE’s THE ANSWER…
Since the pitot probe is blocked, bellows in airspeed indicator will not have any pressure and it will act some what like an evacuated bellow(like that of an altimeter) and since static air is present all around the space in the casing, the bellows will respond to increasing and decreasing of static pressure around it as the aircraft climbs, and through linkage mechanism we will get readings that will somewhat correspond to altitude we were flying at but not as precise as altitude indicator.That’s the reason why airspeed indicator show high readings when probe is blocked on BERGAINAIR flight because it is actually showing height not speed.
“PREVENTION IS BETTER THAN CURE”
Now the question is how to prevent such things from happening ever again(which is what aviation is all about isn’t it?). Several changes are made to procedures and aircraft itself to prevent these unwanted situation to ever happen again.
Changes like..
INSTALLATION OF MULTIPLE PITOT AND STATIC PROBE
MANDATION OF “REMOVE BEFORE FLIGHT CAP ON PROBES”
“80 KNOTS CHECK” rule on take-off roll
AUTO PILOT WILL TAKE DATA FROM MULTIPLE PROBES
PILOT AND CO-PILOT WILL HAVE INDEPENDENT SOURCE OF INFORMATION
PRE-FLIGHT INSPECTION
But anything will be beneficial if the person who is in control will use it correctly, for example in the above mentioned crash after finding out that his airspeed indicator is false during take-off at “80 KNOTS CHECK” the pilot didn’t abort the take-off, if he had done the otherwise instead of continuing those who have lost their life would have lived.
CONCLUSION
Aviation industry is the most sensitive industry of all, one mistake and you will kill hundreds, that’s why each and every person that has some kind of link with it has to perform their best to make sure mistakes are kept to a minimum, so that people will continue to trust their airplane they are travelling at.
A WISE MAN ONCE SAID…
“MAKING MISTAKES IS A SIGN OF BEING AN HUMAN BUT REPEATING THEM IS A SIGN OF FOOL”
We know mistakes can’t be eliminated that’s why SOP’s are kept in place and it is our utmost duty to strictly follow them so that these accidents won’t repeat again.