Aircraft Systems Topic 19.
Pitot/Static Blockage Effects
BLOCKAGE OF THE PITOT OR STATIC
Most large transport aircraft have their pitot and static port heater elements turned on during all phases offlight and this is confirmed as part of the pre-flight check-list. Blockage of pitot or static ports during flight by ice or dirt has caused even big Boeing's to crash before today. The effects of blockages are discussed below.
BLOCKAGE OF THE STATIC PORT
This means the airspeed indicator will still read correctly provided the outside air pressure remains the same.
If however the aircraft was on descent into higher pressure air closer to the earth, you could expect the ASI toOVER-READ. This a a potentially hazardous situation on approach as you are flying close to stall speed, and if the static port is blocked you will be closer to the stall than you think.
If the aircraft is on climb when the static port becomes blocked, then the airspeed indicator willUNDER-READ as the aircraft climbs into thinner and thinner air, which imparts a lesser dynamic force on the pitot capsule. This situation may cause confusion on the flightdeck as the aircraft appears to be under-performing especially in the cruise.
Static port blockage will cause the same under-reading/over-reading errors in the Machmeter.
This caused the loss of a Boeing B727-200 aircraft in the USA some years back, and it’s effect on the airspeed indicator is a very popular question in ATPL exams.
The B727 was flying with crew only aboard, at night in IMC, on their way to pick up a football team.
The pitot’s became blocked by ice whilst on climb, which meant that the only supply of air data being supplied to the airspeed indicator was from the still un-blocked static port. Effectively, the airspeed indicator,whose reading was being referenced for the initial part of the climb, was in fact performing like an altimeter.
As the aircraft gained altitude the airspeed indicator kept OVER-READING to a progressively greater degree as it mimicked an altimeter. The crew commented repeatedly on the fabulous performance the aircraftapparently had when empty.
Progressively the angle of attack was increased to try an hold the climb airspeed and prevent the aircraft over-speeding. Eventually, just below FL200 the aircraft stalled, spun and plummeted into a mountainsidekilling all on board.
Investigators found that the pitot heat had not been selected on by the aircrew during the preflight check-list.
In another incident the wary pilot of a BAe 146 aircraft, whilst conducting his preflight inspection, noticed the pitot tubes and static ports had been covered over with masking tape. Apparently, the engineering personnel had been checking the pitot/static system for leaks and had cleared the aircraft as again fit to fly, forgetting all about the masking tape.
Yet another incident involved a navy pilot who launched off into the night with the pitot covers still on.
Realising his mistake, he turned on the pitot heater element and burnt the covers off. This took some time, and a fair bit of explaining when he landed.
The AIR DATA instruments are essential to flight so be very careful when checking them.
This mini-editorial was taken from the ATPL Navigation Internet course. As in this course, an interactive review assignment can be attempted by clicking HERE. You will be marked on answering all the questions, and clicking on the “submit Answers” button. Your percentage score will be displayed, along with any incorrect responses, and the correct answer. You can undertake the training modules and their associated assignments as often as required.
I trust you found this editorial informative.