Blocked static system

[Eggman]

What effect will a blocked static system have on the ASI and other pressure instruments when an aircraft climbs or descends from the altitude at which the blockage occurs?

Assuming the alternative static system is not available (this has happened more than once) what options are available in IMC?

[franksnbeans]

From my understanding, the VSI has few seconds 'lag' in its reading. This is due to the ambient static pressure on the outside of the aircraft being delayed by the path it is forced to take in order to enter the chamber of the pitot - static system. The system allows for the ambient pressure to slowly bleed into the chamber, which already contains the static pressure that was previously outside the aircraft. A sensitive membraine (spelling?) then measures the rate of change in pressure between the air that is in the chamber and the air that is directly from the outside of the aircraft. It is sortof hard to explain without diagrams :confused: .

Should the static vent become blocked at any stage in the flight, the pressure in the system would become constant, and therefore, according to the VSI, it would appear that the aircraft was flying at a constant altitude (if only this were the case with my flying! :D :D ).

The ASI uses the static pressure from outside the aircraft in a similar chamber, and the total pressure (sum of both the static pressure and the dynamic pressure), that is taken from the pitot tube are pitched up against each other. This cancels the static pressure that is present in the pressure from the pitot tube, and will then leave the system with only the dynamic pressure (ie: the speed through the air)

Should the staic vent become blocked at TOD, but the pitot tube remain clear, the ASI will over read, and in a climb it will under read.

I'm not IFR rated, yet, but I believe there would be a backup set, and I think the instruments must be of a higher standard.

But hypothtically, if no other sorce of static pressure, maybe you could crack the VSI? :)

[Tinstaafl]

Both the altimeter & the VSI rely on static pressure alone and the ASI requires both static & pitot sources.

If the static vent becomes blocked then the altimeter will freeze on whatever altitude at which the the blockage occurred. The VSI will show zero climb or descent.

The ASI will still have pitot pressure. Its mechanism acts to subtract static pressure from the pitot's combined static + dynamic pressures then equating the resultant dynamic to a speed.

(dynamic + static) - static = dynamic

For this to work, both static sources (static vent & pitot) must have the same value.

In a descent with a blocked static vent the effect is to leave the static vent value comparitively lower than the pitot head value. This leads to not quite all of the pitot's static pressure being negated.

End result is the 'left over' bit of static pressure adds to the dynamic pressure causing the ASI to over read.

The reverse happens in a climb. The greater static vent pressure locked into the system is subtracted from the pitot's dynamic+static pressures, leading to an under read.

[sickBocks]

Useful mnemonic for ASI:

SODPUD

Static blockage Overread Descent
Pitot blockage Underread Descent

sB

[Megaton]

To complete sickBocks post:

SOD - static block instruments over-read in descent
SUC - static block instruments under-read in climb
PUD - pitot block instruments under-read in descent
POC - pitot block instruments over-read in climb

Thus: SOD SUC PUD POC

[Eggman]

That's worth knowing. The following accidents have been attributed to:

Blocked static system.
B757, Peru, 1996 (http://www.planecrashinfo.com/1996/1996-38.htm)

Blocked pitot system.
B727, USA, 1974 (http://planecrashinfo.com/1974/1974-34.htm)
B757, Dominican Republic, 1996 (http://www.planecrashinfo.com/1996/1996-6.htm)