Blocked Static and ASI-VSI-ALT question?

[Ejector]

I am confusing my self,
Does anyone have a list of what happens to the basic pressure instruments, ie, ASI, VSI, ALT when the pitot or static source is blocked? I mean which over reads and under reads etc?

Thanks folks

[Reverand Lovejoy]

Ejector,

All 3 of these basic pressure instruments will be affected in different ways.

ASI - ASI shows the difference between the total pressure(dynamic+static) - static pressure, resulting in just the dynamic pressure i.e IAS. If the static port becomes blocked then the ASI will have a fixed static pressure (at the time of the blockage) and any change in static pressure (change in level) will be wrong. If we descend for eg with a blocked static port then the ASI will over read because the trapped static pressure will be lower (it came from a higher level so pressure will be less) resulting in the difference between the total pressure and static pressure being greater than is actually true. In a climb the opposite applies and the ASI will underread

VSI - VSI measures a rate of change in static pressure so if it is blocked it wont be changing so will slowly reset to 0 and stay there whether climbing or descending

ALT - Again, alt only relies on static pressure so a blocked static port will trap the static pressure giving a fixed reading. When you climb the static pressure in the capsule stays the same and gives a fixed reading. The reading at which the port became blocked. If in a non pressurized aircraft the glass can be smashed on the front to obtain an alternative static source but expensive if you get it wrong! This is actually a taught method in the event of a blocked port and no alt static port!!

Probaly the most wordy explanation with the worst spelling but I hope it helps.

Ciao 4 now

The Reverand :ok:

[Gary Lager]

Slightly less wordy way of remembering the ASI bits (not my own, but as I was taught):

PUDSOD

Pitot Blocked, Underreads in Descent (i.e. overreads in climb)
Static Blocked, Overreads in Descent (i.e. underreads in descent)

Also, in contrast to smaller aircraft, most modern jets have a VSI driven by the IRS so are not affected by pitot-static problems. That wasn't your question, I realise.