Flight instruments can be modeled with simple formulas. I find this to be a helpful aid to thinking through failure modes. ## Definitions * $P_p(t)$: Pressure measured at pitot tube at moment $t$ * $P_s(t)$: Pressure measured at static port at moment $t$ * $A$: Altimeter setting * $VSI(t)$: display on Vertical Speed Indicator at moment $t$ * $ALT(t)$: display on Altimeter at moment $t$ * $ASI(t)$: display on Air Speed Indicator at moment $t$ ## Models $ \begin{align} ASI(t) & \propto P_p(t)-P_s(t) \\ ALT(t) & \propto A-P_s(t) \\ VSI(t) & \propto -\frac{d P_s(t)}{dt} \end{align} $ ## Blocked Pitot So, when the pitot tube is blocked, we get ($C$ a constant): $ \begin{align} ASI(t) & \propto C-P_s(t) \\ \end{align} $ ## Partially Blocked Pitot When the pitot tube is blocked but the drain hole remains open, after pressure is equalized, it's approximately: $ \begin{align} ASI(t) & \propto P_s(t)-P_s(t)=0 \\ \end{align} $ ## Blocked Static $ \begin{align} ASI(t) & \propto P_p(t)-C \\ ALT(t) & \propto A-C \\ VSI(t) & \propto -\frac{d C}{dt} = 0 \end{align} $ So the Altimeter freezes, the VSI reads 0, and the ASI will subtract too much (under-read) at higher altitudes and subtract too little (over-read) at lower altitudes.