Loss of Pitot-Static System

16.loss-of-pitot-static. Loss of Pitot-Static System

The pitot-static system supplies ram and ambient air pressure to three primary flight instruments: the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI). Loss of any portion of this system — through ice accretion, water contamination, insect or debris blockage, or a failed pitot heat element — can produce dangerously misleading indications. Recognizing the failure mode and responding correctly is a required emergency competency under the Airman Certification Standards.

System Architecture

• The pitot tube senses ram (impact) air pressure and feeds the airspeed indicator only.
• Static ports, typically flush-mounted on the fuselage, sense undisturbed ambient pressure and feed the ASI, altimeter, and VSI.
• The ASI is essentially a differential pressure gauge: it measures the difference between pitot (total) pressure and static pressure.
• An alternate static source is installed in many airplanes (often inside the cabin) to provide a backup if the primary static system becomes blocked.

Recognizing a Pitot Blockage

If only the pitot ram-air inlet is blocked but the drain hole remains open, ram pressure bleeds off and the ASI drops to zero. The altimeter and VSI continue to read normally.

If both the ram-air inlet and the drain hole are blocked (typical of ice accretion), the trapped air cannot vent. The ASI then behaves like an altimeter:

• Climbing causes the indicated airspeed to increase (because static pressure decreases while trapped pitot pressure stays constant).
• Descending causes the indicated airspeed to decrease.
• This is a particularly dangerous failure because the pilot, chasing a falsely high airspeed in a climb, may pitch up to an unrecognized stall.

Recognizing a Static Blockage

With a blocked static system:

• The altimeter freezes at the altitude where the blockage occurred.
• The VSI reads zero regardless of actual climb or descent.
• The ASI continues to function but is inaccurate: it reads low when above the blocked altitude and high when below it.

Use of the Alternate Static Source

When a static blockage is suspected, select the alternate static source per the POH. In airplanes where the alternate source vents into the cabin, the lower cabin pressure (in unpressurized aircraft with airflow over the fuselage) will typically cause:

• The altimeter to read slightly higher than actual
• The indicated airspeed to read slightly faster than actual
• The VSI to momentarily show a climb when first selected

Consult the airspeed and altimeter correction tables in the POH/AFM. If no alternate static source is installed, the AFH recommends breaking the face of the VSI as a last resort. The VSI is the least critical of the three instruments, and the broken case provides a rough static reference (with reversed VSI indication).

In-Flight Procedure

1. Aviate — fly attitude and power. Use the attitude indicator and known pitch + power = performance combinations from the POH (e.g., cruise pitch and 23" MP yields ~120 KIAS in level flight).
2. Apply pitot heat immediately if icing is suspected.
3. Cross-check the ASI, altimeter, and VSI to identify which instrument(s) are unreliable.
4. Select alternate static if available; otherwise, consider breaking the VSI glass.
5. Declare if needed and request no-gyro or surveillance approach assistance in IMC.
6. Use GPS groundspeed as a cross-reference for airspeed (correcting for wind), and use GPS altitude or ATC altitude readouts as a backup for the altimeter.

Prevention

• During preflight, verify that pitot covers are removed, the pitot tube is unobstructed, and static ports are clear of debris, wax, or insects (mud-dauber wasps are a common culprit).
• Verify pitot heat operation during runup — feel for warmth or check ammeter load.
• In any visible moisture below +5°C, turn pitot heat ON before entering the conditions, not after icing begins.
• Avoid washing or waxing over static ports.

Example

A pilot in cruise at 6,000 ft notices the ASI slowly increasing through 140 KIAS while pitch attitude and power are unchanged. The altimeter and VSI read normally. This is consistent with a completely blocked pitot tube (ram and drain): the trapped pressure is acting like an altimeter, so any climb increases indicated airspeed. The correct response is to ignore the ASI, fly pitch + power for the desired performance, apply pitot heat, and divert.

Understanding the failure signatures — which instruments lie, which still tell the truth, and which knob or breaker restores partial function — is what separates a controlled diversion from a loss-of-control accident.