What Happens to the Airspeed Indicator, Altimeter, and Vertical Speed Indicator When the Alternate Static Source Is Activated
- wifiCFI
- Dec 22, 2025
- 4 min read
Most pilots learn early that the pitot-static system is critical to safe flight. Less obvious—but equally important—is what happens when that system partially fails and the pilot selects the alternate static source.
Activating the alternate static source is a standard response to a blocked or malfunctioning static port, but doing so changes how flight instruments interpret pressure. As a result, the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI) all display different—and slightly inaccurate—values.
Understanding exactly what happens to each instrument when the alternate static source is activated allows pilots to interpret indications correctly and maintain safe control, especially in IMC.
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A Quick Review: The Static Side of the Pitot-Static System
Static pressure is used by:
Altimeter
Vertical Speed Indicator (VSI)
Airspeed Indicator (static side)
Under normal conditions, static pressure is collected from ports on the outside of the fuselage, where airflow is relatively undisturbed. When those ports become blocked, pressure information stops changing—and the instruments become unreliable.
What Is the Alternate Static Source?
The alternate static source provides an alternate path for static pressure to reach the instruments, usually by drawing air from inside the cockpit.
Key characteristics of cabin air:
Slightly lower pressure than outside air in flight
Influenced by ventilation, airflow leaks, and cabin pressurization (if applicable)
Because cabin pressure is not the same as free-stream static pressure, instrument errors occur immediately when the alternate static source is selected.
Why Instrument Errors Occur
When the alternate static source is activated:
Static pressure entering the instruments is lower than true ambient pressure
Instruments interpret this as the aircraft being higher and climbing faster than it actually is
These errors are predictable and consistent, though the exact magnitude varies by aircraft.
Effect on the Altimeter
What the Altimeter Does
The altimeter measures static pressure to determine altitude above mean sea level.
What Happens with Alternate Static
When alternate static is selected:
Cabin pressure is lower than outside pressure
The altimeter senses a pressure drop
It interprets this as an increase in altitude
Result
Altimeter reads higher than actual altitude
Typical error:
+50 to +200 feet, depending on aircraft design
This is why pilots must be cautious when flying near terrain or altitude minimums after activating alternate static.
Effect on the Vertical Speed Indicator (VSI)
What the VSI Measures
The VSI measures the rate of change of static pressure over time.
What Happens with Alternate Static
When alternate static is activated:
Pressure inside the instruments suddenly decreases
The VSI senses a rapid pressure change
Result
VSI initially indicates a climb
After stabilization, it functions normally but with slightly exaggerated indications
This initial “bump” is normal and expected.
Effect on the Airspeed Indicator (ASI)
Why the ASI Is Affected
The ASI compares:
Pitot pressure (unchanged)
Static pressure (now lower due to cabin air)
Lower static pressure increases the pressure differential across the ASI diaphragm.
Result
Airspeed indicator reads higher than actual airspeed
Typical effect:
Indicated airspeed increases slightly
The error grows with altitude
This can be misleading during:
Approaches
Stall-speed margins
Maneuvering near flap or gear limits
Exact values vary—pilots must consult the POH/AFM for aircraft-specific corrections.
When Should Pilots Use the Alternate Static Source?
Pilots should activate alternate static when:
Static port is suspected blocked
Altimeter or VSI freezes
Airspeed behaves inconsistently with pitch and power
Icing conditions affect static ports
In IFR flight, activating alternate static is often preferable to flying with frozen instruments, even with known errors.
Pilot Techniques After Activation
Once alternate static is selected, pilots should:
Expect higher indicated altitude
Maintain extra terrain clearance
Cross-check with GPS altitude (if available)
Use known pitch-and-power settings
Avoid chasing VSI fluctuations
Reference approach minimum corrections (if published)
Training emphasizes instrument interpretation, not blind trust.
Alternate Static Source in Glass Cockpits
In glass cockpits:
Pressure is sensed electronically
Alerts or flags may appear
Corrections may be automatically applied—or not
Despite advanced avionics:
Pressure physics do not change
Instrument interpretation skills remain critical
Why This Knowledge Matters
Many pilots know when to use the alternate static source—but not what it does.
Understanding the effects:
Prevents altitude busts
Reduces confusion in IMC
Improves confidence during abnormal operations
Helps pilots make safer decisions under pressure
This is a classic example of managing degraded systems, not eliminating them.
Conclusion
Activating the alternate static source restores functionality to pitot-static instruments—but at the cost of predictable indication errors. The altimeter and airspeed indicator read higher than actual, while the VSI briefly indicates a climb before stabilizing.
Pilots who understand these effects can confidently continue flight, interpret their instruments correctly, and maintain safe margins—even when part of a critical system has failed.
In aviation, knowing how instruments behave after a failure is just as important as knowing how they work normally—and the alternate static source is a perfect example of that principle.
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