A pilot's aircraft is squawking Mode C and reports an altitude of 5,500 feet to ATC, even though the pilot's altimeter, set to the current local altimeter setting of 30.10, reads 5,700 feet. What is the most likely explanation for this discrepancy?
- AThe pilot's altimeter has a static system leak causing the error
- BThe transponder is malfunctioning and must be immediately deactivated
- CATC radar is displaying true altitude, not pressure altitude, causing the difference
- DMode C transmits pressure altitude referenced to 29.92 inHg, which the ATC computer then corrects using the reported altimeter setting
Show answer & explanationAnswer & explanation
Correct answer: D. Mode C transmits pressure altitude referenced to 29.92 inHg, which the ATC computer then corrects using the reported altimeter setting
The encoding altimeter that drives Mode C always transmits pressure altitude referenced to a standard setting of 29.92 inHg, regardless of the local altimeter setting in the cockpit. ATC's computer system then applies the current local altimeter setting to correct this pressure altitude for display as an approximate MSL altitude, which can create small, normal discrepancies from the pilot's altimeter unless both are cross-checked periodically per 14 CFR 91.217.
Why the other options are wrong
- A. A static leak would typically cause a larger and unpredictable error, not this systematic reference difference.
- B. A small discrepancy from this known reference difference does not indicate a malfunction requiring shutdown.
- C. ATC actually applies a correction to displayed pressure altitude; it does not simply show unadjusted true altitude.
Mode C Altitude Reporting
Mode C transponders transmit pressure altitude referenced to 29.92 inHg (standard setting), which the ATC computer corrects using the current altimeter setting to display an approximate MSL altitude on the controller's scope.
- Mode C always references 29.92 inHg, not the local altimeter setting
- ATC computer applies correction for local pressure to display MSL altitude
- 14 CFR 91.217 requires the transponder-reported altitude and pilot's altimeter to be within specified limits
Memory trick: Mode C always speaks in 29.92—ATC's computer does the translating