This document outlines the functional test procedure for the ATC (Air Traffic Control) Altitude Reporting (Mode C) system on the CRJ-900 aircraft, specifically for PSA Airlines. The primary function of this system is to report the aircraft's altitude to air traffic control, a critical safety feature for air navigation and collision avoidance. The document details the steps required to verify the accurate operation of both transponder 1 and transponder 2, which are integral components of the altitude reporting system.

Function Description

The ATC Altitude Reporting (Mode C) system is responsible for transmitting the aircraft's pressure altitude to ground-based air traffic control radar systems. This information allows controllers to monitor the vertical separation of aircraft and issue appropriate instructions to maintain safe flight. The system relies on transponders, which receive interrogation signals from ground radar and respond with coded information, including the aircraft's identity and altitude.

The functional test ensures that the transponders accurately report altitude data within specified tolerances. This involves simulating various altitudes using an Air Data Test Set (ADTS) and comparing the reported altitude on the aircraft's Primary Flight Displays (PFD) with the readings on a dedicated ATC test set. The test also verifies the proper functioning of associated components such as the pitot-static system, which provides the raw pressure data for altitude calculation, and the tuning select panel, which controls the transponder selection.

The procedure is designed to be performed with the aircraft in a "Weight-On-Wheels" (WOW) condition to prevent incorrect TCAS (Traffic Collision Avoidance System) advisories. This highlights the interconnectedness of various aircraft systems and the importance of maintaining specific configurations during maintenance. The test also includes precautions related to the pitot-static system heaters and the handling of test equipment to prevent damage to aircraft instruments or interference with air traffic control facilities.

Usage Features

The functional test procedure is structured for use by maintenance personnel. It provides clear, step-by-step instructions for preparing the aircraft, setting up test equipment, and performing the actual altitude reporting test for both transponder 1 and transponder 2.

Key usage features include:

Aircraft Preparation: Instructions for configuring the aircraft for maintenance, including connecting electrical power and obeying electrical/electronic safety precautions. This ensures a safe working environment and prevents damage to aircraft systems.
Circuit Breaker Management: Detailed lists of circuit breakers to be opened, safetied, and tagged before the test, and subsequently closed after specific test phases. This is crucial for isolating systems and preventing unintended operation, particularly for the pitot-static system heaters, which can cause burns.
Test Set Setup: Comprehensive guidance for setting up both the ATC-601 and IFR 6000 test sets. This includes physical placement of the test antenna, connecting cables, performing self-tests, and configuring test parameters such as antenna selection, range, gain, and loss indications. The instructions emphasize precise measurements for antenna placement (e.g., 10±1 feet, 30 feet) to ensure accurate signal reception and test results.
Air Data Test Set (ADTS) Preparation: Instructions for installing pitot-static adaptors and connecting test hoses to the aircraft's pitot-static heads. This ensures that the ADTS can accurately simulate various altitudes for the transponders.
Flight Compartment Preparation: Steps for configuring flight deck controls, such as setting the ATC SEL switch to STBY, setting the AIR DATA switch to NORM, pushing the PUSH STD pushbutton on the ARP panels, and verifying barometric pressure on the PFDs. These steps ensure the aircraft's internal systems are correctly configured for the test.
Altitude Simulation and Verification: The core of the test involves slowly adjusting the ADTS to specific altitudes (e.g., -1000, 0, 1000, 10000, 22000, 33000, 41000 feet) and verifying that the altitude displayed on the ATC test set matches the PFD reading within a tolerance of ±125 feet. This direct comparison is essential for confirming the accuracy of the altitude reporting system.
Transponder Selection: The procedure clearly distinguishes between testing transponder 1 and transponder 2, requiring specific adjustments to the ATC SEL switch and repositioning of the test tripod for each transponder.
Time-Sensitive Operations: Notes regarding the ATC transmitting for only 10 seconds and staying off for 30 seconds between transmissions during certain steps. This highlights the need for precise timing during the test to avoid interference or misreadings.
Data Recording: The inclusion of tables for recording ADTS altitude, PFD altitude, and ATC test set altitude values. This provides a structured way to document test results and ensures traceability.
Safety Warnings and Cautions: Prominent warnings regarding the aircraft's WOW condition, the dangers of hot pitot-static system heaters, and the importance of decreasing pitot and static inputs to ambient before connecting/disconnecting equipment. Cautions also address the potential for damage to aircraft instruments and the need to avoid specific ATC codes (7500, 7600, 7700, 7777) during the test to prevent false emergency signals.

Maintenance Features

The document itself serves as a maintenance feature, providing a standardized and detailed procedure for ensuring the continued airworthiness of the CRJ-900's altitude reporting system.

Key maintenance features include:

Standardized Procedure: The document provides a consistent and repeatable method for performing the functional test, ensuring that all technicians follow the same steps and achieve comparable results. This standardization is crucial for quality control and regulatory compliance.
Troubleshooting Guidance (Implicit): While not explicitly a troubleshooting guide, the detailed steps and expected outcomes (e.g., altitude within ±125 feet) implicitly guide technicians in identifying malfunctions. If the reported altitudes do not match the expected values, it indicates a problem within the altitude reporting system, prompting further investigation and repair.
Reference Information: The "Job Set-Up Information" section lists required tools and equipment (e.g., Test Set - Transponder Mode S, Ramp Test Set, Test Set - Air Data, Connector Kit - Pitot Static Pressure Test) and references other relevant manuals and tasks (e.g., Standard Aircraft Configuration for Maintenance, Connect Electrical Power to the Aircraft, Electrical/Electronic Safety Precautions, Put the Aircraft in a Weight-Off-Wheels Configuration). This ensures that technicians have all necessary resources and context before starting the procedure.
Configuration Control: The document specifies the exact revision (Rev A dated 10/12/15) and insertion point within the AMM (Aircraft Maintenance Manual), ensuring that technicians are using the most current and approved procedure. The mention of removing a previous revision (dated Sep 17/14) further emphasizes this.
Aircraft Effectivity: The "ON A/C" lists at the beginning of the document specify the range of aircraft serial numbers to which this procedure applies, ensuring that the correct maintenance is performed on the appropriate aircraft. This is a critical aspect of fleet management and regulatory compliance.
Clear Close-Out Steps: The "Close Out" section provides instructions for safely removing test equipment, restoring the aircraft to its normal configuration, and removing electrical power. This ensures that the aircraft is left in an airworthy condition after the maintenance task is completed.
Documentation Requirement: The requirement to record PFD and ATC test set values in tables serves as a crucial documentation feature. This record provides evidence that the test was performed, the results were within limits, and the system is functioning correctly, which is vital for maintenance logs and regulatory audits.
Preventive Maintenance: By regularly performing this functional test, potential issues with the altitude reporting system can be identified and addressed before they lead to operational failures or safety incidents. This proactive approach is a cornerstone of effective aircraft maintenance.

PSA CRJ-900 - Page 13