Aviation

Boeing Statement on AOA Disagree Alert

On every airplane delivered to our customers, including the MAX, all flight data and information needed to safely operate the aircraft is provided in the flight deck on the primary flight deck displays. This information is provided full-time in the pilots’ primary field of view, and it always has been.

Air speed, attitude, altitude, vertical speed, heading and engine power settings are the primary parameters the flight crews use to safely operate the airplane in normal flight. Stick shaker and the pitch limit indicator are the primary features used for the operation of the airplane at elevated angles of attack. All recommended pilot actions, checklists, and training are based upon these primary indicators. Neither the angle of attack indicator nor the AOA Disagree alert are necessary for the safe operation of the airplane. They provide supplemental information only, and have never been considered safety features on commercial jet transport airplanes.

The Boeing design requirements for the 737 MAX included the AOA Disagree alert as a standard, standalone feature, in keeping with Boeing’s fundamental design philosophy of retaining commonality with the 737NG. In 2017, within several months after beginning 737 MAX deliveries, engineers at Boeing identified that the 737 MAX display system software did not correctly meet the AOA Disagree alert requirements. The software delivered to Boeing linked the AOA Disagree alert to the AOA indicator, which is an optional feature on the MAX and the NG. Accordingly, the software activated the AOA Disagree alert only if an airline opted for the AOA indicator.

When the discrepancy between the requirements and the software was identified, Boeing followed its standard process for determining the appropriate resolution of such issues. That review, which involved multiple company subject matter experts, determined that the absence of the AOA Disagree alert did not adversely impact airplane safety or operation. Accordingly, the review concluded, the existing functionality was acceptable until the alert and the indicator could be delinked in the next planned display system software update. Senior company leadership was not involved in the review and first became aware of this issue in the aftermath of the Lion Air accident.

Approximately a week after the Lion Air accident, on November 6, 2018, Boeing issued an Operations Manual Bulletin (OMB), which was followed a day later by the FAA’s issuance of an Airworthiness Directive (AD). In identifying the AOA Disagree alert as one among a number of indications that could result from erroneous AOA, both the OMB and the AD described the AOA Disagree alert feature as available only if the AOA indicator option is installed.

Boeing discussed the status of the AOA Disagree alert with the FAA in the wake of the Lion Air accident. At that time, Boeing informed the FAA that Boeing engineers had identified the software issue in 2017 and had determined per Boeing’s standard process that the issue did not adversely impact airplane safety or operation. In December 2018, Boeing convened a Safety Review Board (SRB) to consider again whether the absence of the AOA Disagree alert from certain 737 MAX flight displays presented a safety issue. That SRB confirmed Boeing’s prior conclusion that it did not. Boeing shared this conclusion and the supporting SRB analysis with the FAA.

Boeing is issuing a display system software update, to implement the AOA Disagree alert as a standard, standalone feature before the MAX returns to service. When the MAX returns to service, all MAX production aircraft will have an activated and operable AOA Disagree alert and an optional angle of attack indicator. All customers with previously delivered MAX airplanes will have the ability to activate the AOA Disagree alert.

Aviation

Exploring the Different Types of Helicopter Rotor Systems and the Science Behind Them

Helicopters are unique aircraft that use rotating blades, called rotors, to generate lift and enable flight. The design of these rotor systems is crucial because it affects how helicopters perform, maneuver, and respond to different flying conditions.

There are several types of helicopter rotor systems, each with its own advantages and specific uses. Understanding these systems helps us appreciate the engineering behind helicopters and their diverse capabilities, from search and rescue missions to military operations and aerial photography.

In this Video, we will explore the main types of helicopter rotor systems and how they contribute to the helicopter’s functionality and performance.

1. Single Rotor System

The single rotor system is characterized by a single main rotor blade that is responsible for generating lift. To counteract the torque produced by this rotor, a tail rotor is used. This setup is essential for maintaining directional control and stability during flight.

Uses: This design is prevalent in most conventional helicopters, including iconic models such as the Bell 206 and the Robinson R22. The simplicity of the single rotor system not only reduces mechanical complexity but also enhances efficiency. As a result, it is favored for a variety of applications, including aerial tours, law enforcement, and emergency medical services, where reliability and straightforward operation are paramount.

2. Tandem Rotor System

The tandem rotor system features two parallel rotors of equal size that rotate in opposite directions. This counter-rotation helps to cancel out the torque that each rotor would otherwise produce, resulting in a balanced and stable flight profile.

Uses: This configuration is typically employed in heavy-lift helicopters, such as the CH-47 Chinook. The tandem design allows for an increased payload capacity and enhanced stability, making it particularly effective for transporting troops, equipment, and supplies in military operations, as well as for civilian applications like logging and construction, where heavy lifting is required.

3. Coaxial Rotor System

The coaxial rotor system consists of two rotors mounted one above the other on the same mast, rotating in opposite directions. This innovative design minimizes the need for a tail rotor, allowing for a more compact helicopter structure.

Uses: Coaxial rotor systems can be found in helicopters such as the Kamov Ka-50. This design offers several advantages, including enhanced lift capabilities, improved maneuverability, and better control in various flight conditions. These features make it particularly suitable for military applications, where agility and quick response times are crucial, as well as for specific civilian operations that require high performance in tight spaces.

4. Intermeshing Rotor System

The intermeshing rotor system consists of two rotors that rotate in opposite directions while intersecting each other, but without colliding. This unique configuration creates a highly efficient aerodynamic profile.

Uses: This system is utilized in helicopters like the Kaman K-MAX, designed specifically for heavy lifting and aerial work. The intermeshing rotors provide remarkable stability and lift capabilities, making it particularly effective for operations in confined spaces, such as urban environments or dense forests. It is ideal for missions that involve heavy external loads, including construction, firefighting, and disaster relief efforts.

5. Transverse rotor system

The transverse rotor system has two parallel rotors that spin in opposite directions, improving lift and stability. This design enhances the aircraft’s aerodynamic efficiency and maneuverability.

A notable example of this system is the V-22 Osprey, a tiltrotor aircraft that merges helicopter vertical lift with the speed of a fixed-wing plane. allowing the Osprey to operate in tough environments like urban areas and remote locations. It can carry heavy loads and personnel, making it suitable for troop transport, search and rescue, medical evacuation, and logistical support in military operations. Overall, the transverse rotor system enhances the V-22 Osprey’s effectiveness and operational flexibility.

6. Compound Rotor System

The compound rotor system combines traditional rotor systems with fixed wings and other aerodynamic features to enhance efficiency and speed. This hybrid approach allows for greater aerodynamic performance than standard rotorcraft.

Uses: Advanced helicopters like the Sikorsky X2 and Boeing’s DBF (Defiant) utilize the compound rotor system. These helicopters are designed for higher speeds and longer ranges, making them suitable for military operations, search-and-rescue missions, and law enforcement tasks where rapid response and extended operational capabilities are essential.

7. NOTAR system

NOTAR system replaces the traditional tail rotor with a ducted fan and directional airflow to counter the torque from the main rotor. It works by pushing air through the tail boom and out through side vents, creating thrust that stabilizes the helicopter. This design reduces noise, boosts safety, and cuts down on maintenance.

Uses: The NOTAR system is found in helicopters like the MD 520N and MD 902 Explorer. Without an exposed tail rotor, it lowers the risk of rotor strikes, making it safer for operations in tight spaces. Its quieter performance is ideal for missions where low noise is needed, such as urban air operations, police work, and medical evacuations.