What is ALTN on Boeing 737? (Alternate)

The term “Alternate” refers to an alternative airport that pilots can choose as an alternate destination in case the planned landing airport becomes unavailable due to any unforeseen circumstances. It is an essential aspect of flight planning and is particularly important in ensuring the safety and success of every flight. In the case of Boeing 737 aircraft, which is one of the most widely used commercial aircraft around the world, understanding the concept of Alternate is crucial for both pilots and aviation enthusiasts. Let’s delve deeper into what Alternate means in the context of Boeing 737 operations.

Understanding Alternate on Boeing 737

In aviation, Alternate is an airport designated as an alternative landing site for a flight if the primary destination airport becomes unavailable or inaccessible. This situation can arise due to various reasons such as unfavorable weather conditions, runway closures, inoperable navigational aids, or even emergencies happening at the original destination airport.

For Boeing 737 pilots, considering Alternate airports is a vital part of their pre-flight planning process. They carefully evaluate various factors like the distance to the alternate airport, the suitability of the runway for the aircraft’s performance, the availability of navigation aids, and the weather conditions at the alternate airport. All these considerations are crucial for ensuring a safe landing in the event of an unforeseen circumstance at the primary airport.

When determining suitable Alternates, pilots also take into account the fuel required to reach the alternate airport, allowing for holding patterns and approach procedures. This ensures that the aircraft has sufficient fuel reserves to divert to the alternate and potentially even a secondary alternate if needed.

Factors Considered When Selecting an Alternate Airport

Selecting an appropriate alternate airport involves a thorough evaluation of various factors to ensure that it can accommodate the Boeing 737 aircraft and meet the requirements for a safe landing. Some of the critical factors to consider include:

1. Distance: The alternate airport should be within a reasonable distance from the original destination airport, allowing the aircraft to divert and reach the alternate with adequate fuel reserves.

2. Runway Length and Surface: The runway at the alternate airport must be long enough to accommodate the Boeing 737 aircraft’s takeoff and landing requirements. Additionally, the surface condition of the runway should be suitable for safe operations.

3. Availability of Navigation Aids: The alternate airport should have the necessary navigational aids, such as Instrument Landing Systems (ILS) or GPS approaches, to facilitate a safe landing in poor visibility conditions.

4. Weather Conditions: Pilots must consider the weather conditions at the alternate airport, including factors like visibility, crosswind limits, and the presence of thunderstorms or strong winds. The aim is to ensure that the weather at the alternate airport is conducive to a safe landing.

Considering these factors, pilots make informed decisions about which airports to designate as alternates for each flight. It is essential to note that the selection of alternate airports is subject to regulatory requirements and airline-specific guidelines.

Boeing 737’s ALTN Function

In modern Boeing 737 aircraft, the ALTN (Alternate) function is available in the Flight Management System (FMS). The FMS is a sophisticated computerized system that assists pilots in planning and navigating their flights. It helps them calculate the required fuel, optimal flight paths, and even provides guidance for instrument approaches.

The ALTN function within the FMS allows pilots to input the desired alternate airport and plan the diversion accordingly. It takes into account critical factors such as distance, fuel requirements, and airport features to provide the crew with valuable information for decision-making during flight operations.

The ALTN function also takes into consideration the aircraft’s performance capabilities and limitations. It helps the crew determine whether the selected alternate airport is suitable for the aircraft’s landing and takeoff requirements. This feature aids in efficient flight planning and contributes to the safe and successful operations of Boeing 737 aircraft.

Furthermore, the ALTN function allows pilots to monitor any changes in the weather conditions at their alternate airport during the flight. If the weather deteriorates or deviates from the predetermined limits, pilots can use the FMS to identify and select a different alternate airport that better meets the required criteria.


Alternate airports play a critical role in aviation safety, allowing pilots to make informed decisions and safely divert their flights when needed. In the case of Boeing 737 aircraft, the ALTN function within the Flight Management System assists pilots in planning and executing diversions to alternate airports. By considering factors like distance, runway suitability, navigational aids, and weather conditions, pilots can select appropriate alternates that ensure a safe landing for the aircraft and its passengers.

Understanding the concept of Alternate and its significance in the operations of Boeing 737 aircraft provides valuable insights into the complexity and thoroughness of flight planning processes. It highlights the meticulousness and attention to detail that pilots must possess in order to ensure the safety and success of every flight.

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