Altitude vs. Flight Level vs. Height

In aviation, Altitude, Flight Level, and Height are terms used to describe an aircraft’s vertical position relative to various reference points. Each term provides distinct information and is used in specific contexts to maintain clarity and safety in aviation operations. Here’s a breakdown of each term and its usage:

Altitude

Altitude refers to the vertical distance of an aircraft above mean sea level (MSL). It’s a crucial term in aviation, as most navigation and air traffic control operations use altitude to ensure safe separation between aircraft.

• Measured with: Altimeters in the cockpit, which are calibrated to reflect the pressure at sea level (known as QNH).
• Reference Point: Mean Sea Level (MSL).
• Expression: Typically expressed in feet (ft).
• How It’s Determined: The altimeter measures the atmospheric pressure outside the aircraft and converts this into a height above MSL.
• Importance in Aviation: Altitude is a vital measurement when flying below the transition altitude (usually around 18,000 feet in the U.S., though it varies by country). Below the transition altitude, pilots are instructed to set their altimeter to local QNH pressure settings for accuracy relative to local terrain and sea level.
• Key Points on Altitude:
  • Used primarily below the transition altitude.
  • Keeps aircraft vertically separated from terrain and obstacles.
  • Is essential for complying with instrument flight rules (IFR) and visual flight rules (VFR).

Flight Level (FL)

The height of an aircraft above a standard pressure level (1013.25 hPa or 29.92 inHg), measured in hundreds of feet. It is used to standardize altitude measurements and reduce the need for corrections due to changes in atmospheric pressure.

• Measured with: Altimeters, but with a standard pressure setting of 1013.25 hPa or 29.92 inHg (referred to as QNE).
• Reference Point: Standardized atmospheric pressure level.
• Expression: Expressed as “FL” followed by a number. For example, FL350 refers to 35,000 feet under standard pressure settings.
• How It’s Determined: Once the aircraft climbs above the transition altitude, the pilot switches the altimeter to the standard pressure setting. This shift creates a uniform level reference, irrespective of local pressure variations, allowing ATC to maintain safe separation between aircraft.
• Importance in Aviation: Flight levels eliminate regional pressure variations, providing a consistent and stable reference for high-altitude flight. This is particularly important for long-haul, international flights.
• Key Points on Flight Level:
  • Used above the transition altitude, usually 18,000 feet in the U.S.
  • Ensures standardized separation between aircraft at high altitudes.
  • Only affected by atmospheric pressure changes, not regional weather variations.

Height

Height is the vertical distance of an aircraft above a specific ground level, such as an airport or terrain. It’s measured from the aircraft to the ground immediately below it, rather than sea level.

• Measured with: Radio altimeters (particularly close to the ground, as they provide accurate readings based on radar technology).
• Reference Point: Ground level directly below the aircraft.
• Expression: Typically measured in feet, though meters are also used, especially in non-U.S. countries.
• How It’s Determined: A radar or radio altimeter measures height by sending signals that bounce off the ground, calculating the return time to determine distance from the ground.
• Importance in Aviation: Height is crucial during takeoff, landing, and low-level flight (such as helicopter operations or terrain-following flights), as it gives precise distance to the ground. It’s also used in terrain-avoidance systems to prevent potential collisions with the ground.
• Key Points on Height:
  • Specific to the ground directly beneath the aircraft.
  • Most critical for low-altitude operations and in approach phases of flight.
  • Key for operations over uneven terrain, where MSL-based altitudes might be misleading.

Practical Applications of Each Term

• Altitude is commonly used during most of the flight when below the transition altitude, ensuring terrain clearance and separation from other aircraft.
• Flight Level is the preferred measure for aircraft cruising at higher altitudes above the transition altitude, offering standardized vertical separation.
• Height is essential when precise, real-time distance from the ground is necessary, especially during takeoffs, landings, or when flying over rugged terrain.

Transition Between Altitude and Flight Level

The transition altitude and transition level are crucial points where pilots switch from altitude-based (QNH) measurements to flight level-based (QNE) measurements, or vice versa. This change ensures that all aircraft operating in higher altitudes use a standard pressure setting to maintain consistent vertical separation, regardless of changing atmospheric conditions across different regions.

• Below the Transition Altitude: Altitude (based on QNH) is used.
• Above the Transition Altitude: Flight Level (based on QNE) is used.

It’s important to note that the transition altitude, where the use of flight levels begins, varies depending on the location and airspace regulations.

Conclusion

Here’s a table summarizing the key differences:

• Altitude = Distance from Mean Sea Level (MSL) — used below the transition altitude.
• Flight Level = Standardized altitude with reference to standard pressure — used above the transition altitude.
• Height = Distance from the ground directly below — used when the precise ground proximity is necessary.