This article explains about Altimeter Setting and procedure for aviation. More about Altimetry and Altimeter.
What is Altimetry?
Altimetry is the science and technology of measuring altitude or elevation above a reference point, typically the Earth’s surface. It involves the use of instruments known as altimeters to determine an object’s vertical distance above a specific datum, which could be sea level, ground level, or another defined reference point. Altimeters utilize various methods to measure altitude, including barometric pressure, GPS signals, radar, and laser ranging.
Altimetry is essential in various fields, including aviation, meteorology, geography, geodesy, and oceanography. In aviation, accurate altitude measurement is crucial for navigation, flight planning, obstacle clearance, and maintaining safe separation between aircraft. In meteorology, altitude data are used to analyze atmospheric conditions and predict weather patterns. In geography and geodesy, altimetry helps in mapping terrain, determining land elevation, and studying changes in Earth’s surface over time. In oceanography, altimetry is used to measure sea surface height and study ocean currents and tides. Overall, altimetry plays a vital role in understanding and monitoring the vertical dimension of the Earth’s surface and atmosphere.
How does an altimeter work?
An altimeter is a critical instrument in aviation, providing pilots with essential information about an aircraft’s altitude above a reference point, typically mean sea level (MSL). Its operation relies on changes in atmospheric pressure with altitude. As an aircraft ascends, atmospheric pressure decreases, and vice versa. This relationship allows the altimeter to gauge altitude based on pressure readings, making it a vital tool for safe navigation, especially in conditions where visual references to the ground are limited or absent.
To set up an altimeter, pilots calibrate it to a known atmospheric pressure reference point. This reference point is usually the standard atmospheric pressure at sea level, which is commonly set at 29.92 inches of mercury (inHg) or 1013.25 hectopascals (hPa). The altimeter measures the current atmospheric pressure and compares it to this reference pressure to determine the aircraft’s altitude. Inside the altimeter, an aneroid capsule expands or contracts in response to changes in atmospheric pressure as the aircraft climbs or descends.
Altimeter Setting Procedures
Flight crews operating on international routes encounter various units of measurement for setting barometric altimeters, necessitating specific altimeter cross-checking procedures to ensure accurate altitude readings.
QNH and QFE
- QNH: This altimeter setting causes the altimeter to indicate vertical distance above mean sea level, such as the airfield elevation at touchdown on the runway.
- QFE: QFE is the altimeter setting that causes the altimeter to indicate vertical distance above the QFE reference datum, with zero at touchdown on the runway.
Advantages of QNH over QFE
- Transition Levels and Altitudes: QNH eliminates the need to change the altimeter setting during operations below the transition level or transition altitude.
- Missed Approach: QNH also avoids the necessity of changing the altimeter setting during a missed approach, which would typically be required when using QFE.
Operator Practices
- Some operators may choose to set the altimeter to QFE in areas where ATC and the majority of other operators use QNH.
Standard Operating Procedures (SOPs)
- Implementing SOPs can mitigate altimeter setting errors and ensure consistent practices among flight crews.
Units of Measurement
- The primary units of measurement for setting altimeters are:
- Hectopascals (hPa): Also known as millibars (mb) in some countries.
- Inches of Mercury (in. Hg): Commonly used in North America.
Regional Variations
- Throughout Europe, hPa (or mb) is the primary altimeter setting, while in North America, the primary setting is in inches of mercury (in. Hg).
- In other regions, either system may be encountered, leading to potential confusion or misinterpretation.
Impact of Errors
- Errors in altimeter settings, such as mishearing or misinterpreting ATIS or ATC instructions, can lead to significant altitude discrepancies.
- For example, a 10 hPa error in altimeter setting could result in approximately a 300 feet error in indicated altitude when using hPa as the unit of measurement.
Implementing thorough cross-checking procedures and adhering to standardized practices are crucial for ensuring accurate altitude indications and safe flight operations, particularly in international airspace with diverse altimeter setting conventions.
Setting the altimeter
Setting the altimeter correctly is crucial for ensuring accurate altitude readings, especially during flight. To mitigate errors associated with different units of measurement or unusual values, standardized operating procedures (SOPs) should be followed when broadcasting or reading back altimeter settings. Here are some key practices to follow:
- Announce All Digits and Units: When communicating altimeter settings, all digits and the unit of measurement (hectopascals or inches) should be clearly stated. For example, instead of saying “altimeter setting six seven,” specify “altimeter setting nine six seven hectopascals” or “altimeter setting two eight point six seven inches of mercury.” This prevents confusion and allows for the detection and correction of any errors.
- Preventative Measures: Errors in altimeter settings can occur due to factors such as high workload, deviations from defined task sharing, interruptions or distractions, inadequate cross-checking by flight crewmembers, or confusion about units of measurement. Adhering to defined task sharing protocols, both for normal and abnormal conditions, and using normal checklists can serve as effective defenses against altimeter setting errors.
By implementing these SOPs and preventative measures, aviation professionals can minimize the risk of altimeter setting errors and ensure the safety of flight operations.
Metric altimeters
Metric altimeters, used in certain countries like the Commonwealth of Independent States (CIS) and The People’s Republic of China, require specific Standard Operating Procedures (SOPs) or conversion tables to ensure accurate altitude readings.
Crossing the Transition Altitude
Transition altitude, the altitude at which altimeters are set to a standard pressure setting (e.g., 1013.25 hPa or 29.92 inHg), can vary based on location:
- Fixed Transition Altitude: It may be consistent across an entire country, such as 18,000 feet in the United States.
- Airport-Specific Transition Altitude: Some airports have a designated transition altitude specified in approach charts.
- Variable Transition Level: The transition level may vary depending on the reported QNH (altimeter setting) in the Automatic Terminal Information Service (ATIS) broadcast.
Transition between variable and fixed transition altitudes can lead to premature or delayed altimeter reference setting, especially for aircraft transitioning between regions with different practices (e.g., US aircraft flying into Europe or vice versa). Altitude constraints, specified in terms of altitude or flight level, can also affect when altimeter reference changes occur, potentially causing confusion for flight crews.
Altimeter-setting errors are a common cause of altitude deviations and can result in a lack of vertical situational awareness. To minimize these errors, pilots and aircrew should:
a. Be aware of altimeter setting changes required by prevailing weather conditions, such as extreme cold fronts or low-pressure areas.
b. Understand the unit of measurement for setting the altimeter at the destination airport.
c. Anticipate altimeter settings based on aviation weather reports (METARs) and ATIS broadcasts.
d. Cross-check altimeter settings between Pilot Flying (PF) and Pilot Not Flying (PNF).
e. Adhere to SOPs for resetting altimeters at transition altitude/level, using standby altimeters for cross-checking, and making altitude calls.
By following these procedures, pilots can minimize the potential for altimeter-setting errors and maintain accurate altitude awareness during flight operations.