What Does QNH in Aviation Mean? A Pilot’s Complete Guide

The Foundation of Flight Safety: A Clear Conclusion on QNH

Before we delve into the intricate details, let’s get straight to the point. What does QNH in aviation mean? In the simplest terms, QNH is an altimeter pressure setting that, when set on an aircraft’s altimeter, causes it to indicate the aircraft’s altitude above mean sea level (MSL). When an aircraft is on the ground at an airfield, setting the local QNH will make its altimeter display the official elevation of that airfield. This single, crucial number is a cornerstone of flight safety, ensuring pilots have accurate vertical awareness for terrain clearance, obstacle avoidance, and stable approaches. It’s the vital link between the instrument in the cockpit and the true height of the aircraft over the earth’s varied terrain.

Understanding QNH isn’t just about memorizing a definition; it’s about grasping a fundamental principle that keeps aviation safe and orderly. It’s one of a family of “Q-codes,” a legacy shorthand from the early days of radio telegraphy that has endured due to its clarity and importance. This article will guide you through everything you need to know about QNH, from the science behind it to its practical application in the cockpit.

Understanding the Altimeter: The Instrument Behind QNH

To truly appreciate what QNH is, you first have to understand the tool it serves: the barometric altimeter. You might think of an altimeter as a device that magically knows its height, but it’s actually a bit more clever and, in a way, simpler than that. At its core, an altimeter is essentially a sensitive barometer.

It contains a sealed, flexible metal capsule called an “aneroid wafer.” As the aircraft climbs, the outside air pressure decreases, allowing this wafer to expand. As the aircraft descends, the increasing air pressure compresses it. This expansion and contraction is mechanically linked, through a series of gears and levers, to the needles on the face of the altimeter, which we read as an altitude in feet or meters.

However, this presents a problem. The altimeter only knows pressure; it doesn’t inherently know height. To translate pressure into a meaningful altitude, it needs a baseline, a reference point. This is where the International Standard Atmosphere (ISA) comes in. ISA is a theoretical model of the atmosphere with a defined sea-level pressure (1013.25 hectopascals or 29.92 inches of mercury) and a specific temperature and pressure lapse rate. Altimeters are calibrated to this ideal model.

But as any weather forecast will tell you, the real atmosphere is rarely “standard.” The local pressure changes constantly due to weather systems. This is precisely why we need QNH—to tell the altimeter what the *actual* local pressure reference is, so it can display an accurate altitude.

What is QNH? A Detailed Breakdown

Now, let’s zoom in on the definition. QNH is the atmospheric pressure at a specific location, mathematically corrected down to mean sea level (MSL).

This might sound a little abstract, so let’s use an analogy. Imagine you are at an airport in Denver, Colorado, which has an elevation of about 5,400 feet. The actual air pressure measured at the runway will be significantly lower than at sea level. If you simply used that raw pressure, your altimeter wouldn’t be very useful for comparing your altitude to mountains or other airports.

To create the QNH value, meteorologists take the actual station pressure (known as QFE, which we’ll discuss soon) and apply a correction based on the ISA model to calculate what that pressure *would be* if you could take a measurement all the way down at mean sea level. This calculation effectively removes the influence of the airfield’s elevation, creating a standardized sea-level pressure reference for the entire local area.

When a pilot sets this QNH value in their altimeter’s setting window (the Kollsman window), the instrument is calibrated to show the aircraft’s vertical distance above mean sea level. This is called altitude.

This is critically important because all obstacles, terrain, and airfield elevations on aviation charts are depicted in feet or meters above MSL. By using QNH, a pilot can directly compare the reading on their altimeter to the elevations shown on their charts, instantly knowing if they are safely above them.

QNH Units: hPa vs. inHg

You’ll hear QNH given in one of two units, depending on where you are in the world:

• Hectopascals (hPa): Used by most of the world, including Europe, Asia, and Australia. This unit is equivalent to the millibar (mb). A typical value would be “1015”.
• Inches of Mercury (inHg): Primarily used in North America (USA and Canada). A typical value would be “30.00”.

Modern aircraft altimeters are equipped to handle both units, but it’s a key piece of knowledge for any pilot flying internationally.

The Crucial ‘Q’ Family: QNH vs. QFE vs. QNE

QNH is part of a small but vital family of Q-codes that define altimeter settings. Understanding the difference between them is fundamental to being a pilot. The main distinction lies in the reference datum they use, which changes what the altimeter needle actually represents: altitude, height, or flight level.

QNH (Altitude)

As we’ve established, setting QNH references the altimeter to Mean Sea Level (MSL). The reading it provides is your altitude. This is the most common setting used for takeoff, landing, and any flight operations below a specific altitude known as the Transition Altitude.

QFE (Height)

QFE stands for “Query Field Elevation.” It is the actual, uncorrected atmospheric pressure at the airfield datum (usually the highest point on the main runway). When you set QFE on your altimeter while on the ground, your altimeter will read zero (or very close to it). The reading it provides is your height above that specific airfield. While historically used for flying circuits or patterns around an airfield, its use in modern commercial and general aviation has become much less common in favor of the universal consistency of QNH.

QNE (Flight Level)

QNE is not a measured pressure but a worldwide standard pressure setting of 1013.25 hPa or 29.92 inHg. When a pilot sets this standard value, the altimeter no longer reads a true altitude. Instead, it indicates a Flight Level (FL). For example, with the standard setting set, an altimeter reading of 35,000 feet is referred to as “Flight Level three-five-zero” (FL350). This system is used at higher altitudes to ensure all aircraft are using the exact same pressure reference, guaranteeing vertical separation from each other, regardless of the actual atmospheric pressure changes below them.

Comparison Table: QNH vs. QFE vs. QNE

Putting it into Practice: How Pilots Use QNH

Knowing the theory is one thing, but the application of QNH is a constant and critical part of every single flight.

Sourcing the QNH Value

Pilots obtain the current QNH from several sources before and during a flight:

• ATIS (Automatic Terminal Information Service): A pre-recorded broadcast at most airports that provides essential information, including the weather, active runways, and, crucially, the QNH.
• METAR (Aviation Routine Weather Report): A coded text report of the weather at an airport. The QNH is always included (e.g., `Q1023` for 1023 hPa or `A2995` for 29.95 inHg).
• Air Traffic Control (ATC): Pilots can request the QNH directly from controllers. It is also a standard part of takeoff and landing clearances.
• VOLMET: A radio broadcast of meteorological information for aircraft in flight, often covering multiple airfields.

Setting the Altimeter: A Step-by-Step Guide

Setting the QNH is a physical action that is part of a pilot’s core workflow. Here’s how it’s generally done:

1. Obtain the Correct QNH: The pilot first listens to the ATIS or is given the QNH by ATC. For example, the controller might say, “Cleared for takeoff runway two-seven, wind two-niner-zero at ten knots, QNH one-zero-two-one.”
2. Locate the Altimeter Setting Knob: On the altimeter itself, there is a small knob. This knob adjusts the pressure value displayed in a small sub-scale window on the instrument, known as the Kollsman window.
3. Adjust the Kollsman Window: The pilot turns the knob until the correct value—in our example, 1021—is precisely set in the window.
4. Perform a Critical Cross-Check: This is the most important step on the ground. After setting the QNH, the pilot looks at the altimeter’s needles. The indicated altitude should match the known, published elevation of the airport to within a specific tolerance (e.g., +/- 75 feet). If it doesn’t, it indicates a potential instrument error that must be investigated before flight.

Navigating the Skies: Transition Altitude and Transition Level

So, we know that using local QNH is perfect for low-level flight. But what happens when you climb high and fly across the country? The local QNH in New York is going to be different from the QNH in Chicago. If aircraft flying between the two cities all used their local QNH, their altimeters would be referenced to different baselines, creating a serious risk of collision. This is where the transition concept comes in.

The Transition Altitude (TA)

The Transition Altitude is a published, fixed altitude for a given region or country. When an aircraft is climbing through the TA, the pilot must change their altimeter setting from the local QNH to the standard pressure setting (QNE) of 1013.25 hPa / 29.92 inHg. From this point upwards, they will report their vertical position as a Flight Level (e.g., FL250).

In the United States and Canada, the TA is standardized at 18,000 feet. In Europe and other parts of the world, it can be much lower and vary by country, often ranging from 3,000 to 10,000 feet.

The Transition Level (TL)

Conversely, when an aircraft is descending, it must switch back from the standard setting to the local QNH to prepare for approach and landing. The point at which this happens is the Transition Level. Unlike the fixed TA, the TL is not fixed; it is the lowest available Flight Level above the Transition Altitude. Its value depends on the local QNH. If the local QNH is very low, the TL will be higher to ensure there is always a safe buffer of separation between aircraft using Flight Levels and those using QNH altitude.

The space between the TA and the TL is known as the Transition Layer. Pilots are not assigned to fly level within this layer; it is purely a zone for changing altimeter settings.

Why is an Accurate QNH So Critical for Safety?

Mistakes with altimeter settings are among the most dangerous errors in aviation. The consequences of using an incorrect QNH are severe and direct.

Terrain Clearance and CFIT

The single most important role of QNH is to provide accurate terrain clearance. A wrong QNH setting can lead to a Controlled Flight Into Terrain (CFIT) accident, where a perfectly airworthy aircraft is unintentionally flown into the ground, a mountain, or another obstacle.

There’s a famous aviation adage: “High to Low, Look Out Below!” This serves as a vital memory aid:

If you fly from an area of high pressure (e.g., QNH 1030) into an area of lower pressure (e.g., QNH 1005) without updating your altimeter setting, your altimeter will over-read. It will tell you that you are higher than you actually are. You might think you are safely clearing a mountain range at 5,000 feet, when in reality, you are hundreds of feet lower, potentially on a collision course.

Vertical Separation

At lower altitudes (below the TA), all aircraft in a given sector of airspace are using the same QNH provided by ATC. This ensures that when one aircraft is assigned an altitude of 4,000 feet and another is assigned 5,000 feet, there is a genuine 1,000 feet of vertical separation between them, as they are both referenced to the same mean sea level datum.

Stable Approaches and Landings

During an instrument approach, a pilot must cross specific points (fixes) at pre-determined altitudes. An accurate QNH is essential to flying this vertical profile correctly. Being too high or too low on approach can lead to an unstable and unsafe landing attempt.

Conclusion: QNH as a Cornerstone of Aviation Safety

As we’ve seen, QNH in aviation is far more than a random code or a trivial number to dial in. It is a simple yet profoundly effective concept that solves the complex problem of determining an aircraft’s true altitude in an ever-changing atmosphere. It is the agreed-upon language that allows a pilot’s instruments to speak truthfully about their position relative to the earth.

From the pre-flight check on the ground to the final approach for landing, the correct sourcing and setting of QNH is a constant responsibility. It is a fundamental building block of vertical navigation, ensuring that pilots can confidently maintain separation from both the ground and other air traffic. In the world of aviation, where precision and safety are paramount, QNH stands as a perfect example of a simple principle executed with unwavering discipline to keep our skies safe.