This is one more post in the “know your equipment” and “technology is just as good as its user” series. One of the great features of the G1000 and other glass-cockpits is the terrain display. This can be a two dimensional version where the terrain less than 1′000 feet below the plane is displayed in yellow, and the terrain less than 100 feet below the plane is displayed in red. There is also the Synthetic Vision Technology version with a 3D display of the relief, obstacles, runways, and even traffic. Needless to say, these features rely on a precise measurement of aircraft altitude.

Glass-cockpit systems all have barometric inputs to display the altitude to the pilot. Aircraft always used baro-based altitude, simply because it was historically the sole way to measure altitude. Even technologies like radar-altimeters are not usable because of altitude limitations and surface depencence. Our altimeters are indeed barometers with a graduation in feet, and a knob to define the reference pressure and adapt to atmospheric variations. GPS on the other hand provide a rather accurate 4-dimensional position: longitude, latitude, altitude and time. The altitude accuracy is less than the position accuracy because of the satellites dispositions, but it’s not that bad.

If you ever flew with a handheld GPS simulating basic flight instruments you certainly noticed a difference between the indicated altitude and the altimeter reading. This comes from local pressure and temperature differences but also from the the difference in principle. And just as the GPS horizontal position, the GPS altitudes varies over time even if you leave the receiver at the same place.

I don’t think I have to explain here how important altimeter setting is, particularly when it comes to flying IFR approaches. The typical minimum of a category I ILS approach is 200 feet. A difference of 1 millibar in altimeter setting corresponds to 27 feet. An error of 7 millibars simply anihilate the safety margin. In Europe, the transition level is low enough (from FL50 to FL100, depending the places) to be relevant for light aircrafts. If the QNH is more than 7 millibars away from the standard 1013, failing to switch back to QNH can be a fatal error.

Now, here comes the question. Are terrain features of modern glass-cockpits based on GPS or barometric altitude ? There a handful of underlying issues:

• Does an incorrect altimeter setting lead to false alarms or to the absence of alarms ?
• Are these features still available if the static port gets clogged ?
• Can a difference between barometric and GPS altitude lead to false alarms ?

The answer is to be found in the G1000 pilot’s guide: “GPS altitude is derived from satellite position. GPS altitude is then converted to a mean sea level (MSL)-based altitude (GPS-MSL altitude) and is used to determine terrain and obstacle proximity. GPS-MSL altitude accuracy is affected by satellite geometry, but is not subject to variations in pressure and temperature that normally affect pressure altitude sensors. GPS-MSL altitude does not require local altimeter settings to determine MSL altitude. It is a reliable MSL altitude source.”

The manual does however not explain how the GPS altitude is converted to MSL. That being said, it’s good to know that this system is fully independent from the baro system. I usually don’t fly with the terrain enabled on the MFD while on approach, because it is very disturbing to see the whole screen turning red at 100 feet AGL. But having terrain activated on the inset can be a good safety net…

Let’s imagine the following scenario: an ILS approach in IMC and because of stress the pilot did forget to set the altimeter to the local QNH. The altimeter reading is 200 feet higher than the actual altitude. When the pilot reaches 100 feet AGL (below the ILS minima) the inset turns red, attracting the pilot’s eyes. The altimeter says 300 feet AGL (100 feet above minimums). The pilot immediately initiates a go-around, avoiding a crash landing in IMC. This scenario is a bit catastrophic but not impossible. With proper scale setting, the inset should turn yellow when passing 1′000 feet AGL, being a reminder for a cross-check with the altimeter and the approach chart.
If you’re an IFR pilot and never forget an altimeter setting, my hat goes off to you.