ALTITUDES

Surface and Onboard data

The display is divided into two main areas, one for surface station information, in the bottom portion, and one for onboard information, in the top portion of the display. Frames (grey, or white for narrower devices) separate them. A third space, to the right, shows temperature-compensation data, enclosed in a white (or dotted-white) frame.

Surface station information: This area displays surface (ground) station information, i.e. elevation (above sea-level), reference pressure (altimeter setting/QNH, or QFE), and Outside Air Temperature. Also shown are the calculated pressure and density elevation, and the deviation from ISA. Surface station elevation range is from -2 000 feet to 15 000 feet.

Dial-in the aerodrome or surface station elevation, pressure and temperature from weather reports such as METARs. Be sure to know what type of pressure (QNH or QFE) was reported, and to use the appropriate column (QNH or QFE) when setting it. The pressure you set here will be assumed to be the current and correct pressure.

Corresponding QNH and QFE values are shown for the selected surface station elevation.

Onboard data

Onboard altimetry information: This area presents onboard altimetry information, and functions much like an altimeter or air data display. Dial in the indicated altitude/height for which you would like to make calculations. Values can range from -2 000 feet to 100 000 feet.

Setting the altimeter: Like an actual altimeter, you need to set a current reference pressure in order to obtain correct information. For ease-of-use, 3 buttons are provided, which allow quickly setting the pressure and reference. The QNH and QFE buttons will set your “altimeter” to the surface stationʼs pressure and reference (QNH or QFE). The STD button will set standard pressure (29.92 Inches Hg / 1013.25 HPa / 760 mm Hg).

The buttons will show a green dot whenever QNH or QFE is set.

If the altimeter is not set to either surface QNH or QFE, the values will show in yellow, as a reminder that the setting does not correspond to the surface station value you have entered.

All altitudes/heights shown will be recalculated anytime you adjust the altimeter setting. Like a real altimeter, the indicated altitude/height will increase if you increase the altimeter setting, and decrease if you reduce it.

QFE: If you set your altimeter to the QFE value, the onboard values displayed will be heights (above surface), as opposed to altitudes (above sea-level). When QFE is set, labeling for onboard data changes from “Altitudes” to “Heights” (in the top left corner). Where the surface station is located at sea-level, QNH and QFE will be identical, and the labeling will show “Altitudes & Heights”.

Auto QNH/QFE mode: When enabled, it will keep your “altimeter” set to the same pressure that is set for the surface station, and to the reference selected for the altimeter (QNH or QFE). In Auto mode, the thumbwheel for the altimeter setting is greyed-out and will not function. If you wish to manually set the altimeter, simply disable Auto mode.

Note: Auto mode cannot be enabled if a value other than QNH or QFE is set on the altimeter.

Convert barometric pressure units: convert between Hectopascals, Inches (of mercury), and millimetres (of mercury). Simply dial in the value you wish to convert, and immediately see the corresponding values in the other units.

True and compensated altitude/height

In conditions where temperature is below ISA, even when reference pressure is set correctly, your actual altitude/height will be lower than what is shown on your altimeter. Conversely, if temperature is above ISA, actual altitude/height will be higher than shown.

To ensure obstacle clearance in IMC, it is normally recommended to calculate and add a correction to minimum flight altitudes/heights (such as MOCA, MSA, and approach minimum altitudes) when temperatures are low (especially below 0 degrees Celsius / 32 degrees Fahrenheit). The minimum flight altitude/height plus the correction is termed (temperature-) compensated altitude/height.

It is also possible to calculate the actual altitude/height the aircraft is at, using the indicated altitude/height and the temperature at the (nearby) surface station. The result is termed “true” altitude/height.

E6BJet calculates both the (temperature-) compensated and the “true” altitude/height for a given indicated altitude/height.

E6BJet presents True and Compensated altitude/height calculations inside a white frame, on the right side of the display. In order for results to be displayed, the altimeter must be set to either QNH or QFE. Otherwise, compensated, true (and density) values will be removed from view and replaced with dashes.

Please look at the following example:

Here, indicated altitude is 3 000 feet. Aerodrome (surface) elevation is 10 feet, QNH is 30.02 Inches (and correctly set on the altimeter), and surface temperature is -10 Celsius, 25 degrees Celsius colder than the standard temperature at that elevation.

Notice that with this ISA deviation ( -25 degrees Celsius), the SAT at 3 000 feet will be -15 degrees Celsius.

Compensated altitude is calculated to be 3 287 feet. In other words, you should fly at 3 287 feet indicated in order to really be at 3 000 feet ASL.

“True” altitude is calculated to be 2 738 feet, whereas your altimeter is indicating that you are flying at 3 000 feet.

Important! The calculations assume that although the temperature is below ISA, the lapse rate remains standard. The results become less accurate anytime the lapse rate in the actual atmosphere is in fact different from standard. The “true” altitude could be different from what is shown, it is therefore displayed in amber. See below.

Important information regarding the
Accuracy of temperature correction calculations

There are several calculation methods to determine temperature correction. Three of the most common ones are: 1) an approximate correction method, 2) a correction method for specific conditions including tabular data, and 3) an iterative method for more accurate temperature corrections.

E6BJet uses the iterative method, and correction values are valid up to 36 000 feet ASL, provided certain assumptions hold true.

CAUTION! One of the assumptions is that a of a constant and standard temperature lapse rate. The lapse rate in the actual atmosphere might be significantly different, in which case the accuracy of the results will degrade.

Some aircraft FMS also have a temperature correction function. Refer to the manufacturerʼs documentation for more information on how the feature is implemented, and how and when to use it.

For more information about the calculations, refer to:

  • ICAO Document 8168 – Procedures for Air navigation Services – Aircraft operations – Volume 1 – Flight Procedures Part VI, Chapter 3 – Altimeter corrections.
  • Transport Canada Aeronautical Manual, TP 14371E – RAC – Rules of the air and air traffic services – Corrections for Temperature.

Always refer to official country-specific Aeronautical Information Publications for how flight crew should handle cold-temperature corrections in flight. Some countries require the use of different or additional methods to ensure obstacle clearance in cold weather.

iPhones with barometric pressure sensor

The iPhone 6 and later models have a built-in barometric pressure sensor. Pressure readings can be used to calculate device pressure altitude/height, and, with the correct altimeter setting, a “device altitude/height”.

Important: This feature will only work if you have enabled Fitness Tracking on your device, found under Settings – Privacy – Motion & Fitness.

Displaying device altitude. E6BJet can optionally display the sensed pressure and indicated device altitude/height. This feature is disabled by default. If enabled and a sensor is present, its information will be displayed in the Onboard altimetry frame, to the left of indicated altitude/height.

As you would expect, the device barometer is not aviation-authority certified, and cannot be relied on as a substitute for a proper altimeter.

The pressure displayed underneath device altitude/height is the pressure that was most recently sensed by the device. It is not an altimeter setting, and cannot be adjusted by the user.

Recording device altitude and time. You can also optionally enable recording of device pressure altitude readings and altimeter setting to a text file stored on your device. When enabled, a new entry will be written to the file anytime pressure altitude changes, along with date and time of the change. Recording is disabled by default.

Enable the display, and/or the recording of device altitude information via the Settings app. From there, locate the E6BJet settings icon, and tap it to access E6BJet settings. Use the buttons provided to enable display and/or recording.

Display of device altitude must be enabled in order for recording to take place.

When recording is enabled, REC and a steady red dot will be displayed above device altitude.

Caution: the recorded data may allow inferring some information about your movements, so be sure to disable recording if this is undesirable.

E6BJet will record a new entry with the date and time (UTC), pressure altitude, and the currently-set altimeter setting, every time the altitude changes by one foot or more. E6BJet must be in the foreground, but does not need to be displaying the Altitudes tab constantly. Entries will look as follows:

2020-03-24;13:40:48;UTC; press.alt.(ft)=;2511; alt.setting(InHg)=;29.92
2020-03-24;13:40:54;UTC; press.alt.(ft)=;2510; alt.setting(InHg)=;29.92
2020-03-24;13:41:08;UTC; press.alt.(ft)=;2509; alt.setting(InHg)=;29.92

Entries are separated by semicolons, to allow easy importing to other applications such as spreadsheets.

Recorded device altitude data is stored on your device, in a text file named “E6BaltitudeLog.txt”, which can be accessed via the Files app.
(To do so, launch the Files app, select “On my iPhone” from the available locations, and look for a folder named E6BJet. It contains the file “E6BaltitudeLog.txt” (the .txt suffix does not appear). Tap on it to view the contents – you will be asked which app should be used for viewing.

Note that neither the file, nor the folder, will exist if recording has not previously been enabled.

Deleting recorded data. You can delete the file at any time, using the Files app. Do note that if you leave recording enabled, a new copy of the file will be created and logging will start afresh when you reopen E6BJet.


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