ProSim737 IOS - Unconventional Settings — Flaps 2 Approach

Screen capture of the IOS options screen showing several of the parameters that are user configurable

The user interface in the Instructor Operator Station (IOS) allows the user to customise several functions, in addition to enabling or disabling specific options. Whilst most of the functions are straightforward, there are several options that are unconventional and therefore, probably not clearly understood.

In this article, I will explain some of the features/options in the Instructor Operator Station (IOS) that are not succinctly explained elsewhere. The parameters discussed (and others) can be found in the IOS user interface (settings-cockpit setup options-options& FMS/CDU). The information relates to ProSim737 release 3:25b5.

I will examine:

ARP Elevation - Use ARP Elevation as Runway Elevation (S)
Common VNAV (ALO)
ILS Virtualization (S)
Navaid Virtualization (S)
VNAV Energy Compensation (ALO)
VNAV ALT - VNAV Speed Lower and Upper Limit (ACS & S)

Those functions that relate to the simulator are marked with a S, while functions that are an airline option are marked with ALO. The letters ACS are an abbreviation for aircraft systems.

Thanks to ProSim-TS for providing additional information regarding some of the options.

ARP Elevation – Use ARP Elevation as Runway Elevation

ARP is an acronym for airport reference point. In the simulated environment, the scenery has been designed to be flat, however, the approach charts and the Navigraph database reflect the real world (which is not flat).

Scenery design is based on ARP altitude, but the runway data is often based on the actual altitude of the runway. This discrepancy can cause the landing reference bar in the simulator to not display correctly at ground level. To mitigate against this altitude discrepancy, the altitude table must be cross referenced to the correct altitude.

Enabling ARP in IOS applies the correct altitude to the altitude table.

Important Point:

In some situations ARP will need to be disabled. For example, when the scenery database has an altitude error.

Common VNAV

Common VNAV is an airline option that provides commonality with other Boeing aircraft types. Historically, airlines operated one type of aircraft and Common VNAV was not necessary. However, with multiple Boeing aircraft it makes sense for an airline to have each aircraft operating identical VNAV systems when it comes to VNAV descent.

Common VNAV is linked to VNAV descent, and depending whether it is enabled or disabled will alter how the descent (DES) page is displayed in the CDU. When the option is enabled, the pilot cannot select a speed descent from the DES page; there is no manual SPEED/PATH change input and it is not possible to select between VNAV SPD or VNAV PATH mode.

The Basics

Common VNAV is not discussed in the FCOM or FCTM, therefore, further details should be provided.

Simply written, if the aircraft is descending in a Path Descent and is above path the FMC will automatically change to a Speed Descent. The FMC will revert back to a Path Descent once the aircraft is established on or below the required path.

The following information identifies the differences when Common VNAV is enabled or disabled. The topic is very convoluted and by far the easiest way to learn the differences is to use the simulator with Common VNAV enabled and then disabled.

Common VNAV - Enabled

When enabled, it is not possible to change between VNAV SPD and VNAV PTH. Nor is it possible to change the mode using the prompts in the CDU, or use SPD INTV; the path and speed prompts on the descent page are removed.

During a descent, VNAV will revert to VNAV SPD when VNAV is engaged above the VNAV Path. This will initiate a descent with a target speed close to VMN/MMO, and usually within 6 KIAS. This is to prevent overspeed.

If SPD INTV is selected during a descent, VNAV will remain in VNAV PTH. If the throttle system registers the aircraft’s speed is below the target speed (5-15 KIAS) the autothrottle will energize to maintain the target speed.

If the aircraft’s flaps are extended, VNAV will remain in VNAV SPD if the aircraft is above the VNAV profile. When the aircraft descends, VNAV will change to VNAV SPD.

Common VNAV - Disabled

When disabled and in overspeed, the AFDS would revert to LVL CHG.

If the aircraft’s flaps are extended and Common VNAV is disabled, VNAV will revert to VNAV PTH and the aircraft will descend rapidly to recapture the VNAV Path profile.

ProSim737 Compliance

For the most part ProSim737 complies with the above information, however, it does not replicate the system perfectly. Whether this is important is at the discretion of the user.

By default, Common VNAV is enabled (turned on) in the ProSim737 IOS.

Navaid Virtualization

Navaids are part of the scenery located in the scenery database, and when the scenery was released, these navaids were probably correct. However, the navaids used by Navigraph, located in the navigation database are up-to-date and reflect the real world. These files may not be identical to those in the scenery database.

The simulator prioritises the reading of the scenery database and any navaids included with this scenery. Selecting Navaid Virtualization in IOS directs the simulator to reference the navigation database instead of the scenery database; thereby ensuring that ProSim always reads up-to-date navaids.

Important Points:

Navaid Virtualization should be disabled to use ‘older’ navaids (not used today). Doing so will force the simulator to read the scenery database and resultant navaids. For example, when using scenery with older style NDB arrays that are no longer in current use.

Navaid Virtualization will not function with older Navigraph navigation data. To check which Navigraph database version is in use, open the ProSim User Interface (config/database).

By default Navaid Virtualization is enabled.

ILS Virtualization

ILS Virtualization functions similarly to Navaid Virtualization; the ILS signal in the scenery database is merged with the information in the navigation database to generate a ‘virtual signal’.

Essentially the ILS should match the direction of the approach runway. If the ILS antennae in the scenery database is located in the wrong location (such as in older scenery or by a development error), the runway may appear ‘offset’.

In an attempt to rectify offset runways, the data is merged between the two databases, and if the ILS is identical, yet the antennae positions differ, only the antennae position data is updated. In most cases, this resolves any runway offset issues.

It is for this reason, that ILS Virtualization is in beta and no support (at the moment) is provided by ProSim-TS.

Important Point:

Apparently ILS Virtualization functions better when using MSFS. This is probably because the scenery used in MSFS is more recent than that used in other simulation platforms .

VNAV Energy Compensation

VNAV energy compensation (often referred to as VNAV speed reduction) is a specialised program within the FMC software; it is an airline option and can be enabled or disabled in IOS.

Energy Compensation (if enabled) will come into effect during a VNAV path descent, when the MCP has been temporarily set to an altitude above the planned descent path. The airspeed cursor will slowly move toward a slower airspeed while the target speed in the FMC remains constant. The reduction in airspeed improves the likelihood of the aircraft recapturing the planned descent path.

When the aircraft recaptures the planned descent path, the airspeed will slowly build up to the FMC target speed.

The program will also come into effect if the aircraft is too high for the set FMC altitude.

VNAV ALT (VNAV Speed Lower and Upper Limit)

This setting enables you to enter predefined -/+ speeds that will change the lower and upper limits that VNAV uses during a VNAV path idle descent.

If the airspeed is 10 knots or greater the DRAG REQUIRED message will be displayed on the CDU. Conversely, if the airspeed is slower than 15 knots, the ARM mode will change to FMC SPD on the Flight Mode Annunciator (FMA).

In the real aircraft the correct setting to use is -15/+10 knots, but this may differ in the simulator as other variables come into play such as how well VNAV is simulated. Therefore, tweaking these settings may be beneficial when trying to match the performance of the simulated aircraft with the real aircraft.

Final Call

ProSim-TS in its B738 avionics suite does not produce a high-end user manual; rather they use a Wiki manual to explain the basics of their software. This is because their software is in a continual state of flux; updates and changes are common, especially in relation to MSFS-2020.

In this article we have examined a few of the more ‘unconventional’ options that can be found in IOS and have enlarged on the information found in the Wiki manual. A future article may deal with some of the other lesser known options tabs.