Future Service for Future Avionics

Want to understand new technologies and existing mandates? Ken Elliott explores the service and maintenance activity once NextGen avionics have been upgraded or installed.

Ken Elliott  |  25th August 2017
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Ken Elliott
Ken Elliott

Ken Elliott is a veteran with 52 years of aviation experience, focussed on avionics in General and Business...

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NextGen Upgrades

To help operators understand and plan for new technologies and existing mandates, Ken Elliott explores the service and maintenance activity that take place once NextGen avionics have been upgraded or installed.


Begin by understanding the wider impacts of the upgrade process, which drives the need to consider in-service requirements and potential concerns. From an airworthiness perspective, any system ‘touched’ by an aircraft change will require some form of re-validation regarding its integrity and performance.

If disturbed, does the system still perform its intended function, is it still safe and does the certification reflect the current status of both the system itself and its integration into the aircraft?

Most aircraft systems are interdependent, inferring that a change to one will directly or indirectly affect another system or component. Because aircraft are metal tubes with appendages, their real estate is limited. All sorts of devices are packed in together, often requiring technicians to access the part they need by removing another component.

Often items, removed for access, have no system relationship with the item the technician needed to reach. If improperly documented, the process of achieving access may lead to an unrelated defect at a future date. To make matters worse, the access issue may not even involve equipment or a module, but might involve the removal of a fitting, connector or wiring harness.

The upgrade process needs to take fully into account access and reassembly.

Each step should be documented in the aircraft manual. Amendments and disturbances to any system should be validated by a test of its functionality. From an in-service perspective, future maintenance will require the same, or similar, access as the originally installed component. Therefore, the possibility exists for an unrelated system failure.

Many upgrades require specific scheduled checks, either based on calendar dates or aircraft hours flown (times and cycles). These checks are completed under a Type Certificate (TC) or Supplemental Type Certificate (STC) document, titled ‘Instructions for Continued Airworthiness’ (ICA).

Manufacturers and installers include the ICA in the customer’s Flight Manual (Supplements), and the flight department mechanic will ensure its due date is sequenced (along with all other inspection-due items) within the aircraft’s maintenance tracking records.

The environmental impacts to an aircraft change also need to be reviewed. These requirements may include a wide spectrum of considerations from temperature to dissimilar metal corrosion. An ICA may be issued based upon an environmental concern, and they may need to be monitored on a regular basis.

Physical and environmental aspects of an upgrade or new installation are not the only considerations. Also, integrating complex systems electrically can be fraught with potential issues. There are factors such as electro-magnetic interference (EMI), bus loading, bonding, software compatibility and much more to evaluate. These are the areas where it is likely that problems may occur later.

“If it ain’t broke, don’t fix it” is a well-known mantra. Post installation and maintenance checks often require the disturbance of aircraft systems that are functioning quite okay. The mere fact that someone accessed the system, either physically or electrically can be an inadvertent cause for a failure. Worse, tampering with a system can lead to intermittent failures after the aircraft departs.

When upgrading or adding to an aircraft, operators should know all the details of the work undertaken, including systems disturbed.

It will help to understand any additional testing needed, as well as all continued airworthiness and Minimum Equipment List (MEL) requirements.

From time-to-time, both the aircraft and equipment manufacturers will update their systems. This situation is more likely to be the case with NextGen upgrades, such as ADS-B, where in flight exposure of newly designed equipment and integrations may reveal errors, malfunctions or other anomalies. These, once addressed, are resolved by the issuance of Service Letters, Alerts or Bulletins.

In some cases, if the FAA considers the discrepancy to be a safety concern, an Airworthiness Directive (AD) is issued. Updates can be anything from a software change, completed at the customer’s facility, to a full-blown modification on the aircraft at a Maintenance Repair Organization (MRO). The update may be optional, recommended or required. An AD is a mandated, time limited, version of a required update.

Operators on maintenance tracking programs will typically receive notice of factory changes. Aircraft in warranty will always be notified of changes, operational issues and concerns. As you go back further in aircraft age, however, the likelihood of being aware of changes decreases.

Some updates are product improvements and will not be based on safety or operational issues. A good avionics shop will take advantage of these improvements to garner more business from their regular customers. Customer sales representatives from MRO’s, who mostly cover all aspects of maintenance, will likely do the same thing. Operators who stay engaged with the aircraft OEM’s customer workshops and respective websites will always be made aware of product improvements.


NextGen System –ADS-B Out (& In)

Newer and more digital aircraft are less likely to exhibit the potential disruptions of integrated systems. In fact, they have the added advantage of remote system monitoring, via the avionics bus, allowing for systems diagnostics and status reporting. Older analogue aircraft are much more prone to intra-system disruptions and have less status monitoring.

Table A outlines the most likely components that are installed or integrated to, during an ADS-B installation, specifically focusing on a FAR Part 25 turbine aircraft. This information demonstrates a significant number of potential touch points on the aircraft and, by default, a greater possibility for change impact during in-service operations.


Continued airworthiness ground checks for ADS-B include those already undertaken for RVSM, Transponders, Altimeters, Air Data and the aircraft’s pitot static. Just as RVSM can be monitored in flight, so can ADS-B. The additional test of in-flight monitoring and pilot initiated performance reports is a very useful way of ensuring the accuracy of your broadcasted data, while ATC and other aircraft are relying on it.

For ADS-B In aircraft and for ATC, correctly showing your aircraft in relation to others is a situational awareness consideration.

Erroneous display of seemingly authorized data, where no fault indication is present, can be very misleading and potentially an airspace safety concern.

The FAA has a Public ADS-B Performance Report (PAPR), available to operators, that is the means to verify your on-board system, upon request.

Further, the FAA continuously conducts ADS-B Performance Monitoring (APM) as an ongoing procedure for operational, in-flight, ADS-B systems. This is a form of trend monitoring, as data are collected from the last 10-flights, or legs flown, for each aircraft. Operators are notified of any unusual activity, and currently the FAA works with customers to resolve any discrepancies.

As a forward-looking maintenance item for operators, the aircraft Flight ID and Mode S Code, crucial to transponder recognition, will need to reflect the current assigned codes for each aircraft, including individual aircraft codes for fleet operators.

The seven most common ADS-B Out issues that may also become apparent while an aircraft is in-service are as follows:

  • Emitter Category
  • Air/Ground Determination Issues
  • Baro/Geo Altitude Spikes
  • Missing Baro Altitude
  • Duplicate or Wrong ICAO Codes
  • Invalid/Missing Mode 3/A Codes
  • Flight ID issues


Specifically, with ADS-B it would be wise for flight departments to stay engaged with their supplier news reports. Changes are on the horizon, because of:

  • ADS-B In
  • Space-based ADS-B (satellites are already deployed!)
  • ADS-B used for ground operations
  • ADS-B in trail separation
  • ADS-B to facilitate flight and maintenance tracking.


Operations guidance should be included in the Airplane Flight Manual Supplement (AFMS); Aircraft Operating Manual (AOM); Flight Operations Manual (FOM); Pilot Operating Handbook; and associated checklists. Pilot training and refresher training cover the following areas:

  • ADS-B operating procedures
  • Flight planning
  • MEL procedures
  • Human factors
  • ADS-B phraseology applicable to specific regions of operation
  • ADS-B system operation, including normal/abnormal procedures
  • Correct entry of ICAO aircraft ID as applicable to the flight
  • Operational procedures regarding the transmission of the generic emergency code (i.e., 7700) in cases when the flight crew selected a discrete emergency code (e.g., 7500, 7600)
  • Handling of data source errors (e.g., discrepancies between navigation data sources)
  • Incident reporting procedures.

For maintenance personnel involved in supporting in-service ADS-B, it is useful to refer to the FAA’s 2017 version of their Field Approval Job Aid. Embedded within are links to other guidance, some for ADS-B. These links include the latest FAA Policy memo as well as SAFO 17002, which contains flight safety related information for Transponder and ADS-B testing.


CPDLC –VDL Mode 2 & Satcom

It is important to remember that CPDLC has two major parts and then some sub-parts, shown below:

  • Data Link Services–Central Reporting Office (DLS–CRO), formally Link 2000+ and VHF data link (ACARS). The VHF service is VDL Mode 2.
  • The FANS Oceanic and Remote Areas component, which is Satcom-based (when out of VHF range) and relies on two sub elements.

    - Navigation with the appropriate Required Navigation Performance (RNP) accuracy.
    - Automatic Dependent Surveillance – Contract (ADS-C).

Both systems are providing a digital version of what used to be voice (over HF, VHF, Satcom), updating the aircraft position and intention, providing message capability and more. The newer digital piece enhances the aircraft’s intention, provides a more accurate position, allows for tracking and may provide the status of the aircraft. This is further enhanced for ATC by the ADS-C information displayed on the controller’s monitor.

Two key concerns for FANS are the reliability of the integration and the selection of a service provider. Service providers, such as Satcom Direct, offer an Entry-Into-Service module that not only advises clients on upfront selections, but assist in obtaining the maximum system potential when the aircraft enters into service.

As the satellite environment constantly changes, the service provider needs to ensure that your systems are optimized for signal reception and connectivity at all times.

The satellite provider must be able to link the aircraft automatically and appropriately to Ground-Earth Stations, VHF Remote Ground Stations and the Air Traffic Service Unit - no small task.

The service provider needs to know if the aircraft’s Iridium or Inmarsat equipment is altered or changed in any way. The provider also needs to be alerted to new ownership and tail number changes, while component software levels are always crucial to the service provider.

Because the CPDLC (VDL/FANS Satcom) communication is now digital and not voice, the existing aircraft cockpit voice recorder must be able to record the digital interaction, just as it had been capturing information previously for voice. This often requires an update to the aircraft’s recording system. Monitoring of the recording equipment (CVR/FDR/QAR) is a continued airworthiness maintenance requirement.

FANS requires cockpit aural and visual alerts, which should always be operational and not allowed to remain in a failure condition. Some aircraft have a flight deck printer to retain, but not verify, DataCom messages. The continued servicing of this device and of course, need for paper, are requirements.

Table B shows the different DataCom Sublink Designators, indicating various modes of data communication. For an operator the pertinent Sublinks may be found in their DataLink Flight Manual Supplement (FMS).



Table C highlights ADS-C surveillance performance specifications. Note the associated navigation performance requirements that the aircraft needs to meet.



Performance Based Navigation (PBN)

As with CPDLC, en route PBN is only required where air traffic routing specifies its use. PBN is all about separating aircraft while allowing as many vehicles as possible to fly together inside a virtual and narrow tunnel in the sky.

The more operators can ensure the integrity of their vehicles’ flight path, the easier it is to maintain the tighter separation of a PBN route. Approach PBN, such as Wide Area Augmentation System, using Lateral Path Vertical (a satellite derived version of Glideslope), is also an option based on how and where you want to operate.

In both cases, the integrity of the Global Positioning System (GPS) signal is paramount. Once the required hardware and software are installed in the aircraft, the operator must ensure the monitoring capability of the PBN system and the proper functioning of pilot annunciation. This is accomplished by a regular pre-flight annunciation test.

Most GPS sensors are either an integral part of the Flight Management Computer, a remote avionics module circuit card or remote stand-alone sensor.

In all cases, the currency of software is crucial. If a satellite providing position information is updated or changed in any way, the onboard GPS will need to be updated with this new information.

The Flight Management System(s) database is also very time sensitive and will be constantly changing. Obstacles, as a potential safety hazard to pilots, should always be current in any database.

As with FANS, operators need to be approved, and must maintain their approval, to use PBN. Onboard equipment must be conditionally monitored to ensure information presented to the pilot is correct.

As a tip to operators, ask your preferred maintenance provider to inspect all your aircraft antennas during a maintenance trip, including the aircraft’s bonding and electrostatic performance, condition of static wicks and all bonding straps.

Both antennas and an aircraft resistance to electrical interference are key factors in the degradation of navigation performance, and for CPDLC continued communication capability. The aircraft’s satcom antenna, usually tucked away under a hard to access radome, is often the weak link in the satcom chain.




Regarding post-delivery operations and maintenance requirements, only ADS-B Out, DataLink and PBN have been addressed in this article. However, there are many other areas, both outside and within the avionics realm, where service requirements apply. This includes avionics suite upgrades, such as flat panel displays. These often include integrated solutions to allow operation of the three technologies highlighted.

Advanced avionics are smaller, more reliable and longer lasting than their predecessors, but they are still prone to many of the same vulnerabilities as their slightly older cousins.

Continued airworthiness inspections, performance monitoring, regular inspection, and a keen operator-eye can avoid many potential failures.

As with anything relatively new, NextGen equipment will have its teething problems, resulting in revisions, software upgrades, factory alerts and communiques, flowing continuously to an already busy flight department. Also, many upgrades will reflect product improvements, some of direct and others of indirect benefit.

Furthermore, manufacturers like to bundle changes and upgrades where you may receive a fix for the primary issue, along with additional unanticipated product improvements, not all of which may work!

Keep yourself informed. Stay up with the OEM communications, reach out constantly to your service provider and dialogue regularly with your preferred maintenance provider.


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Read More About: ADS-B | NextGen | Avionics Upgrades | Avionics Mandates


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