The Duncan Download Blog: Business Aviation Advice & Observations

Contributed by Scott McKenzie, Avionics Tech Rep and Rod Walther, NAV Team Leader

The Universal Avionics Flight Management System (FMS) has two batteries. One is located on the CPU board that supports Flight Plans and Navigation databases. The other is located on the GPS board and supports the GPS Almanac information. It isn’t unusual for these batteries to begin to lose power or fail with minimal warning resulting in a CDU Data Bus Failure message being displayed.

As previously stated, this failure can happen quite suddenly. The crew may get a “Low Battery” message at start-up on the first leg of a flight, but then have a “CDU Data Bus Failure” message on the start-up of the return flight. When this happens, the FMS becomes unusable until the battery is replaced. Depending on your current flight schedule and where you are located, the downtime could be hours or even days. Especially if it becomes necessary to order a rental unit from the OEM while your unit is sent in for battery replacement.

When there is sudden battery failure, you also lose valuable Pilot Data. After this failure is noted, the only way to replace the data is for pilots to manually re-enter the data after the batteries are replaced.

Below are two simple solutions to avoiding the sudden loss of battery power and potentially all of your stored pilot data.

Two-Year Maintenance Program

It is perfectly fine to retain the original Universal Avionics FMS batteries. However, we recommend that you tie the battery replacement to an airframe maintenance event already on a two-year general maintenance program. The original Universal Avionics FMS batteries are still currently available, however could at some point in the future be discontinued. At that time, operators will need to switch to the new Extended-Life battery modification.

New Extended-Life Battery Modification

Universal Avionics has issued a battery modification that considerably extends the useful life of the batteries. More importantly, this new battery appears to have a steadier decline in voltage, prolonging the low battery indication which in turn, could allow the pilot up to three months to schedule maintenance. However, to ensure continuous battery operation, we recommend you put the FMS with the new battery modification, on a four-year general maintenance program for battery replacement, reducing the likelihood of the sudden loss of battery power and pilot data. If you do this on a dual FMS System, you will save hundreds of dollars of maintenance over the life of the battery.

CPU Board Damage

Waiting too long to replace your FMS batteries also increases the probability of CPU board damage due to battery corrosion. Although a battery may have exceeded its life expectancy, the longer battery replacement is put off, the greater the chances the battery will begin to leak and cause corrosion and damage to the CPU board. When this happens, the CPU board must be replaced at a potential cost of more than $3000.

Both the two-year maintenance cycle of the current FMS battery and the new Extended-Life battery modification can be performed at Duncan Aviation. And by using Duncan Aviation's AOG service, turn time can be as little as one day, if scheduled. To discuss your Universal Avionics FMS Battery maintenance options, call Duncan Aviation’s Avionics Tech Reps.

To schedule your next Universal Avionics FMS battery maintenance event, contact Duncan Aviation’s Avionics Customer Account Reps.

Scott McKenzie is an avionics tech rep located in Duncan Aviation's Lincoln, Nebraska, facility. He specializes in troubleshooting the latest in avionics systems installed on aircraft today. His aviation career began in 1995.

Rod Walther is an avionics team leader located at Duncan Aviation's Lincoln, Nebraska, facility. He leads a team that specializes in NAV systems. His aviation career began in 1993.

Contributed by Gary Harpster, Avionics Installation Sales Rep

For major avionics installations, like upgrading to Honeywell's NZ-2000 FMS, it's important to consider  STC availability and service provider experience with the aircraft and the equipment. Photo source: Honeywell Aerospace.

When you are tasked with an avionics installation and you need to better understand what conveniences you can offer your passengers now, you need to talk to someone well versed in the industry. With a firm understanding of your goals (to minimize capital expenditures, maintain a high level of safety and balance the value of your aircraft against what you’re about to spend), it’s worth taking the time to talk to someone that understands your needs. There are a lot of factors that often times get overlooked when assessing how an upgrade can affect your hull value. It pays to do a through research on your proposed installation.

It's important to evaluate a service provider's experience with the type of aircraft in question, and the equipment that's being considered for this installation. What value does a supplemental type certificate (STC) bring to the equation?

If the installing agency does not have the right STC, they may or may not be willing to take on creating a new one for your aircraft. It's prudent for an installing agency to do its homework as well, before taking on new certifications. Very few installation facilities will generate a new STC unless it meets the criteria with a sound business plan.

It's important to keep in mind that to install new technology, for example a NZ-2000 FMS, a company needs to be familiar with that type of aircraft and the equipment they're considering for installation. A properly researched avionics installation can add value (especially if the aircraft might be sold overseas in the future), and can also minimize support headaches down the road.

For further discussion on items that need to be considered for avionics installation planning purposes, evaluating your service providers, or understanding the decisions behind creating a new STC, read Avionics News' "Taking on the Burden of Generating a New STC."

For more information on Duncan Aviation's NZ-2000 FMS STC, please refer to the press release.

Written by Gary Harpster, Duncan Aviation Avionics Sales Rep.

How does WAAS work?

WAAS uses the existing GPS satellites that are currently in orbit for the geometric aspect of the equation, a basic triangulation from multiple satellites to your position in three dimensional space. The problem is, in using raw GPS signals, is there are various forms of interference (atmospheric disturbances, satellite position error ..etc.) that play a part in the received signal. WAAS Receivers use a very precise timing signal to filter out this error and give you a higher degree of position accuracy, so much so, that the defined vertical accuracy is more precise that your altimeter in unusual temperature (both hot or cold) conditions.

What do all those new acronyms (HAL, VAL, HPL, VPL, LPV) really mean?

These terms really are not something you as a pilot need to be concerned with, these are acronyms the manufacture wants to make sure their box meets during certification. When a manufacturer designs a WAAS receiver they test their components to meet these specifications. As a pilot, you're mostly concerned with the proper WAAS approach plate and the LPV decision altitude (DA)

HAL - Horizontal Alert Limits
VPL - Vertical Alert Limits
HPL - Horizontal Protection Limit
VPL - Vertical Protection Limit
LPV - Localizer Performance with Vertical guidance

What new rules apply to WAAS users?

No new rules really, the FAA recently published there are now over 2,200 WAAS LPV approaches and that Europe is in the process of adding WAAS approaches over there.  

How are WAAS-based approaches constructed

Before any airport is granted a WAAS approach, it must meet the design criteria called out in the FAA Advisory Circular 150/5300-13. Some of the requirements are the amount of space they consider the PFOZ Precision Obstacle Free Zone, Runway Edge Lighting, Parallel Taxiway and a Vertically Guided Airport Airspace Analysis Survey. As you can imagine the airport has to be surveyed to make sure there are no obstacles in your flight path and the decent rate can not be anymore that a typical ILS approach. You will not find a WAAS approach into a valley, where the airport is surrounded by obstacles that could potentially block your GPS signal as you near your minimums.  

What are the similarities and differences between WAAS-based approaches and ILS, VOR, or non-augmented GPS?

A WAAS with LPV approach is very similar to an ILS approach, as the aircraft receives both lateral and vertical guidance just like an ILS with the same or better precision. The main difference is ILS signals are generated from transmitters located on the ground and aimed at the designed flight path, whereas a WAAS with LPV approach is getting the signals from satellites in orbit. VOR approaches provide lateral guidance from signals generated by transmitter on the ground. VOR approaches are commonly referred to by pilots as "dive and drive" approaches, this stems from the fact that during this approach you follow a lateral signal for a certain length of time, then when you cross a predefined waypoint calculated by another external reference point you are allowed to descend again for a period of time. This method continues until you are at a minimum decent allowance where you are then required to make visual identification of the airport for landing.

Written by Dan Magnus, Avionics Tech Rep

Honeywell's GNS-XLS is a Flight Management System that provides control for an aircraft's navigation sensors, computer-based flight planning, fuel management and frequency management, among many other necessary capabilities. When this unit begins to malfunction, it is critical to get it back on line quickly.

I recently fielded a phone call from a DOM asking for troubleshooting advice when the screen on the GNS-XLS went white. Here are the steps I used to determine the cause and provide the solution.

The best way to determine if a unit is faulty is to install another and see if the squawk disappears. I sent out a loaner to arrive the next day. After the unit was installed, the screen remained white, telling me it wasn't the unit causing the squawk.

Next, I had him unhook the antenna. This immediately brought the screen back to its normal state. We replaced the antenna and the system is back on-line.

Squawk solved.  

If you need help troubleshooting or repairing your avionics, contact Avionics Tech Support.

WAAS is a complicated subject. We've written a book, a whitepaper, magazine articles and press releases on the subject, and the questions are still rolling in.

Of the hundreds of WAAS inquires I've seen, this one stood out. Someone asked, "Why?"

The question was, "You mentioned that some installations may require two WAAS receivers to be certified to fly LPV approaches. Why? I don't see that as a requirement in any of the FAA advisory material. What is it about WAAS and LPV that requires two Flight Management Systems and two GPS receivers?"

The answer, of course, is complicated. The following explanation was provided by Avionics expert and AEA Vice Chairman, Gary Harpster: 

Prior to WAAS receivers, the Flight Management Systems were certified under TSO C129. TSO C129 allowed an operator to use a single FMS for a GPS approach as long as it was an overlay approach. This also required alternate navigational imputs, such as VOR, DME, ADF, etc.

The new WAAS receivers and Flight Management Systems are certified under TSO C145 and C146, allowing a very precise GPS approach without any additional inputs. These systems also have a unique feature: they are constantly doing an analysis of the GPS integrity. Part of this analysis is a cross check of the secondary system, which adds a high degree of reliability. 

One of the challenges of Air Traffic Control is to put multiple aircraft into a confined corridor and keep them properly separated. The new WAAS receivers are designed to accomplish this- the secondary system is a necessary backup in case of the failure of the primary system.  

Some manufacturers have designed WAAS monitors to act as a secondary FMS without a Cockpit Display Unit. This configuration provides the required redundancy without the expense of an entire second system. 

Do you have a question about WAAS? Ask it here.