How does CPDLC work?

From the Canadian Aviation Safety Letter (Canadian Aviation Safety Letter 2014)

CPDLC employs a series of standardized text messages for most routine communications. These include over 200 “uplink messages” (from ATC to the cockpit) and more than 100 “downlink messages” (from the flight crew to controllers). Pilots and controllers also have the option of sending
free-text messages. For the most part, CPDLC works with the click of a mouse. Controllers have drop-down menus on their screens with the standard messages. Menus are divided into different categories to make the appropriate message easier to find. Each ACC can modify their drop-down menus and choose which messages are contained in each message group.

For instance, the Maintain (Alt), Climb to and Maintain (Alt), Descend to and Maintain (Alt), and At (POS) Climb to and Maintain (Alt) would likely go under the Altitude drop-down menu. Other common drop-down menus are Radio, Route, Speed, and Free text.

There are also quick-response buttons for Unable, Roger, Negative, Standby and Deferred.
For downlink messages that require a response, the controller just has to click on that message and the appropriate drop-down menu, and the response is highlighted in green (as opposed to white), making the messages easier to find.

Usage and Equipage
The number of monthly CPDLC contacts in domestic airspace has almost tripled in the 14-month period from November 2012 to December 2013: from just under 18 000 to nearly 53 000. Those numbers are expected to continue to rise as the CPDLC equipage rate increases. The percentage of CPDLC-equipped flights in Canada’s domestic high level airspace varies according to geographical location, from 27 percent in the central Canadian FIRs, to 79 percent near the country’s east coast.

In addition to the many safety enhancements, CPDLC adds an important efficiency benefit. As the need for voice communications decreases, the problem of radio frequency congestion becomes less of an issue. CPDLC also has a multiplier effect on alleviating frequency congestion when you calculate the number of flights using CPDLC. If one data link message can eliminate even 30 seconds of airtime, that can translate to 15 or more hours per day of voice communications
taken off the airwaves.

And finally, for those of you who are wondering about the safety of pilots “texting while flying”, it is always the pilot monitoring (PM) who sends and receives the messages.

ICAO Definition of Safety

The state in which the possibility of harm to persons or of property damge is reduced to, and maintained at or below, an acceptable level through a continuing process of:

• hazard identification and 
• safety risk management.

What are LNAV/VNAV, LPV and LP approaches?

What are LNAV/VNAV, LPV and LP approaches?

LNAV is a non-precision approach. It uses GPS and/or WAAS for lateral navigation, but there’s no vertical guidance. Typically it takes you down to 400 ft.

LNAV/VNAV again is a non-precision approach. It provides lateral guidance from the GPS or WAAS receiver and vertical guidance from a barometric altimeter or the WAAS. Without WAAS, you must have a VNAV altimeter. Decision altitude is typically around 350 ft.

LPV? LPV is a non-precision approach. It stands for Localizer Performance with Vertical Guidance and uses the WAAS GPS only. It is the most desired approach that you can be offered. It typically takes you down to 200-250 ft decision height.

LP. It is a future approach that will use the high precision of LPV for lateral guidance, and a barometric altimeter for vertical. Runways where obstacles or infrastructure limits are, vertically guided approaches cannot be published.

LPV Operational Benefits

Currently over 1,975 runways across the United States and more to come. Many approaches have minimums to 200 ft height above touchdown and ½ mile visibility with greater accuracy and consistency.

Once you’ve flown a GPS approach with LPV you’ll be amazed at how the aircraft performs and how stable it is. Electronic glide paths eliminate intermediate step down approaches, or dive and drive approaches. This type of approach provides more comfort for your passengers, less ear popping, and just more comfort and safety overall. The glide path become independent of ground or barometric equipment. Everything is calculated internal to the aircraft.

WAAS and LPV eliminate:

• cold temperature effects,
• incorrect altimeter settings,
• and lack of local altimeter source

If I install WAAS-capable equipment, am I ready to fly LPV approaches?

Questions that we’re often asked are if I install WAAS-capable equipment, am I ready to fly LPV approaches? The answer is no. WAAS receivers cannot be installed under a straight field approval. There’s a lot more to the field approval process. Once it’s installed in the aircraft, the installing agency needs to make sure that all equipment in the airplane is properly functioning. That means the autopilot, the scaling, everything that becomes a part of this equation needs to be checked, so it’s a lot more stringent than a straight field approval. Most WAAS receivers are installed under an STC.

WAAS-capable avionics do not automatically mean that you can fly to an LPV minimum. To accomplish the LPV minimums, you need dual WAAS receivers. They must be certified under TSO 145/146.

Can I upgrade my existing navigational system to a WAAS receiver?

We’re often asked the question can I upgrade my existing navigational system to a WAAS receiver and the answer is no. Current systems are certified under TSO C129, a completely different criteria. TSO C145 and 146 means that the units are certified as a standalone receiver. No other signal needs to go into that box in order to give it the accuracy that it will present on your aircraft instruments.

It also requires an antenna change. The antenna’s are different from the TSO 129 box to what’s certified on a 145/146. Installation is currently being done by STC and requires:

• dual GPS receivers,
• other equipment mods, such as the scaling and autopilot,
• annunciation, whether it’s external or on an EFIS system,
• and a flight test procedure are all required.

Airports with WAAS approaches

This map shows some of the airports (see presentation slides) that are across the United States.

The WAAS/LPV Approach Plate

The LPV approach on an approach plate is called out (see presentation slides). You’ll see that it says “WAAS Approach” on it. So just like any other approach, GPS approach, VOR approach, ILS… it calls out particular criteria. So you take the approach appropriate to that airport, channel it up in your receiver and fly the approach.

Contact Gary Harpster or a member of Duncan Aviation's Avionics Installation Sales team for more information.

Understanding WAAS/LPV Video Series

	WAAS Considerations: Introduction July 1, 2010 - Gary introduces the video series with a brief overview of key topics to consider if your aircraft isn't equipped with WAAS, including legacy interface problems, field approvals, annunciators, Next-Gen criteria, and how to determine if your aircraft is a candidate for an upgrade. » Watch "WAAS Considerations"
	Part 1: What is WAAS? July 8, 2010 - Find out how WAAS is a part of the FAA's Next-Gen initiative, the system network and coverage, and why installing WAAS-capable equipment doesn't mean you're ready to fly LPV approaches.. » Watch "What is WAAS?"
	Part 2: How WAAS Works July 15, 2010 - Learn how WAAS interacts with GPS signals, what augmentation messages do, and how they're processed. » Watch "How WAAS Works"
	Part 3: WAAS Benefits July 22, 2010 - Learn how WAAS changes navigational system accuracy, approaches, flexibility, precision, fuel savings and government and airport costs. Operational benefits and Next-Gen considerations are also discussed. » Watch "WAAS Benefits"
	Part 4: What is LPV? July 29, 2010 - Learn what LPV approaches are, how they benefit operators, and common equipment questions. » Watch "What is LPV?"