Flying (and Filing) Direct v. Airways

Many pilots who use GPS as their primary navigation tool, whether operating VFR or IFR, now plan their flights assuming a direct course from departure to destination. They do so for several common reasons:

  • To save time and fuel by trimming miles from long flights
  • To simplify the creation of flight plans (lists of waypoints) in GPS navigators, especially units that don’t support entry of airway designations
  • To simplify navigation in flight by reducing the number of turn points
  • To avoid major changes from a filed IFR flight plan when they receive their actual clearance from ATC. Pilots who file direct often say, “Filing a specific route is futile. ATC assigns a different route when you get your clearance, so why bother planning a detailed route?”
Direct or…

Direct

…Airways?

Airway

While it’s true that direct routes are by definition shorter than those that include zigs and zags, as we’ll see shortly, the difference, especially on trips of the length typically flown by piston-powered aircraft operating below the flight levels, is usually much smaller than most pilots assume.

In the event, terrain, airspace, and preferred routes (under IFR) can make direct routes ill-advised, impractical, or impossible, especially in the mountainous regions of the West and within the congested airspace of areas like the Northeast, the Bay Area, and the LA Basin.

A route with several waypoints or which travels at least partially along airways has several advantages, including:

  • Making it easier to track your progress on charts
  • Helping you remain aware of alternatives should you need to stop for fuel, accommodate passengers, avoid weather, or deal with a malfunction

It’s also far easier to amend a flight plan than it is to build a complete flight plan from scratch, and having a detailed route “in the box” before you leave the ramp also helps reduce heads-down time while taxiing and during high-workload phases of flight like departure and arrival, especially when you’re trying to navigate unfamiliar airspace.

(Keep in mind that even if you prefer to file and fly direct under IFR, your route usually must include several waypoints. The procedures and requirements for filing direct and RNAV routes are described in AIM 5-1-8, especially sections C and D.)

But for the purposes of this exercise, assume that direct routes are feasible.

Direct v. airway: distance, time, and fuel

Today, many tools make laying out routes an easy, no-math proposition, so planning a flight with course changes is hardly a chore. Flight-planning/navigation apps like ForeFlight (version 4.6 is described here) now include flight-planning/route-building features that make it easy to compare direct and airway routings and to include preferred and TEC routes, and SIDs and STARs. The flight log features in iPad apps like ForeFlight also help you efficiently build flight plans in GPS navigators (such as the popular Garmin GNS 400/500 series boxes) that require manual entry of at least the fixes that define entry, turn, and exit points of airways or the initial fixes of SIDs and STARs.

Let’s use ForeFlight to explore some of the issues outlined above in more detail.

A typical flight: KBFI-S78

Here’s route between Seattle and Boise that I fly several times a year to support Pilots N Paws. It’s from Boeing Field (KBFI) to Emmett, ID (S78).

As you can see below, the direct route is 327 nm. In this example, at 160 KTAS, estimated time in en route is 02:10, and the trip requires about 32.5 gallons of fuel, not including time and fuel to climb.

020

And climb we must to fly this route, because under IFR we must cruise at or above 10,000 ft. to clear terrain (probably at least 11,000 ft. when flying southeast). The direct route also clips or comes close to a couple of restricted areas.

My typical, mostly airways, route, at 341 nm, is only 14 nm longer, about 5 minutes at 160 knots. This route follows the airway between SEA and ELN (MEA 8000) and clears the big restricted area near YKM. In fact, the total mileage is a bit less, because after takeoff, ATC typically vectors me to join V2. The KBFI-SEA leg is in the flight plan (and the box) just to make it easier to join the airway.

This route also passes near many airports and VORs, giving me options and backup navigation sources.

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Another common route, via V4 out of Seattle, is, at 330 nm, practically the same as the others, although it requires a cruise altitude of at least 10,000 ft. under IFR.

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Another example: Seattle to northern California

Here’s another trip that I make a few times a year: KBFI to the Bay Area with a fuel stop (for relatively inexpensive fuel) at KOVE (Oroville, CA). The direct route is 483 nm.

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That route spends a lot of time over the mountains and clips some MOAs. Because of traffic between Seattle and Portland, ATC isn’t likely to offer it under IFR.

My usual filed route avoids those issues and comes in at 511 nm, a difference of 28 nm (about 10 minutes at 160 knots). It clears the busy corridor between Seattle and Portland, and ATC usually accommodates it. Again, this route remains close to many airports, and the majority of the flight is over terrain that allows reasonable MEAs. Because I have stored this route (including the fixes that define V23) in my GNS530W, it’s easy to include changes or shave miles if ATC approves shortcuts (e.g., between OLM and EUG) or deviations for weather. But as we’ve seen, on trips of this length, trimming a leg or two doesn’t typically offer significant savings in time or fuel.

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One more example: KBFI-KOSH

But what about a much longer flight, say the annual trip to EAA AirVenture in Oshkosh, WI? Leaving aside the need to stop for fuel and spend the night, the direct route from KBFI to KOSH is 1,417 nm, about 08:38 of flying time at 160 KTAS.

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A typical, mostly airway route (subject, of course, to changes for weather) is about 100 nm (some 7 percent) longer, but it adds less than 1 hour of flying time (winds aloft and other factors being equal). Total block time, of course, would be much greater to accommodate fuel and overnight stops, but those considerations also apply to the direct route.

029

More importantly, the mostly airways route guides you through mountain passes and keeps you relatively close to airports and highways (which means you’re also nearer towns and services).

Flight plans and flight planning

As I’ve suggested above, filing and filing a route that includes airway segments and multiple waypoints can also help you track your progress more easily than when following long direct segments. The act of laying out a route and building a flight log encourages you to become familiar with navigation fixes and to think about decision points along the way. What’s the weather like at nearby alternates? Which airports offer the best services? What types of instrument approaches are available along the way? What are good points en route to evaluate your actual flight time and fuel burn?

It’s certainly possible to ask and answer such questions when laying out a basic direct route, but there’s often real value—and little additional cost in time or money–in following another path.

Microsoft Ends Work on Microsoft Flight

Microsoft has stopped all work on Microsoft Flight, the successor to Microsoft Flight Simulator. No official announcement at the product’s website yet, but various sources, including Kotaku, have posted a statement from the company:

Microsoft Studios is always evaluating its portfolio of products to determine what is best for gamers, families and the company, and this decision was the result of the natural ebb and flow of our portfolio management. Many factors were considered in the difficult decision to stop development on “Microsoft Flight” and “Project Columbia,” but we feel it will help us better align with our long-term goals and development plans. For “Microsoft Flight,” we will continue to support the community that has embraced the title and the game will still be available to download for free at http://www.microsoft.com/games/flight/.

Apparently, most of the team that was working on Microsoft Flight has been laid off, so it’s not clear if Microsoft has any plans for its line of flight (and flight simulation) products. For now, the core of Microsoft Flight Simulator X lives on in Prepar3D, developed by Lockheed-Martin.

Update July 28, 2012:
Microsoft has posted the following statement on the Microsoft Flight website:

We know there are a number of questions out there in the community about the discontinuation of development for Microsoft Flight. We wanted to make to be sure to clarify a few things. While we will not be continuing active development, we are committed to keeping Flight available for our community to enjoy. All the content you have paid for is still valid, and the content that is available for sale will continue to be available on http://www.microsoft.com/games/flight/. If any further information becomes available for us to share, we will do so.

Ceiling and Visibility Analysis Product from ADDS

The NWS Aviation Weather Center is releasing a new tool on 24 July, when the Ceiling and Visibility Analysis Product goes live on ADDS.

CVA presents simplified area maps of ceiling, visibility and flight category outlined as:

Ceiling estimates are displayed as:

(i) Possible Terrain Obscuration (pale orange) for ceilings less than 200 ft agl, (ii) less than or equal to 1000 ft agl (pale yellow) from 200 to 999 ft, agl, and (iii) 1000 ft agl or greater (pale blue).

Visibility estimates are displayed as:

(i) less than 3 miles (pale yellow), and (ii) 3 miles or greater (pale blue).

CVA is intended to accompany other aviation weather products such as METARS, AIRMETS, TAFs and Area Forecasts to help the general aviation pilot (particularly the VFR-only pilot) avoid IFR conditions. To remain current in rapidly changing conditions, CVA is updated every 5 minutes using the latest observations. The use of CVA should be followed by further examination of METARs, TAFs, AIRMETs, Area Forecasts and other weather information.

CVA primarily is intended to help the general aviation pilot; however, CVA’s quick-glance overview of ceiling and visibility conditions can be useful to others involved in flight planning or weather briefing.

CVA initially will be available only via the ADDS website at:

http://www.aviationweather.gov/adds/cv

RNP Procedures and Typical Part 91 Pilots

Some pilots remain confused about GPS-based RNAV approaches, especially procedures with “RNP” (Required Navigation Performance) in the title and “AUTHORIZATION REQUIRED” notes on the chart. For many reasons, those approaches are currently off-limits to a typical IFR pilot flying an airplane equipped with a WAAS-capable, IFR-approved GPS.

The situation is becoming more complicated with the forthcoming publication of RNAV approaches such as the RNAV (GPS) Y RWY 24 at KCRQ (see below). Although that procedure is not labeled RNP, one transition (from VISTA) includes an RF (radius-to-fix) leg, a curved path that resembles a DME arc, but which requires specific equipment and training to fly–see below. Most of us flying with GPS units like the Garmin GNS400/500W and new GTN-series navigators may not see this procedure in the database, but it may appear among the updated charts you receive from Jeppesen or FAA AeroNav Products.

image

For a basic explanation of why you need specific authorization to fly RNP procedures, see the Instrument Procedures Handbook:

In the United States, all RNP procedures are in the category of Special Aircraft and Aircrew Authorization Required (SAAAR). Operators who seek to take advantage of RNP approach procedures must meet the special RNP requirements outlined in FAA AC 90-101, Approval Guidance for RNP Procedures with SAAAR. Currently, most new transport category airplanes receive an airworthiness approval for RNP operations. However, differences can exist in the level of precision that each system is qualified to meet. Each individual operator is responsible for obtaining the necessary approval and authorization to use these instrument flight procedures with navigation databases. (5-22)

Note that authorities have dropped the use of “Special Aircraft and Aircrew Authorization Required (SAAAR)” in favor of AUTHORIZATION REQUIRED notes on charts, in ACs, and the like. And the current advisory circular is AC 90-101A.

That AC explains in part:

a. AR. RNP AR approaches include unique capabilities that require special aircraft and aircrew authorization similar to Category (CAT) II/III instrument landing system (ILS) operations. All RNP AR approaches have reduced lateral obstacle evaluation areas and vertical obstacle clearance surfaces predicated on the aircraft and aircrew performance requirements of this AC. In addition, selected procedures may require the capability to fly an RF leg and/or a missed approach, which requires RNP less than 1.0. Appendix 2 of this AC identifies specific aircraft requirements that apply to these capabilities.

b. Navigation Performance Monitoring. A critical component of RNP is the ability of the aircraft navigation system to monitor its achieved performance and to identify for the pilot whether the performance meets set standards during an operation.

A more detailed description of an RNP procedure (from AC 90-101A), helps you understand why a typical WAAS-capable piston flown by a single pilot doesn’t qualify:

2. Characteristics of RNP AR Approaches.
a. RNP Value. Each published line of minima has an associated RNP value. For example, Figure 2 shows both RNP 0.3 and RNP 0.15 lines of minima. Each operator’s RNP AR authorization documents a minimum RNP value, and this value may vary depending on aircraft configuration or operational procedures (e.g., use of flight director (FD) with or without autopilot). RNP AR approaches will have an RNP value of RNP 0.3 or less.

b. Procedures with Radius to a Fix (RF) Legs. Some RNP AR approaches include RF legs. The instrument approach charts will indicate requirements for RF legs in the notes section or at the applicable initial approach fix (IAF). Figures 1 and 2 provide examples of procedures with an RF leg segment (e.g., between SKYCO and CATMI).

c. Missed Approaches Requiring Less Than RNP 1.0. At certain locations, the airspace or obstacle environment may require RNP capability of less than 1.0 during a missed approach. Operation on these approaches typically requires redundant equipment. This requirement ensures that no single point of failure can cause loss of RNP capability. Figure 2 provides an example of a missed approach requiring RNP less than 1.0. The notes section of the chart indicates this requirement.

d. Non-Standard Speeds or Climb Gradients. Normally, RNP AR approach procedure design relies on standard approach speeds and climb gradients including on the Missed Approach Segment (MAS). The approach procedure will indicate any exceptions to these standards, and the operator must ensure it can comply with any published restrictions before conducting these approach operations. Figure 2 provides an example of a non-standard climb gradient and speed restrictions.

AC 90-101A also describes the training necessary for crews authorized to fly RNP procedures and the equipment requirements (beyond a WAAS-capable GPS receiver) necessary to fly those approaches.

With all that in mind, it’s important to understand that if your airplane is equipped with, say, a GNS530W or one of the new GTN boxes (with an appropriate AFM supplement), you can fly RNAV SIDs and STARs based on RNP-1 criteria. And you can, of course, fly en route segments that require RNP-2 accuracy. No special authorization is required at these RNP levels. For an explanation of RNP levels, see “Required Navigation Performance” in the Instrument Procedures Handbook (1-13).

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For more information about operations approved for different IFR-approved GPS units, see AC 90-100A, “U.S. Terminal and En Route Area Navigation (RNAV) Operations,” and the associated Excel compliance worksheet. A good summary of AC 90-100A is available here.

New Videos at BruceAirFlying’s YouTube Channel

I have posted several new videos on my YouTube channel. Most of the videos, like the sample below, are short clips that demonstrate basic aerobatic maneuvers. I captured the maneuvers during aerobatic rides and training flights in the Extra 300L.

Changes to ICAO Flight Plans Coming in November

ICAO is making several changes to its flight plan form, effective November 15, 2012. You can read about the changes on the FAA website, here.

Although most U.S. pilots still use the FAA flight plan form, ICAO IFR flight plans are required if you want automatic assignment of RNAV SIDs and STARs.

Changes coming this fall include:

  • Additional alphanumeric qualifiers in Item 10 (Equipment and Capabilities) that reflect enhancements to operational capabilities in ground-based and satellite-based navigation and surveillance equipment. The new qualifiers cover equipment for Automatic Dependent Surveillance-Broadcast (ADS-B), Automatic Dependent Surveillance-Contract (ADS-C), and Controller Pilot Data Link Future Air Navigation System (CPDLC-FANS), as well as, several other qualifier additions and deletions.
  • Significant changes to Item 18 (Other Information), including formatting, indicator definitions, specific grammar for special handling, and new Performance Based Navigation (PBN/) qualifiers for Area Navigation (RNAV) and Required Navigation Performance (RNP).
  • Specific format for aerodromes not listed in the Aeronautical Information Publication (AIP).
  • Delay (DLA), Change (CHG), Cancel (CNL) and Arrival (ARR) message formats.
  • Ability to file a flight plan up to 120 hours prior to departure.

A Perfect Day to Fly

Here’s a still captured from video during a stall/spin/URT lesson I gave this morning in the Extra 300L.

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