Use of RNAV/GPS inside the Final Approach Fix

There is a lot of debate among professional pilots on whether or not they are allowed to use their FMS or GPS receiver to fly a VOR or NDB approach.  All instrument pilots should know they are required to use the NAVAIDs associated with an instrument inside the FAF.  For example, if the crew is flying a VOR 14L approach, they are required to follow the VOR inside the final approach fix through the missed approach point.  There are approaches with a “or GPS” in the title allowing the flight crew to use a GPS receiver as the only NAVAID inside the final approach fix.  Most of these approaches are being phased out for separate RNAV (GPS) approaches and conventional NAVAID approaches.

VOR or GPS-A title

The question is whether or not we can use the RNAV system inside the final approach fix.  There are three good reason to allow an RNAV system to fly the airplane on these types of approaches.

  • Stabilized constant descent angle approaches (SCDA) are something the FAA wants pilots to perform and that is creating a glidepath that will result in the aircraft descending and crossing the MEA for the next segment without leveling off.  The FAA really doesn’t want dive and drive approaches anymore.  An RNAV system with baro-altimeter inputs can create a baro-VNAV path that is similar to a glideslope on an ILS.
  • The airplane will not chase the needle as it enters and exits the cone of confusion.
  • The autopilot will follow an RNAV system signal where not many are able to be coupled to a NDB signal. 

A lot of flight crews want the benefits of the RNAV system but may not know the specifics of what is allowed or not allowed.

AC 90-108 allows the flight crew certain freedoms when using an RNAV / GPS system on conventional routes and procedures.  Specifically this AC allows a flight crew to use “a suitable RNAV system as a substitute means of navigation” or as “an alternate means of navigation” for all VOR, DME, TACAN, NDB routes or compass locators.  It does not matter if the underlying NAVAID is operational or not.  A suitable RNAV system can be used as a replacement for such equipment.  There are some specific allowances found in section 7 of the AC.  A suitable RNAV system can be used in the following four ways

  1. Determine aircraft position relative to or distance from a VOR, TACAN, NDB, compass locator, DME fix or named fix defined by a VOR radial, TACAN course, NDB bearing or compass locating bearing intersection a VOR or localizer course
  2. Navigate to or from a VOR, TACAN, NDB or compass locator.
  3. Hold over a VOR, TACAN, NDB, compass locator, or DME fix.
  4. Fly an arc based upon DME.

With these allowances, we can fly using a RNAV system as our only means of navigation all non-RNAV SIDs, STARs, airways dependent on VOR (victor and jet routes) or NDB (colored airways).

This AC prohibits using an RNAV receiver for three specific instances on non-RNAV procedures.

  1. NOTAMed procedures.  If the approach is NOTAMed out of service or “NA”.  An RNAV system cannot fly that approach.
  2. Substitution on a final approach segment.  This term is defined in the FAA’s Pilot/Controller Glossary as the segment between the final, approach fix or point and the runway, airport or missed approach point.
  3. Lateral navigation on LOC-Based courses.  The key is lateral navigation and not vertical navigation.  This includes all approaches based on an ILS, LOC, LOC/BC, SDF and LDA.

Number two above is the issue of the debate among professional pilots.  This AC does not allow a flight crew to use the RNAV system as “a substitute or alternate means of navigation” inside the FAF through the missed approach point.  However, some crews will still use an RNAV system inside the airplane with some extra responsibilities whether it was legal or not.

AIM 1-2-3(c) note #5 was added in 2016.  This note stipulates:

Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.

This settles that debate.  Flight crews are allowed to use an RNAV system inside the final approach fix with some caveats.  The underlying NAVAID must be operational and the NAVAID monitored.  This means that if a flight crew is shooting a VOR or NDB approach without “or GPS” in the title, they must be monitoring that NAVAID inside the FAF and follow it wherever it leads.  Most flight crews that I work with will bring up a RMI bearing pointer for the VOR or ADF and display that on the same screen as the RNAV HSI to make monitoring easier.

The flight crew will couple the autopilot to the FMS and set it up to follow a baro-VNAV path.  This will allow the aircraft to fly a specific lateral and vertical profile.  The flight crew must bring up the underlying NAVAID, on their bearing pointer or secondary navigational display and watch both the RNAV system and underlying NAVAID.  This type of approach has been coined an “FMS overlay approach”.  If the RNAV system starts to disagree with the underlying VOR or NDB signal the pilot is required to click off the autopilot and follow the underlying NAVAID (VOR or NDB).  I don’t know if I would be prepared under such a circumstance… so I choose to go missed.  I will follow the underlying NAVAID till the missed approach point and then use the RNAV system (it is now allowable to be used) to finish flying the missed approach.  I would then select a different approach to brief and fly.  If at anytime the underlying NAVAID becomes inoperative the approach must be abandoned and a new approach requested.

In the end, here are the steps that are necessary to fly an FMS overlay approach.

  1. Brief the approach and get the current weather at the airport
  2. Load the VOR or NDB approach into the RNAV system
  3. Setup the navigational radios to monitor the VOR or NDB signal.
  4. Display the underlying NAVAID (VOR or NDB) on your screen
  5. Follow the RNAV system to the final approach fix.
  6. Allow the autopilot to fly the approach using RNAV system guidance inside the FAF but be prepared to click off the autopilot if the RNAV system disagrees with the underlying NAVAID.
  7. If a disagreement occurs or the underlying NAVAID becomes inoperative, execute a missed approach.

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Runway Declared Distances

This was originally taken from an aviation forum I peruse.  The question was from a user and the following is my answer.

How do pilots use stopway, EMAS, clearway and displaced threshold?

RSAs are defined in AC 150/5300-13A

Runway Safety Area (RSA). A defined surface surrounding the runway prepared or suitable for reducing the risk of damage to aircraft in the event of an undershoot, overshoot, or excursion from the runway.

RSA’s were developed for specific reasons and are discussed in section 307 of the AC.

In the early years of aviation, all aircraft operated from relatively unimproved airfields. As aviation developed, the alignment of takeoff and landing paths centered on a well-defined area known as a landing strip. Thereafter, the requirements of more advanced aircraft necessitated improving or paving the center portion of the landing strip. While the term “landing strip” was retained to describe the graded area surrounding and upon which the runway or improved surface was constructed, the role of the landing strip changed to that of a safety area surrounding the runway.  This area had to be capable under normal (dry) conditions of supporting aircraft without causing structural damage to the aircraft or injury to their occupants.  Later, the designation of the area was changed to “runway safety area” to reflect its functional role.  The RSA enhances the safety of aircraft which undershoot, overrun, or veer off the runway, and it provides greater accessibility for fire-fighting and rescue equipment during such incidents.

Runway safety areas surrounding a runway is typically twice the width of the runway and 1000′ longer than the runway on both ends. The 1000′ of additional length is referred to as a runway end safety area (RESA). Objects inside the RSA must have specific function and be frangible (fall down) if hit by an aircraft)

AC 150/5300-13A Figure 3-5

Let’s talk about runway end safety areas (RESA). The 1000′ feet of additional space beyond the end of each runway is now mandatory. Every airport must have a RESA at the end of the runway. The only exception to that rule is runways with an EMAS installed. The 1000′ is not an arbitrary number either. Through analysis of aircraft overrun incidents from 1975 to 1987 it was determined that approximately 90% of the overruns stopped within a 1000′ past the end of the runway at speeds less than 70 knots. See AC 150/5220-22a for reference.

There are airports that cannot comply with the RESA requirement due to topography, roads, railroads, houses, etc. In these situations, the FAA has established a concept called declared distances that will push the RESA onto the runway. This will limit the accelerate-stop distance available (ASDA) and landing distance available (LDA) values. Airports that cannot comply with the RESA standard are limited to a runway length which is available and suitable for preflight planning purposes. It would be impractical for the FAA to tear up perfectly good runway.

This means pilots must reference declared distance information from the current chart supplements booklet to take advantage of runway safety protections.

Take a look at KSAN runway 09 end.

otakrGoogle imagery from KSAN Airport

A 1000′ RESA lands in the middle of shopping center. In this case, the RESA is moved onto the runway. The pilots must reference the chart supplements to determine usable lengths for takeoff and landing. Here is the declared distances from the chart supplements booklet. The runway length is 9400′ but the ASDA and LDA values 1,121 feet shorter due to no space for a RESA at the end of the runway.

Chart supplements information for KSAN

Other parts of a RSA include a runway protection zone (RPZ). This area extends up to 1700 feet past the end of the runway and can be as wide as 500 feet depending on the type of aircraft the runway was designed for. A little history on RPZs

Approach protection zones were originally established to define land areas underneath aircraft approach paths in which control by the airport operator was highly desirable to prevent the creation of air navigation hazards. Subsequently, a 1952 report by the President’s Airport Commission (chaired by James Doolittle), entitled The Airport and Its Neighbors, recommended the establishment of clear areas beyond runway ends. **Provision of these clear areas was not only to preclude obstructions potentially hazardous to aircraft, but also to control building construction as a protection from nuisance and hazard to people on the ground.** The Department of Commerce concurred with the recommendation on the basis that this area was “primarily for the purpose of safety and convenience to people on the ground.” The FAA adopted “Clear Zones” with dimensional standards to implement the Doolittle Commission’s recommendation. Guidelines were developed recommending that clear zones be kept free of structures and any development that would create a place of public assembly.

AC 150/5300-13A Figure 3-17

Take a look at KSAN runway 9 again and you will see that the runway does not have space for a runway protection zone at the end of the runway. A RPZ is mandatory and so the RPZ is moved onto the runway. The takeoff runway available (TORA) is also 1,121 feet shorter than the runway for just that reason.

For takeoff performance calculations, pilots are required to ensure the airplane takes off and climbs to at least 35 feet within the TODA distance to achieve the designed safety in the RPZ.

Clearways are a special type of RPZ that will not allow any obstacles other than lights that are less than 26 inches in height. The clearway must be at “least 500 feet wide centered on the runway centerline and the length may not be more than 1/2 the runway length.”

A clearway increases the allowable aircraft operating takeoff weight without increasing runway length.

In other words, takeoff distance available (TODA) is increased beyond the runway when a clearway is installed.

Displaced thresholds are created for four reasons that are beyond the power of the airport owner / operator.

  1. A means for obtaining additional RSA prior to the threshold. See  paragraph 307.
  2. A means for obtaining additional runway object free area (ROFA) prior to the threshold. See paragraph 309.
  3. A means for locating the RPZ to mitigate unacceptable incompatible land uses. See paragraph 310.
  4. Mitigation of environmental impacts, including noise impacts.

Let me answer your questions to the best of my ability

What is the difference from a regulation point of view?

A stopway is designed to allow an aircraft to stop during an aborted takeoff. Interestingly enough the AC 150/5300-13A does not say it is designed for runway overruns during landing or undershoots on the opposite runway. By design it will increase the ASDA value for the runway

An EMAS system replaces a stopway and is usable for undershoots while landing on the opposite runway and overruns during both takeoff and landing and is not intended to meet the definition of a stopway. An EMAS will not increase the ASDA value for the runway

How does the pilot know if this is a stopway where braking is desired, or an EMAS where thrust reversers might be hazardous?

The chart supplements book contains information on all EMAS systems. Looking at the declared distances for the airport will indicate the presence of a stopway when the ASDA value is greater than the runway length. Take a look again at KSAN and you will see a note for an EMAS off of runway 27.

Chart supplements information for KSAN

Stopways are a little harder to notice but can be spotted by noticing the ASDA value is greater than the runway length.

How do pilots use non regular runway portions such as clearway, stopway, EMAS, displaced threshold, and possibly more of this kind if it exists and is there some regulation which limits them for taking off, taxiing, or landing?

What are Runway Declared Distances?

This is the easiest question of the bunch. Pilots are required to use the applicable declared distances for takeoff and landing performance calculations. By design declared distances take into account all of the airport design standards such as clearway, stopway, EMAS and displaced thresholds.

Declared distances bridges the gap between runway design standards and aircraft performance.

§91.103   Preflight action.

Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This information must include—

(a) For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts, fuel requirements, alternatives available if the planned flight cannot be completed, and any known traffic delays of which the pilot in command has been advised by ATC;

(b) For any flight, runway lengths at airports of intended use, and the following takeoff and landing distance information:

(1) For civil aircraft for which an approved Airplane or Rotorcraft Flight Manual containing takeoff and landing distance data is required, the takeoff and landing distance data contained therein; and

(2) For civil aircraft other than those specified in paragraph (b)(1) of this section, other reliable information appropriate to the aircraft, relating to aircraft performance under expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature.


The AIM has information concerning runway lengths.

4−3−6. Use of Runways/Declared Distances

4-3-6(c)(1) Declared distances for a runway represent the maximum distances available and suitable for meeting takeoff and landing distance performance requirements. These distances are determined in accordance with FAA runway design standards by adding to the physical length of paved runway any clearway or stopway and subtracting from that sum any lengths necessary to obtain the standard runway safety areas, runway object free areas, or runway protection zones. As a result of these additions and subtractions, the declared distances for a runway may be more or less than the physical length of the runway as depicted on aeronautical charts and related publications, or available in electronic navigation databases provided by either the U.S. Government or commercial companies.

4-3-6(c)(2)(c) When considering the amount of runway available for use in takeoff or landing performance calculations, the declared distances published for a runway must always be used in lieu of the runway’s physical length.

4-3-6(c)(2)(d) An aircraft is not prohibited from operating beyond a declared distance limit during the takeoff, landing, or taxi operation provided the runway surface is appropriately marked as usable runway.

AIM Figure 4-3-6

How do you use declared distances for multi-engine pilots?

How do runway declared distances affect my takeoff distance?

The short version is aircraft utilizing balance field length calculations typically provide a single distance for takeoff that must be lower than both the TORA and ASDA value.

Aircraft with unbalanced field length calculations typically provide separate performance calculations for accelerate-go and accelerate-stop. The TODA and ASDA values must be greater than the calculated accelerate-go and accelerate-stop distances, respectfully.


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Runway Declared Distances

All FAR Part 139 certificated airports have been updated to provide runway declared distances.  What are these declared distances and how are they applied to everyday flying.  Unfortunately many professional pilots haven’t kept up with the latest information from the FAA and could be in violation if they takeoff from a runway with a takeoff distance greater than these values.  We will take a look at each of the four declared distances and see how they are applied practically to a flight.

  • Takeoff Run Available (TORA) value:  The runway length declared available and suitable for the ground run of an airplane taking off.

  • Takeoff Distance Available (TODA) value: The takeoff run available plus the length of any remaining runway or clearway beyond the far end of the takeoff run available.

  • Accelerate-Stop Distance Available (ASDA) value: The runway plus stopway length declared available and suitable for the acceleration and deceleration of an airplane aborting a takeoff.

  • Landing Distance Available (LDA) value:  The runway length declared available and suitable for a landing airplane.

Definitions taken from FAA Pilot / Controller Glossary.

These declared distances can be found in the A/FD.  Here is an excerpt for Four Corners Regional Airport in Farmington, NM (KFMN).

KFMN AFD Excerpt

All multi-engine aircraft have two calculations to perform for their takeoff distance.  They are required to determine the accelerate-go distance and the accelerate-stop distance.  Just like the names imply, it is the distance required to either continue the takeoff or abort the takeoff, respectively.  Depending on the regulations the airplane was certified under, Part 23 or Part 25, determines the height of the airplane at the end of the accelerate-go distance.  Part 23 certified aircraft must climb to a height of 50 feet while Part 25 aircraft, 35 feet.

We use these takeoff performance numbers and they must be lower than the respective runway declared distances.  Accelerate-go must be lower than the TODA value and the accelerate-stop distance must be lower than the ASDA value.  As a side note, those aircraft that use a balanced field length single takeoff distance number must be lower than both the TORA and ASDA values.

Why can’t we use the whole length of the runway anyways?  Why do we need to have declared distances?  Good question.  A Boeing 737 landed fast in Burbank, CA and overran the runway.  It went through the metal blast fence and an airport perimeter wall coming to rest near a gas station.  This is one accident in many where the aircraft departs the end of the runway.

The FAA has determined in AC 150/5300-13 Airport Design that not all pilots are able to stop on the runway while either taking off and aborting the takeoff or landing.  To make this safer for the pilots and passengers they have created runway safety areas (RSA).

A runway safety area or runway end safety area (RESA) is defined as “the surface surrounding the runway prepared or suitable for reducing the risk of damage to airplanes in the event of an undershoot, overshoot, or excursion from the runway.

If the runway doesn’t have the required RESA, the airport operator could install an engineered materials arresting system (EMAS).  An EMAS system is defined as high energy absorbing materials of selected strength, which will reliably and predictably crush under the weight of an aircraft.

Photo of Functional EMAS Bed - NTSB Docket Photo

Picture taken from FAA Lessons Learned Website.

EMAS systems are shown on Airport Diagrams

KMDW Airport Diagram

As a result, every Part 139 certificated airport is required to have a RESA or EMAS system installed.  See the ends of runway 4R and 22L, 13C and 31C at Midway.  Some airports cannot expand the RESA past the end of the runway so they take the 1,000’ required for the RESA from the end of the runway.  This is easily seen in the TORA value.

I think we learned something here,  The TORA value may not equal the runway length.  In fact, the definition for TODA implies this.  The length of the usable runway may not be the actual pavement length of the runway.  Amazingly enough most of the professional pilots use Jeppesen plates and on their 10-9 chart will show takeoff and landing lengths.  These are necessarily equal to TORA and LDA, respectively, but they are close enough (within 1 or 2 feet) from what I can find.  As a result, I treat those values, takeoff as the TORA value and LDA and the landing value.  What you won’t find on Jeppesen charts legend is the ASDA value.  We have already shown that multi-engine aircraft must have a takeoff distance less than the TORA, TODA and ASDA value.  See KTUS, KTVL, KSAN for TORA values less than runway lengths.  These are only three of many more.


See page 65 of the Jeppesen Glossary Legends Manual.

So the question becomes, are there any airports where the ASDA is lower than the TORA.  The answer is yes.  In fact, Four Corners regional has one such runway.  Runway 7 and 25 have ASDA values lower than the TORA value.  See the picture at the top.  Are there other runways.  YES!

I was able to download the FAA airport and runway information text files.  I put those text delimited files into my favorite database program and learned there are over 2,300 airport where declared distances have been created.  Of those runway 324 have an ASDA shorter than the TORA value.  If you fly with Jeppesen charts, you can potentially takeoff overweight, which I am sure, at least in my airplane flight manuals, makes it illegal.

When I downloaded the FAA airport and runway information, I have found inconsistencies with the data displayed there and the AF/D.  For example, the AF/D shows KFMN above with ASDA lower than TORA values but you won’t find that airport in my spreadsheet.  The chart below won’t be kept up to date so, in all cases determine the declared distances from the proper sources.  The table below lists all the runways where the ASDA is more than 10% shorter than the TORA value.


Download the XLS file here

As a recap, declared distances are important for pilots to use while calculating takeoff and landing performance.  Understanding how they play into the performance calculation is key.  Without a proper understanding, pilots can cause unnecessary damage to their airplane, passengers and other on the ground by taking off at a weight greater than allowed.

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