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20 to 1 Airport Obstructions

 20 to 1

The Issue

According to the FAA, obstacles penetrating the Visual Area Surface, which is commonly referred to as the 20:1 slope (spoken as “twenty to one”), represent a hazard to air navigation. The 20:1 slope normally begins 200 feet prior to the runway threshold and rises one foot for every 20 feet horizontally along the runway’s extended centerline. This slope is the minimum area that must be protected from penetrations for the FAA to allow an airport to have unrestricted instrument approach procedures.

If there is a penetrating hazard, the FAA can restrict instrument minimums to that runway, which then can impact airport access. All aircraft must have a safe path to descend and land regardless of whether they are VFR or on an instrument approach procedure. In the latter situation, the pilot still must be able to see a certain distance in order to visually acquire the runway and any obstruction that must be avoided prior to landing. There can be a negative economic impact for the airport and aircraft operators when restrictions are imposed.


The FAA’s 20:1 policy historically allowed for penetrations to be mitigated by raising the visibility minima to one statute mile. The terminal procedures (TERPS) design criteria changed in 2002 to require penetrations to be marked and/or lit so as to make them more conspicuous to pilots, or a notam issued for the procedure that would not authorize the approach at night. Airport compliance and oversight was poor during this time, with the FAA placing little emphasis on enforcement.

In 2003, the FAA Flight Standards Service issued a memo providing relief from the negative operational impacts caused by increased visibility requirements and procedures not being able to be utilized at night. A three-year process for airports to become compliant and have access to airport funds was created. Airport funds can take considerable time to reach airports, and many general aviation airports rely on this outside funding to negotiate with adjacent land owners, such as for avigation easements, and then to remove obstructions.

The Flight Standards memo providing extended relief to the applicable TERPS policy was rescinded in 2010. Certain airports continued to receive relief, but most were required to abide by the published TERPS criteria. The criteria were modified in 2012 to address unlit 20:1 obstacles and increase the notam requirements to ensure procedures were not authorized at night as required. An inaccurate obstacle database resulted in misidentification for many airports and no advance notice prior to a notam being issued for numerous others.

In 2013, the FAA, AOPA, and other industry stakeholders met as part of the Visual Approach Surface Task Group to offer recommendations as to what the new guidance should be for a proactive system and a risk-based enforcement philosophy. The subsequent policy allowed sufficient time for the airport operator to verify the obstacle and, if it was verified to be valid, more time to mitigate it (by removal, lowering, or another method).

The interim policy guidance took effect January 2014 and was viewed favorably by the FAA, airport operators, and pilots because the hazard level of the obstruction determined the level of impact on the utility of instrument procedures. Concurrently, the FAA’s airport office took a more proactive approach by increasing the tools and access to obstacle data available to airports to better empower them to deal with future or existing 20:1 obstructions. The FAA also conducted additional outreach in 2015 to its airport offices and to airport sponsors to remind them of their responsibilities to keep approach and departure surfaces clear of obstacles.

The above diagram is from the FAA’s interim policy guidance. The risk-based approach that made that policy so successful is being retained in the new 20:1 policy.

The interim policy guidance expired in early 2016. In the two years the interim policy was in effect, 3,002 airports were studied, and all 16,518 instrument approach procedures (about 32,000 lines of minima) in the United States were reviewed. During this period, more than 900 airports and about 2,000 procedures were limited by a 20:1 obstacle but, of all of the airports affected, 565 eventually saw the removal or mitigation of the obstruction by the airport and the impact to the procedure removed.

Current FAA 20:1 Policy

In March 2016, the FAA presented its new 20:1 policy and enforcement philosophy to the industry. The new philosophy retains the risk-based approach, and the FAA has committed to maintaining that policy into the future.

As new 20:1 obstructions are identified by the FAA and validated by the airport, they will be submitted to the FAA Flight Standards Procedure Review Board (PRB). This group meets weekly and includes representatives from the Flight Standards Service, Operations Support Group Flight Procedures Teams, Airport Standards Division, and Mission Support Services. The PRB will discuss the airport and the obstructions from the standpoint of the risk of the penetration (for example, its location, height, type of object, whether it is lighted) and may grant a restricted waiver to complement an airport compliance plan, or initiate corrective action through a notam or procedure amendment.

After the two-year effective term of the interim policy, the FAA is hopeful airports will be aggressive and better educated regarding obstructions so there will be fewer penetrations identified. The new policy will allow a better understanding of why obstructions are still being identified and the most effective way to prevent future penetrations.

AOPA’s Position

AOPA has been actively engaged in advocating for a risk-based approach that allows GA airport operators enough time to secure funding and remove the obstruction while not impacting safety or unnecessarily affecting airport access. The association was very involved in crafting the interim policy guidance that was favorably viewed by operators and regulators.

Airport operator awareness and aggressive obstruction removal philosophy are key to preventing future 20:1 penetrations. Mitigations put in place unilaterally by the FAA have resulted in confusion for airport management and little to no notice for operators. Contributing to the issue is inaccurate obstacle data in the FAA database, lack of awareness on the part of airport operators of their responsibilities of, and a lack of consistent enforcement of the 20:1 surface.

AOPA will provide support to the FAA in the critical components of outreach and education. AOPA will continue to work closely with airport operators to ensure they have the tools they need to deal effectively with surrounding obstructions.

Airport Responsibilities

Airports that accept federal funds and agree to the grant assurances are required to keep their approach surfaces clear. Airports that do not have grant assurance requirements still must keep the approach areas clear of obstacles or risk losing their instrument approaches and increase their liability due to a known unsafe obstacle. Aircraft can still land on a runway with 20:1 obstacles, but the airport could be held liable for any incident involving an aircraft, whether IFR or VFR.

Any solution that would allow an obstacle to permanently penetrate the 20:1 is problematic from various FAA policy requirements and from the safety standpoint. Airport access could be degraded by penetrations or lead to a runway becoming no longer viable if the airport operator is not responsible for 20:1 obstacle clearance. The obstruction’s hazard level determines the timeline for the airport, but being proactive and preventing penetrations is the most effective approach. Additional mitigations, as detailed below, also are available to airport operators.

Tools for Airport Managers

Obstacle survey information can be found on the Airport GIS Surface Analysis and Visualization (AGIS SAV) tool; however, many airports are unaware of its existence. AOPA is working with the FAA to improve this tool and to advertise its availability. Most Part 139 airports (commercial service) use this tool, but many smaller GA airports do not. You must be an official affiliated with the airport to register and be approved for access to the obstacle data.

Erroneous obstacle information could be caught more quickly using the SAV tool and thus avoid the issuance of erroneous notifications or notams. The tool also facilitates a more efficient and standardized process as airport operators communicate with the Flight Procedures Teams and the Airports District Office when dealing with a 20:1 obstruction. A consolidated obstacle database is key to providing the best information to the airport operator and improving data integrity.

The AGIS SAV tool overlays the obstructions on an interactive map and displays the obstacle using symbols corresponding to the level of the hazard. Airport operators can use this tool to access FAA guidance, view current obstacles, file mitigation plans, or correspond with the FAA.

Available Mitigations for 20:1 Obstacles

The FAA details in its TERPS criteria and other policies the following available mitigations for verified 20:1 penetrations:

  • Removal of obstacle
  • Lowering of obstacle (This mitigation normally requires a surveyor’s verification.)
  • Lighting and/or marking of obstacle (For example, the FAA may allow poles to be be erected and lit to mark trees.)
  • Relocation of runway threshold (This mitigation is likely not eligible for Airport Improvement plan funding.)
  • Visual Glide Slope Indicator (A VGSI may qualify as a mitigation that precludes the issuance of a notam not authorizing a procedure at night.)
  • Notam preventing certain operations, such as operations at night
  • Notam temporarily raising minimum descent altitude (MDA)/visibility (This mitigation is normally used when a crane is temporarily penetrating the Visual Area Surface and erected only during the day.)
  • Permanent modification to the instrument approach procedure, such as addition of a step-down fix (The 20:1 obstruction would normally need to be further away from the runway for this mitigation to be considered.)
  • Waiver (A waiver may be available if the obstacle is outside the ILS, LOC, LPV, or LP full-scale deflection.)
  • Request by the airport for a special instrument approach procedure to be designed that would allow approved users to fly unrestricted instrument procedures to that runway despite 20:1 obstructions (This request is several-year process with burden of funding on the airport.)

It is important to note that each penetration may not qualify for one of the above mitigations. Additionally, the FAA Flight Procedure Teams normally look at what mitigations may be applicable and will work with the airport operator to determine whether paperwork is required to document the mitigation’s presence. The airport operator is responsible for preventing 20:1 obstructions and, should they become an issue, removing them quickly to maintain airport access.

Frequently Asked Questions

Why can’t the MDA be increased to accommodate a new obstruction, such as requiring the approach to have VFR minimums?

Raising the MDA will not have an impact or provide mitigation for the presence and threat caused by a 20:1 obstruction. For example, if a tree grows tall enough to impact the Visual Area Surface it may prompt the FAA to issue a notam indicating that the instrument approach procedure is not authorized at night. This is because a pilot cannot see the tree at night and avoid it because it is not lit. The mitigations and actions taken by the FAA vary based on the hazard (amount of penetration) and type of obstruction (building versus vegetation). The 20:1 surface is the minimum surface the FAA expects to be protected by airports.

Below are some policy and philosophical reasons why the FAA also does not allow permanent 20:1 obstructions:

  • There is industry consensus that new obstacles should not lead to permanent procedural changes except in limited circumstance. As a general rule, obstacle clearance surfaces for instrument procedures are not increased to accommodate new obstacles. “Changing airspace procedures in order to accommodate proposed structures sets the precedent that further changes could be implemented, which would lead to a gradual degradation of the safety, utility, and efficiency of the airspace surrounding airports.” (Understanding Airspace, Objects, and Their Effects on Airports; ACRP Report 38)
  • The FAA’s enforcement of the 20:1 surface relates to 14 CFR Part 77, AC 150/5300-13, and TERPS criteria.
  • A greater allowance for 20:1 obstructions would affect the ability to chart vertical descent angles and visual descent points on approach charts. Both of these notes are important for many operators to ensure a stable approach.

My airport’s approaches were just designated not authorized at night by a notam. Who can I talk to about the obstructions?

The FAA will be in direct contact with the airport manager and the airport’s sponsor when 20:1 obstructions are identified. Normally a notification letter is sent to the point of contact for the airport as identified on FAA Form 5010. This individual has 30 days to validate the obstruction before action is taken by the FAA to restrict the procedure as required by the agency’s guidance. The airport manager is the best point of contact as he or she will be best informed and best able to describe plans to remove or mitigate the obstruction. Various organizations, including AOPA, can help airports when 20:1 obstructions are identified and when a removal strategy must be drafted.

I am an airport operator. The FAA has not visited the airport recently. How do they know there is a 20:1 obstruction?

New obstacles can be identified to the FAA via several methods:

  • A new obstacle survey (can take some time for the data to enter the system)
  • FAA Form 5010 (normally part of a routine airport inspection)
  • FAA Form 7460 (new manmade obstruction near the airport)
  • Flight inspection (You can see when your procedure is scheduled for inspection by checking the periodic review list.)

The FAA also uses a formula to account for annual tree growth, so it is important for airport operators to be proactive and follow FAA guidance as it pertains to managing vegetation. When notified of a new 20:1 obstruction, be sure to verify its validity as errors in the obstacle database do occur.

The 20:1 obstruction is located on an uncooperative neighbor’s land. How can I remove it?

There is no mitigation proposed for a newly identified obstacle that would result in a long-term restriction for the airport due to impracticality for removal (such as an uncooperative neighbor). The airport and its sponsor should negotiate with the landowner and discuss a temporary easement or long-term avigation easement to ensure the obstacle is removed in a timely manner. An easement may require financial or other type of compensation. Typically, airports will remove the offending tree and its stump, provide lawn care service for a period of time, and plant a height-compatible replacement tree at no cost to the landowner as compensation for removing a tree. Negotiations can take a considerable amount of time and an ensuing lengthy period of impact to airport access. Airport operators should be proactive and have a plan to deal with known 20:1 obstructions prior to the FAA notification.

I manage a GA airport. How can I stay aware of obstructions and be proactive?

Below are some best practices:

  • The FAA recommends airport operators take a proactive approach by reviewing all approach surfaces in advance of any flight check. The FAA does publish the flight inspection schedule for the year.
  • All airports should sign up for the AGIS SAV tool, which graphically depicts obstructions on an interactive map. This tool can be used to find FAA guidance and to correspond with the FAA regarding mitigation plans.
  • Airports should be particularly attentive to vegetation and document when trees on and off airport property will need to be lowered or removed. It is important to start the planning process early to allow sufficient time for funding and negotiations with land owners.
  • Review the FAA and industry guidance on obstacle mitigation. There is a wealth of literature available on this topic that details case studies, creative solutions, and points of contact.

Will air traffic control advise me whether the requested approach procedure is NOTAMed or charted as not authorized at night?

Air traffic controllers will clear an aircraft for an approach procedure despite a charted note or NOTAM not authorizing that procedure to be flown at night. According to a recent air traffic interpretation, it is the pilot's job to determine if they are able to fly that procedure, not the controller's. This interpretation is consistent with other FAA determinations which maintain it is the pilot's responsibility to fly the correct line of minima, understand when "night" begins, and abide by published notes on an approach chart.


To offer feedback on the information provided or on the FAA’s 20:1 policy, please send an email to AOPA.

Updated June 2, 2016