5-1 CHAPTER 5. RUNWAY OVERRUN HAZARDS
5.1 Background. In addition to being able to predict and assess aircraft performance on a runway, pilots need to understand and account for other hazards that can significantly affect the landing distance required.
5.2 Overrun Hazards. The following hazards have been shown to increase the risk of a runway overrun.
5.2.1 Unstabilized Approach. Deviations in airspeed, altitude, descent rate, glideslope, runway aim point, and localizer control place pilots in a position where recovery to the desired flight path is unlikely. It is the pilot’s responsibility to inform ATC when compliance with an instruction will result in an unstabilized approach due to speed or descent rate.
5.2.2 E xcess Airspeed . It is critical that the pilot be aware of airspeed during the approach. The recommended approach reference landing airspeed (Vref), plus wind and gust additives, should be maintained until 50 ft over the runway threshold. Excessive approach speed may result in extra speed over the runway threshold and landing beyond the intended touchdown point. In addition to using more runway to touchdown, once the aircraft is on the runway, excessive speed will require more runway to stop, which could lead to a runway overrun. As a rule of thumb, on dry or wet runways, a 10 percent increase in final approach speed results in a 20 percent increase in landing distance. For example, published Vref is 120 knots (kt) with a landing distance of 3,000 ft. If the aircraft crosses the runway threshold at 132 kts, the aircraft will require 3,600 ft to stop.
5.2.3 E xcessive Height Over the Runway Threshold. The landing distances provided in the AFM are based on the aircraft being at 50 ft over the runway threshold. For every 10 ft above the standard 50 ft TCH, landing distance will increase 200 ft. For example, if the aircraft crosses the threshold at 100 ft, the required landing distance will increase by 1,000 ft. (Refer to FSF ALAR Briefing Note 8.3, Landing Distances.)
5.2.4 Landing Beyond the Intended Touchdown Point. AFM/POH distances are based on a touchdown point determined through flight testing procedures outlined in AC 23-8, AC 25-7, and AC 25-32. If the airplane does not touch down within the air distance included in the AFM/POH landing distance, it may not be possible to achieve the calculated landing distance.
5.2.5 H igh Airport Elevation . High airport elevation or high-density altitude results in a higher true airspeed, groundspeed, and a corresponding longer landing distance, compared to a lower airport elevation. Pilots should be aware that the performance penalty for high airport elevation can be significant. The AFM/POH usually includes an adjustment for this factor. (Refer to § 25.125(a).)
5.2.6 Airplane Landing Weight. Any item that affects the landing speed or deceleration rate during landing affects the landing distance, so the gross weight of the aircraft has a significant effect on landing distance. In other words, the required landing distance varies in direct proportion to the gross weight. For example, a 10 percent increase in landing weight results in a 5 percent increase in landing speed and a 10 percent increase in
5-2 landing distance. (Refer to the Pilot’s Handbook of Aeronautical Knowledge (PHAK) for more information.)
5.2.7 D ownhill Runway Slope . Runway slope has a direct effect on landing distance.
A 1-percent downhill slope increases landing distance by 10 percent (factor of 1.1). This effect is usually only accounted for in performance computations if the downhill runway slope exceeds 2 percent. (Refer to FSF ALAR Briefing Note 8.3.)
5.2.8 Delayed Use of Deceleration Devices/Maximum Braking.
5.2.8.1 For those airplanes so equipped, deceleration devices consist of spoilers/speed brakes, thrust reversers, and brakes. Since wheel brakes are much more effective in slowing the airplane once on the ground than air drag during the airborne part of the landing, pilots should prioritize achieving the intended touchdown point. The sooner the airplane touches down and starts braking, the shorter the total distance will be. Delayed braking action by the flightcrew during the landing rollout has been the causal factor in several accidents and serious incidents. Similarly, improper use of speed brakes, wheel brakes, and reverse thrust have been significant factors in a number of runway excursion landing accidents.
5.2.8.2 Prompt and proper operation of all means of deceleration has a major effect on landing distances. For aircraft with automatic ground spoilers/speed brake deployment but with manual arming, it is of paramount importance that these devices are effectively armed before landing. In addition, once on the ground, deployment should be verified. Spoilers/speed brakes greatly decrease lift, dump the weight on the wheels, and make the brakes effective. It should be noted that manual spoilers/speed brakes operated by the pilot involve a delay.
A 2-second delay at speeds of 200 ft/second (118 kts) increases the airplane stopping distance by almost 400 ft. Landing distance data in the AFM is typically based on a 1-second delay between successive actions to manually deploy/engage the deceleration devices. A conservative approach is to add 200 ft to the landing distance for every second in excess of 2 seconds to deploy the airplane’s deceleration devices.
5.2.9 E ffect of a Tailwind on Landing Distance. The effect of a tailwind on landing distance is significant. Given that the airplane will land at a particular airspeed independent of the wind, the principal effect of a tailwind on landing distance is the change in the groundspeed at which the airplane touches down. The effect of a tailwind increases the landing distance by 21 percent for the first 10 kts of tailwind. (Refer to the PHAK for additional general information and the aircraft’s AFM/POH data to specifically determine if tailwind-landing data is available for your airplane.)
5.2.9.1 T ailwind landings affect all types of airplanes. For transport category airplanes, the effect of tailwind is shown in the AFM landing distance information. For small airplanes, tailwind-landing data may not be provided.
The Aircraft Certification Service (AIR) provided the following tailwind performance information for a few small airplanes:
5-3
1. Cessna 150 and 152: Note on the landing distance chart, “For operation with tailwinds up to 10 knots, increase distances by 10 percent for each 2 knots.”
2. TBM 850: Note under landing distance table to “increase total distances of 30 percent for every 10 knots of tailwind.”
5.2.9.2 Tailwind example: Available runway of 5,000 ft, AFM landing distance of 3,000 ft, 50 ft TCH, and at the correct airspeed with a 10 kts tailwind results in an increase in the operational landing distance of 21 percent. This increase equates to an additional 630 ft, which increases the operational landing distance to 3,630 ft.
5.2.10 Cumulative Effect. The LDA can be exceeded when a cumulative effect of the above conditions exists. Pilots need to be aware of this cumulative effect. An example of the compound effects is in Figure 5-1, Effects of Compound Factors, below.
Figure 5-1. Effects of Compound Factors
1. Base line: At target landing speed, 50 ft at the threshold crossing height, dry runway, zero wind = 0% increase in AFM/POH landing distance.
2. With a 10-ft increase in threshold crossing height = 200-ft increase in landing distance.
3. With a 10% increase in landing speed = 20% increase in landing distance.
4. Plus a 10 kts tailwind, 21% increase in landing distance.
5. Add 20% for less-than-maximum braking and 200 ft for each second greater than 2 seconds to use deceleration devices.
6. Delay to employ deceleration devices. For each second beyond 2 seconds, add 200 ft to the landing distance
7. Add 15% safety margin to the resultant required runway length.
Total landing distance, including safety margin The End 5,000 ft of runway
5.3 Risk Mitigation.
5.3.1 SOPS. Specific SOPs to prevent a runway overrun are a primary risk mitigation tool.
Once SOPs are developed, it is imperative that the pilot/flightcrew execute them faithfully to mitigate a runway overrun. At a minimum, the SOPs should contain the factors presented in this AC.
5.3.2 Runway Overrun Mitigation Training Curriculum. An effective training program provides the knowledge and skill to increase the pilot's awareness of the factors that can cause a runway overrun. At a minimum, the operator's training program should include the same elements contained in their SOPs. Also, the go-around maneuver and the 5-4
5-5 reasons to initiate a go-around should be part of the training program. (Refer to SAFO 10005.)
5.3.3 Certificated Air Operator’s Flight Checking and Recurrent Training. Checking, testing, and recurrent training that emphasize airplane-landing performance are essential tools to mitigate runway overruns. Pilot examiners, flight and ground instructors, check airmen, and dispatch examiners should specifically stress aeronautical decision-making (ADM), runway overrun risk management, and the elements within this AC as they qualify and train pilots and dispatchers. Instructors and examiners should include runway overrun mitigation strategies during training and checking to ensure the pilot/applicant can apply the principles in a real-world environment.
Appendix A A-1 APPENDIX A. AIRCRAFT BRAKING ACTION REPORT (ABAR) SYSTEMS A.1 Overview. The information in this appendix is intended for consideration by flight operations management personnel responsible for creating policies and procedures regarding the use of automated systems that generate ABAR. As such, it provides background information for management purposes and is not intended as an operational guide for pilots.
A.2 Design Philosophy. An ABAR system is a report given describing a level of braking action using data from the aircraft. It will reflect to what degree the weight of an aircraft can be transferred to a deceleration force via the wheel brakes when the anti-skid activates during a friction-limited event. An ABAR system can be located either remotely or onboard an aircraft and uses the aircraft’s data to produce a standardized indication of wheel braking performance.
A.2.1 ABAR systems are the most accurate and objective means of observing and reporting aircraft braking action. The use of aircraft data to generate an ABAR can mitigate the human errors resulting from inadequate training, inexperience, or cognitive bias that may occur with a Pilot Braking Action Report (PBAR).
A.2.2 The data recorded by the ABAR system during the landing roll is used to:
1. Isolate the mechanical wheel braking forces from all other forces contributing to deceleration, as well as identify when wheel braking is friction limited; and
2. Map those forces in friction-limited conditions to the scale used to make braking action reports.
A.2.3 ABAR systems may come from different vendors and apply to different aircraft. A means of assuring that all systems operate within minimum, industry-based engineering standards, is through demonstrated compliance with ASTM E3266, Standard Guide for Friction-Limited Aircraft Braking Measurements and Reporting. Compliance ensures a minimum degree of confidence in all ABAR systems and facilitates a harmonized communication of wheel braking capability between aircraft with similar performance characteristics. (Refer to Advisory Circular (AC) 25-32, Landing Performance Data for Time-of-Arrival Landing Performance Assessments.)
A.2.4 A ircraft systems may also transmit data that is not visible to the flightcrew. Data not visible to the pilot is not considered part of the ABAR, and may be utilized for other data analysis purposes.
A.3 Aircraft Considerations.
A.3.1 ABAR generating systems are considered applicable to a wide range of aircraft types and manufacturers whose design allows for such an analysis.
A.3.2 ABAR systems installed on aircraft may require appropriate supplemental type certification approval.
Appendix A A-2 A.3.3 ABAR generating systems are designed to be used on aircraft that utilize landing performance data derived in accordance with AC 25-32; International Civil Aviation Organization (ICAO) Doc 10064, Aeroplane Performance Manual; or other equivalent standards.
A.4 Acceptance Criteria for ABAR S ystems.
A.4.1 ABAR systems are not required for certification of transport category aircraft and their intended function is not a regulatory requirement.
A.4.2 Operators who utilize ABAR related technology to transmit information with the label “ABAR,” should only do so if it can be determined that the system meets its intended function and complies with a minimum engineering standard published for such a system.
Entities seeking such a determination for an ABAR producing system should contact their designated FAA representative for guidance.
A.4.3 ASTM E3266 provides a suitable acceptance criterion for operators wishing to transmit information from an ABAR system.
A.5 Confidence of Reports.
A.5.1 ABAR generating systems may rely on aircraft data that is inferred from other sources to calculate the required values. Because these systems may not rely on sensors taking direct measurements of certain values, these systems cannot be calibrated in a manner similar to other flight deck indicators. Therefore, while the ASTM standard sets minimum levels of accuracy and precision for these systems, it should be recognized that there exists a statistical difference between what a system calculates and what a theoretical “real” value could be. For that reason, these systems are described as having a “confidence factor.”
A.5.2 ABAR generating system standards require the systems to demonstrate a confidence factor of 95 percent that the system will fall within one level of braking on the Runway Condition Assessment Matrix (RCAM) five level scale (i.e., good, good to medium, medium, medium to poor, and poor.)
A.6 Development of Operational Guidelines. Operators who utilize an ABAR system to provide and/or use such reports should develop operational guidelines and flightcrew training. This should include submitting Pilot Reports (PIREP) based on ABAR information after landing as well as the use of ABAR reports during TOA assessments.
ABAR systems whose intended function entails timely recognition from the flightcrew should comply with AC 25.1322-1, Flightcrew Alerting.
Note: Pilots and operators need to understand that an ABAR is a decision support tool rather than a decision-making tool. Pilots should use an ABAR as one piece of information along with the current weather, runway surface conditions, and other factors to make the decision as to whether or not it is safe to land.
Appendix A A-3 A.6.1 The following guidelines are considered acceptable for the operational use of information provided by ABAR systems:
1. When an ABAR confirms the accuracy of the Runway Condition Code (RwyCC)
(i.e., both values are the same), the time of arrival (TOA) landing distance assessment should be based on this confirmed value; and
2. When an ABAR and the reported RwyCC disagree (i.e., one value indicates worse runway conditions than the other), the TOA landing distance assessment should be based on the worse (most conservative) report.
Advisory Circular Feedback Form If you find an error in this AC, have recommendations for improving it, or have suggestions for new items/subjects to be added, you may let us know by contacting the Air Transportation Division at 9-AFS-200-Correspondence@faa.gov, the General Aviation and Commercial Division at 9-AFS-800-Correspondence@faa.gov, or the Flight Standards Directives Management Officer at 9-AWA-AFB-120-Directives@faa.gov.
Subject: AC 91-79B, Aircraft Landing Performance and Runway Excursion Mitigation Date: _____________________ Please check all appropriate line items:
An error (procedural or typographical) has been noted in paragraph ____________ on page _______.
Recommend paragraph _____________ on page __________ be changed as follows:
______________________________________________________________________ ______________________________________________________________________ In a future change to this AC, please cover the following subject:
(Briefly describe what you want added.)
______________________________________________________________________ ______________________________________________________________________ Other comments:
______________________________________________________________________ ______________________________________________________________________ I would like to discuss the above. Please contact me.
Submitted by: Date: ______________________