Monday, June 21, 2010

Limits of See and Avoid

On November 16, 2000, at 1548 eastern standard time, a U.S. Air Force F-16CG, operated by the 347th Wing, Air Combat Command, collided in mid air with a Cessna 172, N73829, near Bradenton, Florida. The F-16, based at Moody Air Force Base (AFB), Valdosta, Georgia, was on a low-altitude training mission. The Cessna 172, registered to Crystal Aero Group, was operating as a 14 CFR Part 91 personal flight. The airline transport (ATP)-rated Cessna pilot was killed. The F-16 pilot, who held a commercial pilot's certificate, ejected from the airplane and sustained minor injuries. Visual meteorological conditions prevailed at the time of the accident.

This accident offers a dramatic illustration of the risks of fast-moving military aircraft operating in the vicinity of slow-moving GA aircraft. And as you'll see, the accident also illustrates that military personnel are human: They can and do make mistakes. Safely sharing airspace with military aircraft requires an understanding of regulatory and non-regulatory airspace, including special use and other airspace. It's also important to recognize the limits of the "see and avoid" collision avoidance technique. Last but not least, knowing where military aircraft are scheduled to be operating can help you avoid coming into close proximity with fast moving hardware.

Rules or No Rules?

The national airspace system is divided into two basic categories: Regulatory and Non-regulatory. Regulatory includes Class A, B, C, D, and E (aka Controlled Airspace and defined in 14 CFR 71) as well as Restricted and Prohibited areas (aka Special Use Airspace and defined in 14 CFR 73). As implied by its name, regulatory airspace has specific regulations that define who can enter and operate in that airspace. Break those regulations and you'll find yourself in trouble with the FAA.

Non-regulatory airspace includes Military Operation Areas (MOAs) as well as Warning, Alert, and Controlled Firing areas and these are defined in FAA order JO 7400.8: Special Use Airspace, which describes the location, dimensions, times of use, and identifies the controlling agency. There are no specific regulations restricting flight into these areas, other than those for VFR cloud clearance and visibility. There are some common sense operating procedures that pilots should follow and failing to use those procedures may get you some unwanted adventure, or worse.

Mysterious MTRs

The above-cited accident involved, at least peripherally, a Military Training Route (MTR), a type of non-regulatory airspace that has been given the innocuous classification of other airspace. MTRs are depicted on VFR charts as subtle (some say nearly invisible) charcoal lines with a two-character prefix followed by a three or four digit number. Flights on MTRs with an IF prefix are conducted under IFR while those with a VF are usually conducted under VFR. A three digit number following the prefix indicates the military aircraft should be operating above 1500' AGL and, to the maximum extent possible, under IFR. A four digit number indicates the aircraft may be operating below 1500' AGL. In general, you can expect military aircraft on these routes will be operating in excess of 250 knots. They may be operating close to the surface or they may be at higher altitudes during descent to or climb out from the route.

Other that what's encoded in the MTR identifier, there is precious little information available to the average pilot who does not possess a current copy of the DoD's flight information publication Area Planning: Military Training Routes: North and South America, also known as the AP/1B. About the size of a small phone book, the AP/1B is published every 56 days and defines each MTR's location, hours of operation, route description, operating procedures (VMC or IMC), route width, and entry/exit points. Many routes have specific restrictions and can only be flown by certain types of military aircraft, but I know of no way for civilian pilots to get their hands on a current copy of AP/1B.

The flight lead stated that the two F-16s were assigned a block altitude of between 25,000 feet and 26,000 feet en route to the entry point of visual military training route (MRT) VR-1098. As the flight approached the SRQ area, Miami Air Route Traffic Control Center (ARTCC) cleared the F-16s to descend to 13,000 feet. At 1543:39, the Miami ARTCC controller instructed the flight lead to contact Tampa Terminal Radar Approach Control (TRACON) controllers. The flight lead was not successful (because he was given an incorrect frequency), and he reestablished contact with Miami ARTCC and canceled IFR. Miami ARTCC advised him of traffic at 10,000 feet, which was acquired on radar. The controller accepted the cancellation and asked the pilot if he wished to continue receiving radar traffic advisory services. The flight lead declined. According to the air traffic control (ATC) transcripts, the controller then stated, "radar service terminated, squawk VFR [transponder code 1200], frequency change approved, but before you go you have traffic ten o'clock about 15 miles northwest bound, a Beech 1900 at ten thousand [feet]." The flight then began a VFR descent to enter VR-1098.

See and Avoid

In addition to the laissez-faire character of special use airspace is the fact that most traffic collision avoidance is accomplished with the see and avoid technique. When a mid-air accident occurs, the NTSB always states that the cause of a mid-air was the pilots' failure to maintain adequate lookout for other traffic.

There have been several studies that attempted to determine how pilots scan for traffic, what a good scan is and what a bad scan is, how head-down time (due to complex avionics and pretty colored moving maps) affects the scan. Traffic collision avoidance hardware (whether installed or portable) can be a help and I always fly with a Zaon PCAS affixed above the instrument glare shield. The problem is that in spite of training, admonishments, and collision avoidance hardware, mid-air collisions continue to occur at about the same rate and most occur in visual conditions.

Speed can Kill

When quizzing pilots about 14 CFR 91.117 (airspeed limits) during a flight review, I hear comments that these regulations don't apply because they fly slow moving aircraft. I usually counter that it's important to know how fast the other guy might be going and then add that there is an escape clause which can result in the other guy going even faster (emphasis added):

(d) If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed.

The minimum recommended cruising speed for an F-16 below 10,000 feet is 300 knots, but this flight of two was going even faster (emphasis added):

The F-16 flight entered the top of the class B airspace about 380 knots airspeed and left the airspace at 6,000 feet about a minute later at 360 knots. Speeds of up to 450 knots were noted during the descent. The airspace between Tampa class B airspace and Sarasota class C airspace is Class E airspace, with a lower floor at 700 feet. About 30 seconds after leaving the Tampa class B airspace, the flight entered the Sarasota class C airspace at 380 knots. The flight remained in the Sarasota class C airspace where the midair collision took place. The flight lead's speed remained above 300 knots until the accident F-16's collision with the Cessna. 

A Cessna 172 cruises between 110 to 120 knots, but a normal climb speed is around 80 knots. Needless to say the closure rate between the two accident aircraft was very rapid with just over 20 seconds between the time ATC received a conflict alert and the impact.

A review of altitude data and ground track data (and airspace boundaries) determined that Tampa TRACON's intruder conflict detection software noted a conflict between the flight lead and the Cessna, and generated an aural conflict alert in the TRACON facility at 1547:39 that continued until 1548:03.
Hazardous Training

It's interesting to note that training activities figure prominently in this accident. The flight of F-16s were training and the Tampa TRACON controller was receiving instruction from another controller.

After continuing to descend, the flight lead looked back to the left and observed the accident F-16 slightly below him at the 7 o' clock position and about 4,000 feet to 5,000 feet behind him. The flight lead also observed a white, high-wing white airplane (the Cessna) in a 30 to 45-degree right turn. The Cessna and the accident F-16 collided in a left-to-left impact at the flight lead's 10 o' clock position, he stated. After the collision, the flight lead observed vaporizing fuel on the F-16's right side. The flight lead did not see the Cessna. The flight lead called the accident pilot and stated, "it appears you have had a mid air and are streaming fuel." There was no response. 

The Cessna pilot had departed Sarasota-Bradenton, was inside class C airspace, and was in communication with Tampa approach.

The Miami ARTCC controller contacted Tampa TRACON at 15:47:55 and asked Tampa TRACON for the flight lead's altitude because he had lost radar contact with the lead F-16 (only the flight lead had his transponder activated because formation flights are handled as a single aircraft by ATC). Tampa TRACON replied at 20:48:00, stating "ahh hang on I see him down at two thousand." At 15:48:09, Tampa TRACON informed N73829 that he had traffic off his left side at 2,000 feet. N73829 did not respond.

The biggest single factor in this accident may have been that collision occurred inside Class C airspace where the Cessna pilot least expected encountering fast moving fighters because the F-16s were off course by several miles. The lead pilot may have thought he was at the entry point for VR1098, but he was mistaken.

Lockheed Martin examined the download data from the crash survivable flight data recorder (CSFDR), the SDR, data printouts from the general avionics computer (GAC), the global positioning system (GPS), the inertia navigation system (INS) and the AVTR tapes from the flight lead's airplane. Lockheed Martin's examination report stated that M Aero stated that GPS "was removed from the navigation solution at some time prior to the midair. It cannot be determined from the data why the GPS was removed from the navigation solution." The report added: "A position error of approximately 9-11 nm was entered into the navigation system at some time on the mishap flight prior to the video recording. It can not be determined from the data what caused this position error."

Avoiding Military Activity

There is no regulation that prevents you from operating in or around an MOA or MTR, but the conventional wisdom is to avoid these areas when they are active. If you must operate in or around these areas, then by all means get flight following from ATC. This accident illustrates that ATC traffic advisories are no guarantee that you'll be safe, but you want as much help as you can get.

To determine if an area is scheduled for use, contact Flight Service before you fly or talk the controlling agency. The FAA's VFR charts require that you first look up the identifier for the area you're interested in, then look on the edge of your chart to find information about times of use, altitudes, and the relevant radio frequencies. It would be a heck of a lot simpler if they just put that information next the depiction of the area, but they don't.

You can also reference this web site to get a graphical representation of regulatory and non-regulatory airspace, though there's a disclaimer that the most up-to-date information is only available through Flight Service.

The conventional wisdom I've heard from fighter pilots is that best strategy for avoiding an imminent collision with a fast moving military aircraft is descend. The fighter aircraft is likely going to climb to convert their fast airspeed into altitude, so you're best bet may to descend rather that turn away.

If you think military pilots can see you on radar and will avoid you, think again. They are focused on their training mission and don't have a lot of extra time to look for you. If you think military pilots are so highly-trained that they never make mistakes or errors in judgement, read the NTSB's probable cause (emphasis added).

The National Transportation Safety Board determines the probable cause(s) of this accident as follows: the failure of the F-16 flight lead pilot and F-16 accident pilot to maintain an adequate visual lookout while maneuvering. Factors contributing to the accident were: the F-16 flight lead pilot’s decision to discontinue radar traffic advisory service, the F-16 flight lead pilot’s failure to identify a position error in his aircraft’s navigational system, the F-16 pilots subsequent inadvertent entry into class C airspace without establishing and maintaining required communications with air traffic control (ATC); and ATC’s lack of awareness that there was more than one F-16 aircraft in the formation flight, which reduced the ATC controllers ability to detect and resolve the conflict that resulted in the collision.

And if you think you're Super Pilot, you can operate in these areas whenever you want, and that you'll be able to see a flight of two fighter jets operating close to the ground at high speed, well good luck to you. You're going to need it.


Frank Van Haste said...

John -

Concur w/ all. Back in '91 the Aussie ATSB issued this report on the limitations of "See-and-Avoid". I commend it to your attention.



Ron said...

Great article. I have experienced the limitations of "see and avoid" first-hand, and am a big believer in utilizing all the available tools in the quest to avoid the Skyhawk's fate: radar services, TIS/TCAD devices, extra sets of eyes, and a careful thought or two about one's route.

While this accident was certainly tragic, it's relatively rare for military and civilian aircraft to collide like that. Much more common is a civilian/civilian midair collision. We've had many of those around the L.A. basin over the years. I have to say, I feel better when I've got some sort of traffic avoidance device on board. They are not a panacea by any means, but they certainly don't hurt.

JetAviator7 said...

Many a student I have taught thinks that radar alone will save your bacon, even if you aren't talking to TRACON.

When I learned to fly my instructor carried a rolled up newspaper and when my head stopped moving he smacked me.

I think pilots today rely way too much on the instruments inside the aircraft and not enough on what is going on outside the aircraft.


John Ewing said...


I'll take your comment with grain of salt ...

Rather than punish a student, I think a more effective use of an instructor's time is looking outside the cockpit since 1) having more eyes scanning for traffic will increase the safety of the flight and 2) students learn more by the example set by their instructor and their peers than they do by brute force.

The research I've read would seem to indicate that the type of negative reinforcement you describe enduring at the hands of your instructor does not enhance learning. In fact, negative reinforcement often teaches something that the instructor never intended: The student's head may be moving and their eyes are outside the cockpit, but who knows if they have actually learned how to effectively scan for traffic?

One of the main points is that the visual scan is limited. If technology can help avoid midair collisions, it's silly to reject it's use out of hand.