October 17, 2010
The picture above is Pacific Southwest Airlines (PSA) flight 182, a Boeing 727, after a mid-air collision with a Cessna 172 over San Diego, California on September 25, 1978. PSA 182 was on a downwind leg for runway 27 at the same time that the 172, N7711G, was flying a practice ILS approach to runway 9. After the impact, both aircraft crashed, killing a total of 144 people…135 passengers and crew aboard PSA 182, a student and his instructor in the 172 and 7 people including 2 children on the ground. An additional 9 people on the ground were injured and 22 homes were destroyed.
This accident and other similar incidents gave impetus to the creation of such technology as the Traffic Collision Avoidance System (TCAS), policies like those requiring altitude reporting transponders while inside the airspace surrounding major airports and procedures inside the cockpit and within air traffic control centers that would help prevent such disasters from occurring in the future.
Since it relates specifically to the story I’m about to tell, let me provide a short explanation of TCAS.
The picture above is a good representation of a typical TCAS display. TCAS involves communication between all aircraft equipped with an appropriate transponder (provided the transponder is enabled and set up properly). Each TCAS-equipped aircraft "interrogates" all other aircraft in a determined range about their position, and all other craft reply to other interrogations. This interrogation-and-response cycle may occur several times per second.
Through this constant back-and-forth communication, the TCAS system builds a three dimensional map of aircraft in the airspace, incorporating their bearing, altitude and range. Then, by extrapolating current range and altitude difference to anticipated future values, it determines if a potential collision threat exists.
The next step beyond identifying potential collisions is automatically negotiating a mutual avoidance maneuver (currently, maneuvers are restricted to changes in altitude and modification of climb/sink rates) between the two (or more) conflicting aircraft. These avoidance maneuvers are communicated to the flight crew by a cockpit display and by synthesized voice instructions.
In basic terms, if I am sharing airspace with an aircraft that is not equipped with TCAS, the TCAS onboard my aircraft will tell me to climb or descend to avoid a collision. If I am sharing airspace with an aircraft equipped with TCAS, the systems onboard my aircraft will communicated and coordinate with the other aircraft and our respective TCAS systems will provide instructions to climb or descend to avoid a conflict.
So here we go…Ontario, California to Dallas - Ft. Worth, Texas on a crisp, cool Saturday afternoon in November. We were planning to have a few open seats on our flight home, but non-revs and last minute travelers seem to come out of the woodwork at the last minute and we managed to leave full. We left the gate a few minutes ahead of schedule and began the short taxi to runway 26R for takeoff as I briefed the Captain on the final weight and balance information and ran the taxi and before takeoff checklists.
The departure procedure out of Ontario is far from the most complicated we fly, but it can be a challenge for a few reasons. First, Ontario has a noise abatement procedure that is put in place to minimize the impact of noise pollution in the area surrounding the airport. The MD80 that I fly is not exactly known for it’s quiet engines and is notorious for setting off noise sensors when the procedure is not followed correctly. They don’t just issue the procedure and ask us to fly as quietly as possible, they actually listen.
Many airports around the country have installed noise sensors in the neighborhoods surrounding major airports. If the pilots exceed the allowable decibel level on one of these sensors, the airline is fined. In this case, the procedure calls for an early left turn with the flaps and slats extended to allow for a sharper turn and a reduction from takeoff to climb power at a lower than normal altitude. By itself, not a huge deal.
The procedure is further complicated by the routing and altitude requirements on the SID (Standard Instrument Departure). After liftoff, we are required to make an early and sharp turn to the southeast and proceed directly to the Paradise VOR. Once direct to Paradise, we must be careful to cross 6 miles from the VOR at or below 4000 feet and then cross over the VOR at or above 2700. We then cross the next fix between 4500 and 9000, the next fix at or below 11000 and the fix after that above 6000. It’s all very confusing and a lot to think about in the first few minutes of the flight, especially when you consider the pilots are also retracting gear and flaps, accelerating to 250 knots, running the after takeoff checklists and watching for traffic.
Traffic. It’s that last little thing that has the potential to make this departure so interesting. Southern California and specifically the LA basin hosts a significant number of small, general aviation aircraft sharing the same airspace as large jets easily operating 2-3 times their speed. The LA basin is also home to a number of major airports…LAX, ONT, BUR, LGB and SNA…to name a few. The combination of large jets and small general aviation aircraft operating in significant numbers in a relatively small geographic area makes the possibility of disaster is a very real concern.
I was flying, so after receiving our takeoff clearance, I pushed the throttles up to stabilize the engines, then called for “auto throttle” and allowed the automatic system to set takeoff thrust. We accelerated normally down the runway and the Captain called out V1, Rotate and we were airborne. As we climbed through 100 feet, we received our first traffic advisory from the tower. I don’t know exactly what was going on in the controllers mind at the time…maybe he was distracted by something else…maybe he was previously unaware of the traffic, but I can tell you that 100 feet is an extremely unusual time to receive a traffic advisory. There was a helicopter two miles west of the airport heading east at 1000 feet. That put him directly in front of us, heading right at us, at an altitude we were going to climb through very shortly. Our TCAS called out an audible warning “Traffic, Traffic” to warn us of the target and displayed a solid white diamond shape on the TCAS display that immediately became an amber circle, indicating the increasingly close proximity of the traffic. I elected to begin the turn as depicted on the departure procedure and increased our rate of climb to clear the traffic as soon as possible. As we passed through 800 feet the TCAS once again sounded…this time with the words “Climb, Climb” as the amber circle became a red square and a green arc appeared on the VSI (Vertical Speed Indicator) indicating the rate of climb needed to resolve the event. We continued our turn and climbed as fast as we could with the nose well above the 20 degrees. We safely passed through 1000 feet and could see the helicopter clearly as he passed behind us…much closer than I would have liked. Then as we passed through 1500 feet and were moving away from the target, the TCAS made it’s final announcement… “Clear of Conflict” and the event was over. No more than 20 seconds had elapsed since rotation.
As we continued to climb, we shifted our attention to completing the after takeoff checklist and monitoring the various altitude restrictions on the departure. Approaching the JUMPA intersection, so named for parachute jumping activity in the area (another target to watch for and a story for another day) we received another traffic advisory from ATC. As we passed through 9,000 feet climbing at about 2000 feet per minute, ATC advised us of opposite direction traffic ahead at 10,500 feet. “Advise you stop climb until passing traffic” we were told. I had already started to push the nose over before receiving the instruction and we level off at 10,000 feet just before the TCAS announced “Traffic, Traffic”. By the time I looked down at the TCAS, the target was already a red square on the display and we received our second Resolution Advisory of the day as the TCAS announced “monitor vertical speed” and a green arc illuminated on the VSI indicating that we should maintain level flight or descend to avoid the target.
Level at 10,000 we visually acquired the traffic ahead, a twin engine Cessna 421. As the traffic passed by, the TCAS announced “clear of conflict” once again and two other targets appeared on the TCAS screen, both below us, one crossing right to left and the other crossing left to right, both less than 1000 feet below us. I wondered if they were aware of each other. With the overhead traffic now behind us, we happily continued our climb out of what is almost always a very busy environment on a nice Saturday afternoon. Once above 18,000 we were once again in no man’s land for small planes enjoying visual flight and could once again relax a bit.
Clearly, TCAS was a tremendously important tool to us on this flight. The ATC controller watching over our flight was responsible for separating us from other traffic in the area, the Captain and I were also responsible for separation and were paying close attention to local traffic, and I’m sure the other pilots involved were as well. That said, TCAS provided a level of safety and protection for our flight that was not available in 1978. With respect to the PSA accident over San Diego, TCAS would almost certainly have prevented the collision. While I am saddened by the fact that it took a death count to bring about this change, I am hopeful that those who lost loved ones in this accident and other like it gain some solace in knowing that the death of those they cared for may very likely have saved the lives of thousands.
Posted by APC at 12:28 PM