For many of us, the road we follow is deliberate but sometimes three forces play into life’s outcomes: luck, choice, or fate. This is likely truer in aviation than in other vocations, at least if you stay long enough. And that is certainly the case with me and a near fatal event during an F-16 Radar Terrain Following test flying over the Gulf of Mexico when the jet suffered an ‘anomaly’ in the flight control system and pushed hard over when flying at 600 knots and 100 feet above the water.
The newest and most impressive version of the F-16 was the E/F model, Block 60 in F-16 parlance. No longer the lightweight fighter; this version built exclusively for the United Arab Emirates (UAE) Air Force was a monster compared to legacy F-16 models. It had the biggest F-16 engine yet at 32,000 lbs. of thrust, conformal fuel tanks, an Active Electronically Scanned Array (AESA), an incredible electronic warfare suite, new cockpit, Gucci flight control systems and more. It was a beast, developed by Lockheed Martin (LM) engineers and test flown exclusively by LM test pilots, without any US government ‘help’. We flew mostly from the factory location in Fort Worth, Texas but had a semi-permanent detachment at Holloman Air Force Base (HMN) in New Mexico to fly those tests that could not be conducted in the Texas airspace. We built three identical 2-seat F models as test jets, with all the flight test instrumentation needed to complete the program testing.
Radar Terrain Following
The F-16 E/F was developed to support the threat faced by the UAE Air Force. Radar Terrain Following (RTF) was required for one of the mission sets of this customer. While low level tactics were no longer flown by western air forces, the very peculiar threat landscape of the Middle East warranted this capability for the Block 60 fleet. Terrain Following (TF) had been developed for the General Dynamics F-111 and the F-16 Block 40/42 Low Altitude Navigation and Targeting InfraRed for Night (LANTIRN) system in the distant past (as depicted in Schematic 1). Incorporating this capability with the AESA APG-80 was to be a technical challenge and not merely adding a legacy capability.
Schematic 1: Radar Terrain Following
Based on the testing experience gained during the legacy TF systems, the RTF phase was scoped to be a comprehensive and thorough effort. In a commercially focused test program, efficiency is directly linked to profits and attention is placed on completing the program as quickly and efficiently as possible. Time is money in a commercial program; different than the luxury of time often allowed in military programs. While there was pressure to trim the RTF test phase and test points, the Lead Engineer and his team ensured that all required test conditions were included in the test plan for this phase of development.
Radar Terrain Following included both a manual system (to be hand flown by the pilot) and an automatic capability (flown hands-off linked to the Block 60 autopilot / autothrottle) at 1000 down to 100 feet above ground and as fast as 600 miles per hour.
RTF Test Progression
RTF development with the APG-80 progressed remarkably well considering the technical challenge of incorporating an AESA versus a legacy radar system. Testing profiles for were completed in the Manual mode, Automatic mode, at all altitude settings and airspeeds without any major incidents or technical difficulty. RTF testing over the many terrain differences, from flat desert, hilly terrain, aggressive mountains, against rock cliffs, and over water were all flown with behavior as expected. One last set of test points, called ‘dwell points’ designed to be flown at max airspeed, minimum altitude remained. Build-up test points were included leading to a final test point of 600 Knots Calibrated Air Speed (KCAS), 100 feet Above Sea Level (ft ASL), non-turning for a sustained period of time (hence the term ‘dwell’). The intent was to stabilize at down low where the RTF system would have to look forward at a very low grazing angle at very high speed.
The 2-seat F model test aircraft had been located at the detachment site at Holloman AFB, NM. The flight profile was to transit high altitude from New Mexico, flying over the state of Texas down to the Gulf of Mexico, descend to open airspace and conduct the testing more than 12 miles offshore in international waters, then landing at Ellington Field near Houston, Texas for refueling before ferrying the jet back to HMN once complete. The large fuel capacity of Block 60 with conformal plus external fuel tanks supported the flexibility of being able to travel long distances to find the needed airspace for certain peculiar test points. There was an added bonus for me. My family lived in Houston, and I used to commute home back and forth on weekends after flying / work was done. Landing in Houston midweek allowed me to have lunch with my youngest son, spoil him with food from the airport where we landed and to hang out for a bit.
The F-16F was flown beyond 12 NM of the Texas coast into open airspace and descended VFR to begin the long, straight test run. As per normal operations, a series of RTF engagement safety checks was completed to ensure that the system fail-safes had been exercised. Once the forward flight path had been cleared of obstacles, initial test runs were flown to prepare for the last test runs. We then descended to 100 ft ASL and accelerated to 540 KCAS. Once content at 540 KCAS, we accelerated to the final test point at 600 KCAS, 100 ft ASL with the Automatic mode engaged intended to allow the terrain following system to dwell at this condition. I looked ahead to clear the flight path for the non-turning run and noticed 2 shrimp boats to the right of the aircraft flight path. I had hit a big hawk in the front of the canopy at 540 KCAS earlier in the Block 60 test program and was conscious of the bird strike risk from the gulls that circle behind fishing boats as they troll in open waters (see photo). I moved my right hand and placed it behind the sidestick in case I needed to take control and avoid a bird strike. Normally, my right hand might have been writing down test notes and data during the test run or manipulating cockpit screen settings for the RTF test. In this case, the hand was set at the sidestick ready to take control if needed.
Shrimp Fishing Boat
What we felt was the aircraft pulling away from our seats as an uncommanded full nose down pushover by the flight control system in the Auto TF mode was initiated. The aircraft was driving itself towards the water without any human interaction from 100 feet. I recall diving towards the water then grabbing the sidestick, pulling back to arrest the descent and climbing away from the water. I then disconnected the autopilot which had been engaged with Auto TF. The discussion between cockpits was pretty short. We had no idea what had happened and were not going to stick around in case something else bit us. The mission was terminated, we climbed up out of the low-level environment, slowed down, contacted Air Traffic Control and recovered uneventfully into Ellington Field. Once on the ground, we contacted our test team at Holloman AFB and then the large engineering team at the ‘Mothership’ in Fort Worth, TX to discuss the next steps. After many hours of deliberations, it was decided that the best way to get the aircraft and radar data to the test team to be analyzed was to fly the aircraft back to HMN without the use of autopilot, and have the data extracted and expedited back to Fort Worth to be analyzed.
BTW, the lunch with my son was not nearly as much fun as normally was the case. We were so busy with phone calls and trying to understand what had just happened to our jet that there was a lot less time to spoil him with Skittles, chips and junk food. I did get a photo of him in the cockpit though.
F-16 Father / Son Hero Shot
Once data had been retrieved, the analysis was focused on determining why the aircraft command had been full nose down. The day of the test event there had been no wind and the water over the Gulf of Mexico was dead, flat calm, the equivalent of a 3-sigma glass-water day. With the extreme speed and low grazing angle based on the very low altitude, the forward radar energy bounced forward off the mirror glass water surface and not back to the aircraft as per normal. Without radar energy, an anomaly in the gap filling logic took the Radalt data point, measured at 93 ft AGL and then anticipated subsequent points, drawn at the maximum nose down angle that the radar could measure which was 55 degrees nose down. That logic anomaly calculated the next data point to be -4100 feet below the sea at 1100 feet in front of the aircraft and the next data point past that further down the -55 degree line. The aircraft was commanded to follow these points, hence the maximum nose down push to join the profile.
The solution was to fit the gap filling logic such that when there were no forward-looking radar measurements, the logic would use Radalt return data and project forward the profile for TF to follow until forward radar returns could be read. Ignoring ‘No Measurement’ data allowed the logic to use the near-range Radalt data to build the profile until radar returns were measured and integrated into the profile.
According to the system design, when the aircraft crossed the 75% of our set altitude threshold (called the Set Clearance Plane or SCP), a fly-up would have been triggered. At the instant of the uncommanded pushover, the aircraft was actually at 93 ft ASL which meant that the 75% SCP threshold was at 84 ft ASL. Crossing that altitude would have triggered a +3 additive G command nose up. Theoretically, the aircraft would have recovered on its own without actually impacting the water. A desktop computer calculation of the recovery profile estimated that the aircraft would have bottomed out at 40 feet. This incident could not be duplicated in the simulator and that profile was never matched or certainly not flown for real. Dynamics of a real recovery with increasing negative G at the crossover altitude, arresting the downward motion, correcting it (-1G + +3G = + 2G) would have begun the recovery. However, it remains unproven if the nose down motion could have been arrested and the aircraft recovered before impact. I have yet to be convinced that we would not have hit the water on that day.
- Perfect is the Enemy of Good
Engineers are always accused of insisting on developing the 98% solution, wasting time and resources, when an 86% solution would be good enough. In the Block 60 RTF test plan, our engineers had fought to preserve the thoroughness and completeness of the test plan. They were unwilling to sacrifice seemingly odd endpoint testing, arguing that these extremes were needed to potentially ferret out issues. As was the case in my event, intentional build-up test points were added to drive the aircraft to unique conditions where the RTF system would be stressed. If problems were hidden and dormant in the system, these end points might lead to potential discoveries. The test team did not actually know that something would happen flying at 600 KCAS, 100 feet with Auto TF, but they did realize that there needed to be a dwell test point at these end conditions to allow the system to function and potentially expose a malfunction. In the end, they were correct, a true failure point lay dormant.
What are the consequences of not finding issues during testing? For so many programs and test phases, ‘good enough’ really is. In this case, had the test team not fought and won the argument to include the high speed, minimum altitude Dwell end point, the Auto RTF capability would have been released to the fleet and one day in the future, with a pilot flying alone at night, over the water, the aircraft would have dove into the ocean, the pilot killed, and the wreckage potentially never found. The ‘What If’ in this case would have been a dead pilot. So, when program managers argue that ‘Good Enough Is’, we point to examples like the 600 knots, 100 feet Dwell point to show that ‘Good Enough Isn’t’. Perfection really isn’t the enemy of good after all.
Lesson 1: Good Enough Isn’t
At our disposal in this day and age are the most sophisticated engineering tools ever. We have extraordinary computing power, incredibly capable system integration labs (SIL) and vehicle system integration facilities (VSIF) which put all our aircraft systems together on the ground in simulation facilities that almost exactly predict what we will see airborne. In most cases, feeding back data from actual test flights allows us to enhance our tools even more as flight testing progresses. But simulation will always be just that…simulation and not the real thing. Simulation can never be the same as flying in real air…’God’s Wind Tunnel’ as I call it.
In the case of Block 60 RTF, we sent a very experienced, capable test crew to drive the aircraft to its end points. We wrung out the aircraft at its extremes within the margins of a well-run test phase. What was not done was unleashing an advanced capability to an operational pilot to be flown in operational conditions before all corners of the flight envelope had been evaluated. The Auto TF system was meant for nighttime use, low over the desert or water, with pilots flying alone and completely relying on the TF system to keep them safe.
What could have happened? Our engineers spent a long-time post-event investigating the root cause of the negative G pushover and the near fatality. What did not happen was a recall years later to give testimony at a fatal crash linked to a hard over negative G pushover to impact which would have occurred at some point in the operational life of the Block 60 fleet. The real lesson is that, in spite of amazingly capable engineering tools, test pilots still have to fly in real conditions to ensure the robustness of the capabilities that we unleash to the operational world. It will be some time yet before test pilots are no longer needed.
Lesson 2: We still need to fly and test in God’s Wind Tunnel
The Block 60 RTF system was so smooth and so well developed I used to say that it was like riding the Monorail at Disneyland (see photo). In every test program, so much attention to risk is focused at the beginning of testing. As time and testing progresses, we gain confidence in our methods and our own understanding of the system. While we don’t become complacent, we certainly become more familiar and comfortable, even in high-risk testing. In this case of the hard over push incident, my focus was on the shrimp boats ahead and the risk of a bird strike. I was lucky. I chose to move my hand behind the side stick because of the bird risk and a previous bird strike during RTF testing. On many other occasions in RTF testing, I had been writing test notes, changing radar system selections, tracking air-to-air targets, and manipulating cockpit screens. This time, at the right time, my hand was where it needed to be to save us.
We were not complacent, nor cavalier. We understood the environment that we were testing in but clearly, I made the right choice; and also, luck played in our favor. The incident reminded me that the risks remain until the very end of testing, just as they were at the beginning and all along. A test flight is not finished until the aircraft is back on the ground, parked on the ramp and the aircraft shut down. There are so many stories where the tests had been completed, the aircraft returned to base only to have a major event happen on landing. Diligence is the key; maintaining it is the difficult element. Reminding ourselves always that testing demands staying focused right until the end of the marathon is paramount. In the oft-quoted pop-culture phrase of baseball great Yogi Berra, former New York Yankees catcher, manager, coach, 18 time all-star, and 10-time World Series winner: “It Ain’t Over Till It’s Over”.
Lesson 3: It Ain’t Over Till It’s Over
- Passing on the Message
Most professions have some form of continuing education that strives to educate peer groups on the technological advancements in their professions and in some cases, the lessons that have been learned along the way. Test pilots do the same; meeting every year to tell the tales of what went wrong in our test programs. In spite of these efforts, and the very professional briefings that are given, we seem to fail at cementing the lessons with our peer groups. The same mistakes are being made, time after time, year after year.
The popular Apple TV+ Series “Ted Lasso” often used quirky tales from the characters to pass on valuable lessons of life and leadership. In one episode, Ted Lasso as coach tells one of his star players, who is frustrated by his play, that he should be like a Goldfish, which apparently has a 10 second memory. The implication is that if the soccer player can forget all the bad plays from earlier, he won’t get bogged down in a bad mental state. We need to take on the lessons from all the other test programs before us and never forget what has been learned before our time. Whatever the forum, it is clear that lessons learned are not being cemented in the minds of our audiences and we must strive to be more compelling in our story telling so that this trend changes course. We need to remember the lessons told from other test programs and be just the opposite of what Ted Lasso says…Don’t be a Goldfish.
As I wrote in the beginning, our lives so often are governed by luck, choice, or fate. In this case, our lives were spared by my decision to move my hand behind the sidestick because of the bird strike risk…or maybe it was luck?
What is the lesson to be gained from this and why waste everyone’s time in this tale with just another pilot “There I Was” story? It applies to all of us, almost every day. Our good fortunes in life don’t just happen because we study hard, focus on being good at our jobs and live the good life. Sometimes, we succeed and survive because we make the right choices at the right time. We learn from previous events and apply the lessons to the next scenario. I moved my hand behind the sidestick during the high speed, low altitude test because I had already had a bird strike on my F-16 canopy earlier in this test program. I learned a lesson and applied it to the next similar scenario.
Sometimes we are just lucky, and we do the right thing at the right time. We need to be humble enough to appreciate being fortunate that things go our way every now and then. One of NASA’s great test pilots, Bill Dana, last man to fly the X-15 rocket plane, used to say, “take your job seriously, never take yourself seriously”. We should remember that more often. Sometimes it is better to be lucky than good.