Supersonic Flight by Gregory Matte

by BGen (Ret’d) GCP, CD, PhD

Supersonic Flight

I’ve been asked on numerous occasions to relate my experiences in the supersonic flight regime, in part based on the story I shared at the 2019 Wings Awards Banquet about flying supersonic at 100’ AGL. In fact, many if not most of the non-aviators that I took up for a backseat ride in the CF-18 asked to experience breaking the sound barrier. Spoiler alert… for most it was a non-event as they didn’t feel a thing, so I had to direct their eyes to the Mach meter so that they could see for themselves that they were hurtling through the sky faster than the speed of sound. Prior to Chuck Yeager’s flight in the Bell X-1 on 26 April 1948, many believed that supersonic flight was impossible.

The reason for this belief was the underlying physics. While air is far less dense then water or hydraulic fluid, it too can be compressed to the point of incompressibility…a barrier that would destroy any plane that attempted to pass through it. However, at altitude the air is far less dense than at sea level. When Yeager broke the sound barrier for the first time, it was in the stratosphere at 45,000’ MSL. At lower altitudes, the air becomes increasingly dense, and the “experience” of going supersonic much different. Violent might be one of several adjectives that I would use, having once taken the CF-18 supersonic at 100’ AGL (approximately 2000’ MSL) over the Cold Lake Air Weapons Range (CLAWR).

While the CF-18 was designed for supersonic flight, with an upper limit of Mach 1.8 (which I explored on a separate occasion), my flight that day in May 1988 was an experience that I would never forget, nor repeat! To put things in perspective, the speed of sound (aka Mach 1) at 2000’ MSL on a standard day (15 degrees Celsius) is about 1100 feet per second, or 760 miles per hour. By way of a comparison, driving down the Pat Bay Highway at 122 kilometers per hour would equate to 76 miles per hour…one tenth the speed of sound. As you might imagine, the sense of speed as one gets closer and closer to the earth seems to increase exponentially. Flying supersonic at 100 feet above the treetops feels VERY fast. Naturally, the margin of error for a slight dip from level flight is negligible, so it’s important to focus on maintaining level flight.

To fly faster than the speed of sound at low altitude, I started at about 20,000’ MSL where it’s easier for the Hornet to go super, than pushed the nose over and dove towards the earth in full afterburner while maintaining about 1.1 Mach, as I was concerned that I might slow back into the transonic regime as the air density increased. My plan worked, and a carefully pulled out of the descent as I transitioned below 1000’ AGL while remaining supersonic, leveling out at 100’ AGL (about 70’ AGL on my radar altimeter, given the trees). Fortunately, the terrain is relatively flat in the weapons range, so level flight was relatively easy to maintain…if it wasn’t for the thermals as my flight path criss-crossed over the cold water of the various lakes and the warmer land. But I’ll come back to that unexpected part of the “experience”.

While the total duration of my remaining super at 100’ AGL, it only lasted about 45 seconds, but that was as long as I could stand it, as I thought that at any moment the plane might suddenly

disintegrate out from under me do to the stress on the fuselage, wings and twin vertical stabilators. As I said early, “violent” is probably the best adjective I could use to describe the experience. In addition to disintegration, I was also concerned about the engines exceeding the placard limit of 852 degrees Celsius for exhaust gas temperatures EGT), as this could lead to engine fires or explosion.

What caught my attention first once I leveled out above the treetops was the manner in which the aircraft felt like it was struggling, as if was pulling a heavy weight behind it on a two rope. The plane seemed to almost be yawing slightly due to the minor asymmetries of airflow around the aircraft and the external pylons. The sound of the air rushing by the canopy was deafening. Then the wings caught my eye through my peripheral vision. Recall that the wings on the CF-18 are folding so that they can be closely parked together on board an aircraft carrier. I could see that the wings tips beyond the wing fold were flapping up and down by a foot or two at about 60 hertz. Was it possible for the wing lock to release due to the strain? Then I noticed the vertical stabilators in my mirrors. They were oscillating side to side at about 120 hertz.

Then there were the thermals. Ever thermal caused a sudden and significant bounce. If it wasn’t for the 5-point harness (including the negative G strap), I probably would have cracked the canopy with my helmet as I bounced with each thermal in my seat. A quick glance at the EGT gauges indicated that the engines were running hot and getting too close for comfort.

Although this “experience” only lasted 45 seconds at the most, by this time my “spider sense” was telling me that maybe this wasn’t the best idea. I gently pulled away from “terra firma” to reduce the strain on the aircraft while slowing my speed, then came out of afterburners. Once I returned to a safer subsonic speed with a few thousand feet of air under me, I suddenly realized that my body was completely tensed up and my hands were trembling. As you might expect, I had a recurring dream for some time thereafter wherein I was flying low level across an open field with a stand of trees at the far end of the field. Each dream ended with the realization that I waited to late to pull up and was destined to crash through the trees. Maybe it was my subconscious reminding me of the danger of low-level flight, and the foolishness of flying supersonic at 100’ AGL.

We have a saying in the fighter pilot world. There are bold pilots and there are old pilots, but there are no old, bold pilots. We have another saying as well…I’d rather by lucky than good. In my case I was lucky that I survived my bold phase to both enjoy many more years of flying the Hornet and to eventually become the “old man”, as the Wing Commander of 4 Wing Cold Lake at the ripe old age of 41.