Why did the pilots in Top Gun: Maverick invert at the crest of the mountain before descending?

Question

In Top Gun: Maverick, the pilots plan to perform a bombing run in the valley of a craterous mountain range in F/A-18s. At the crest of the first side of the crater, the pilots were instructed to roll to an inverted attitude, start their descent, and, then, roll back over. The in-movie explanation was something like "to allow you to maintain the lowest possible altitude", which was of concern to avoid SAM targeting.

The maneuver looked very graceful and cool, but is there any real-world merit to flying this way? Maybe you can descend more responsively this way?

I drew a little animation that may demonstrate better what I'm explaining. (F-18 credit)

Answer 1

It’s to maintain a positive loading throughout the maneuver. The F/A-18E is load limited to +7.5G/-3.0G, depending on loadout and flight operations. With a designed in factor of safety of 1.5, the airplane could potentially experience structural damage at negative load factors in excess of -4.5Gs. That’s not going to be acceptable in order to hug the terrain over a mountain ridge after a 30 or 40° pitch up to hug the terrain, causing Mav’s F-18 to go sailing right over the target, still climbing, and straight into enemy air defenses.

And then there’s the pilot - the meat servo at the controls. High negative G pushovers are quite painful. Blood rushes into the brain causing redouts and potentially could damage the ultrafine capillaries of the cerebral vascular system. Combat is definitely not a place where you want to have a intracranial aneurysm. In addition, a hard negative G pushover followed by a hard positive G pull can result in a GLOC (G induced Loss of Consciousness) during the positive G pullup as your blood goes from being smashed up in your head to rushing into your abdomen and extremities.

And that’s probably about the only realistic thing I can think of in regards to that whole attack run.

Answer 2

Human negative G limits for pushing the nose down are about 3G, vs. maybe 9G positive G limit for pulling the nose up (or downward while inverted), very roughly, and as other answers explain, negative Gs right before positive Gs could be even worse. Airframe design limits for the F/A-18 are about -3 .. +7.5 G, per @max's answer.

This is how real fighter pilots fly when following terrain, even when it's less dramatic than a mountain peak. When cresting a smaller ridge, you might not roll all the way inverted, but rolling to 90 degrees as you approach the crest removes the vertical component of lift. And rolling past lets you pull some Gs to get the nose down while turning some. (Or just go fully inverted if you want to keep your heading while dropping the nose by any significant angle.)

C.W. "Mover" Lemoine flew the F/A-18 for the US navy (and other jets for the USAF). On his youtube channel, he posted cockpit video from his first low-level training flight (looking forward over his shoulder), adding voiceover to talk through what was going on and why.

[4:21] ... [my wingman was] very aggressive in doing the ridge-crossing and stuff which you just saw. Instead of bunting, instead of pushing negative G, you roll to get the positive G on the aircraft so you're not pushing over every time you cross a ridge.

On another ridge, at 6:13, he rolls all the way inverted to pull the nose down after crossing the ridge to see how much drop off there actually is. That one didn't involve much of a change of heading, unlike some of the intentional turns at ridges in the training flight (which is another reason not to roll fully inverted).

This was his first low-level training flight doing stuff like this close to terrain. He says at the start that he stayed a bit high while getting comfortable with the whole thing. At 17:52, he rolls to about 120 or 135 degrees: getting to about 90 before reaching a ridge, continuing the roll a bit more after he can see past it. And he's low enough to see details on trees before cresting the ridge.

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If staying tight to the terrain over one specific big crest was critical to a real mission, you likely would roll inverted before the crest. (Unless it was a really wide round curved hilltop, not a ridge at all. If pushing the nose over to follow the curve would only leave the pilot feeling between 0 and 1 G, they might stay upright.)

I haven't seen Top Gun: Maverick yet, but if they only roll inverted at the crest, after pushing the nose down to horizontal, that's probably a movie mistake unless that part happened gradually. Like a shallow curve and then a steep drop.

I'd guess that if you'd scoped out the terrain for a specific ridge and practiced in a simulator, you could be much more aggressive. But if in real life you're just shooting footage for a movie, you're not going to risk your life more than usual. So it makes sense that what you see in the movie is rolling and pulling down at the crest of the ridge, not much before. Assuming that shot was 100% practical, and actually done close to terrain, not a separate shot of a jet composited onto a shot following terrain.

Answer 3

Positive G vs Negative G.

The pilot and aircraft can sustain greater pos G than neg. This would allow them follow a sharper arc going over the top, and stay closer to the landscape.

Of course, an actual fighter pilot will have more insight into this.

Answer 4

Virtually every aircraft is designed with a positive-G limit that is between 2 and 3 times the negative G-limit. Most FA-18 models have a positive G limit of 7.5G and a negative G limit of -3.0G.

For comparison, a typical light general aviation aircraft like a Cessna 182 will have an envelope of +3.8G to -1.52G for what are called "normal category" maneuvers. Some, but certainly not all civilian aircraft have an additional approval for specific maneuvers between +4.4G and -1.76G, but only under a more limited set of weights. A 1,500 pound plane puts the same stress on the wing and tail structures at 4G as a 2,000 pound airplane places on them at 3G. (6,000 pounds of force in both cases.)

These limits are used as part of the design process, so the aircraft's elevator is designed with more nose-up authority and less nose-down. And further, when the pilot is trying to get into a high-G maneuver quickly, he/she wants the lift vector of the wings to be adding to, not fighting against, the entry into the maneuver. So in maximum performance maneuver, a fighter pilot rolls the aircraft into whatever orientation aligns the wing lift vector with the desired performance.

So in the case of a high-G maneuver, it's not about pilot preference or what's "easiest" on the airframe, it's that the aircraft's design may not make a -7G maneuver possible.

( P.S. I don't mean to sound critical, but please keep in mind that on SE there is a difference between an answer that directly answers the question and a comment that provides commentary within the context of the question. I suggest that the other answers provided would have been more suitable for comments.)

Answer 5

Also worthy of mention is that the pilot may have a significantly better view of the terrain while inverted.

Answer 6

Inverting gives you two free negative G's

Take a Southwest 737 in cruise. It has 1 gee operating on it from regular old gravity. Then it has another gee from the normal trim of its wings.

These cancel out and it holds altitude.

Now if the aircraft inverted while doing their best to sustain "perceived" 1.0 gee in the cabin, those two gee-forces would stack instead of cancel out. Thus giving a -2 gee downward force, whilst in the cabin, the passengers would feel the entirely normal one gee.

A skilled pilot might not even spill the passengers' drinks.

Cresting the peak of a hill, you need negative gee's to keep hugging the terrain. Imagine you have been climbing the hill at 3 gees and need -5 gees to crest the hill, then back to plus 3 gees to hug terrain. If you make that extreme transition from +3 to -5 in level flight, you'll have extreme change of acceleration, or colloqually "jerk", which will break the airplane and the pilot.

But if you simply invert, you get a free -2 gee and you simply continue pulling 3 gees, netting out to -5 gees. It's a smooth transition with no jerk and no disorientation.

Probably not a perfect "aileron roll at 3 gees" because of terrain... but you are adjusting in a limited range of say 1.5 to 3.5 gees, which is manageable and you do it all the time as a fighter pilot.

A Youtube channel called Grim Reapers regularly simulates aerial feats, real or hypothetical, relating to recent wars, and often penetrating first-rate air defense networks such as Aegis, S-400 or Pantsir. They regularly make use of this technique. On one fictional helicopter raid, all were picked off by the S-400 system except one pilot who made great use of this technique. With a helicopter.

Answer 7

Negative Gz's are dangerous and Navy pilots are trained to reduce or eliminate negative Gz's. Look up the push-pull effect and you will understand why they went inverted over the ridge-line in order to accomplish the mission set.