The backdraft (afterdraft?) of a fighter jet

Factual Questions
1

OK, I have weird dreams and they induce weird questions. (I only post a fraction of them.)

Say I lived on a wide open plain with no close neighbors and I had a friend in the armed services who flew a fighter jet. Say that, as a prank, he buzzed my home.

  1. How low to the ground (and house) can a fighter jet fly?
  2. How much backdraft is generated from said jet? Would it tear shingles off the house? Would it do other damage?
  3. Is it indeed backdraft, afterdraft or some other word that I am searching for?
2

The word you are looking for is wake.

It’s severity is mostly related to how heavy the aircraft prodicing it is. It is strongest when the aircraft is near stall. If a fighter jock wanted to do the most damage, he would pull up at maximum G just before he got to the house.

But I don’t really know how strong that would be.

3

I used to fly fighters.

In appropriately cleared territory the USAF rules said we could operate at 100 feet above the ground. At speed. Like >600 mph speed.

Those areas don’t tend to have almost zero houses, they tend to have exacly zero houses.

In areas where there were/are occasional houses, the rules provided we had to keep 1000 feet (~1/4 mile) away, laterally or vertically or some combination. The general FAA rules for civilians are to keep >500 feet distant from people and buildings when in a sparsely populated area. We were stuck with 1000 becasue we were a hell of a lot noisier than a typical Cessna.

As a practical matter, the routes we flew when not over Gov’t-owned totally-house-free ground were set up to minimize even the occasional farmhouse and often had higher altitude restrictions in sections where there were farm houses.

So that’s the legalities.

Now the practicalities …

100 feet above the ground is only doable on really smooth terrain, *a la *Kansas or the Southwest desert. Over really smooth terrain (dry lake bed, water, etc) you could get lower than 100 ft, but at that point it took 100% of your concentration to not hit the surface. Even a moment’s attention to anything else risked tying, yet again, the record for low altitude flight in a fighter. Nobody’s ever broken that record, which stands at zero feet, and nobody since the olden days of very slow fighters has survived the attempt. So we didn’t do that.

With even gently rolling hiills of a couple hundred feet peak to trough you’d be hard-pressed to get much below 3-400’ AGL when cutting *across *the small valleys without hitting the trees on the way in or out.

You can run *along * even a narrow valley a good long way at 100 ft. But with our turn radius, we’d often have to squirt out the top when the valley turned more sharply than we could.
Finally, as to effects on the ground. The term you’re looking for is “wake” or “wake turbulence”. Some guys in my squadron did overfly a campsite at speed at pretty close to zero altitude one day. Tents went everywhere and the camper’s horses apparently didn’t stop running until they got to the next time zone. The Boss was not impressed.

I have standing out on the ground and got buzzed by somebody doing about 450 mph at about 50’ The noise was very impressive, and the wake shook the shit out of the 15’ saplings nearby. The wake made a very loud eeery whistling sound and was a strong gusty wind for 10-15 seconds. But I’ve felt far stronger just before a garden-variety thunderstorm opened up on me.

You can do some searching and come up with videos of low altitude high speed passes over water which include kicking up a bit of a water wake. same idea.

4

Here’s a video of an F-14 low over the water.

5

LSLGuy, how low do the Blue Angels/Thunderbirds/Snowbirds go when they do their spectacular looking low flight passes across the runways? It looks lower than 100’, but they are always a goodly distance away (farther than you think; airports are BIG) so it might be a scale problem.

6

Not an entirely facetious question, but what if you had to sneeze at that altitude? About, what, a half mile would go by while your eyes were closed.

7

I believe things like the “what if I had to sneeze?” scenario is a reason why such low altitude flying is considered risky, hazardous, dangerous, etc.

Don’t forget the exhaust from a jet’s back end is also quite hot. You could potentially start a fire with it, just for some extra mayhem.

8

Obligatory link

9

I’d ballpark it at 200 ft. Next time you see one, or a picture of one, do some amatuer photo analysis: An F-16 is 50’ long and an F/A-18 is 56’ long. They usually seem 3-4 airplane lengths above the ground. Extreme low altitude isn’t necessary for dramatic effect. 200’ at 450+ mph is pretty outrageous compared to what the crowd sees on a daily basis.

Rule One at very low latitude is: no matter what, climb. Think you might sneeze? Climb. Hear the computer announce a possible problem with the jet or some bad guy astern? Climb.

Even at just 500 ft you have 5x as much time for diverted attention before the ground arrives. Once you’re used to it, 500’ is quite leasurely and you have plenty of spare brain cycles for working the radar, looking behind you for bad guys, setting up switches for the attack or for navigation, checking fuel, etc.

Down at 100’, that’s doing your damndest to hide for that last couple minutes = 20 miles while you sneak up on the target. Total focus forward, find the target, & do your worst to it.

The fire is hot, but traveling at around 1000 ft/second it doesn’t put much energy on any given spot.

Standing right behind when we release brakes for takeoff is another matter. In full afterburner, the exhaust at 50’ directly behind was IIRC, some 750 degrees F and going about 400 mph. Not a healthy place to stand.

10

I used to launch my little sailboat at the N end of the runway at National Airport (77.04032 W, 38.86337 N) and it was no fun to be overflown at very low altitude by a jet taking off just as I was sailing out to the river. Anyone who wasn’t an experienced sailor would probably end up in the water.

11

I’ve only seen the Angels once since they went to YA/18s. I’ve got very fond memories of F-4s (and much less fond memories of when they flew Corsairs :mad:).

12

A few anecdotes re wakes:

I think the whistling sounds were the wingtip vortices descending from the aircraft and reaching your postion. The San Diego Aerospace Museum in San Diego is located just below the approach path for runway 27 at Lindbergh Field. Here you can see the the nearly circular museum (the atrium used to be open-air) and to its left a 737 (maybe Alaskan?) on final. During a visit there would be a ten second pause after a flight passed by and then as the vortices came down they would whistle and blow around the inside of the circular atrium for 5 or 6 seconds: heaven! They have since roofed over the opening and it saddens me. There should have been a big placard out there explaining wingtip vortices and the nature of lift, with live demonstrations!

When the opposing solo takes off in the Blue Angels demonstration flight there is a perfect picture of the OP’s scenario. You can see Blue Angel 6 perform a standard Navy departure (;)) at 40’ AGL here. There’s a pretty good amount of spray from downwash and mostly jet blast, but not enough to blow your shingles off; mostly because the jet is simply not in the vicinity for very long!

And my childhood neighbor trained in P-51s. Though not a jet per the OP they still have a little excess HP. One training flight his wingman and he decided it would be interesting to see if they could blow down the freshly stacked hay in a farmer’s fields not too far from the airbase. It turned out that they could: Apply full military power, then put it on the deck and yank it up just before the haystack. They both had good sport of it and then returned to base. Of course the CO knew about it even before they had landed and they were detailed to un-wreak their havoc the next day. I still remember my neighbor telling me that the farmer only said one thing while handing the two of them pitchforks: “Why dintcha bring gloves?”

So after a long day of “farm training” with blistered and bloodied hands the two were treated to a home cooked banquet. He said it was still the best meal of his life.

13

Mythbusters did something similar a season or so back, but with a large-ish sedan, as well as a schoolbus, and identical results.

14

200 feet wouldn’t be legal. It’s 500 feet AGL over non-populated areas and 1000 feet over an open air assemply of people.

15

Airshows are routinely granted TFRs, or Temporary Flight Restrictions, that allow demonstration teams and show aircraft to perform low passes over the show area. Those same TFRs restrict flight by other aircraft into or through the show area.

So the participants are not bound by normal FAA or FAR regs.

16

Air show TFR’s are flight restrictions for pilots to keep them out of the air show flying area.

You’re referring to a waiver. The only reference I could find was on the Blue Angels sight which says they fly at 50 feet AGL. That would certainly be a runway pass. there is no way they are flying over people at that altitude.

17

Found the waiver informationregarding military airshow displays:

F. Flight over Primary Spectator Area. 1) Civilian and Military Aircraft. Other than those described in Table 3-2, flight over the primary spectator area is permitted by single aircraft, aircraft in a single formation, or multiple aircraft in trail when at or above 1,000 feet above the spectators. 2) Military Demonstration Teams. When authorized by AFS-800, military demonstration teams are permitted to fly at a minimum altitude of **500 feet **over the primary spectator area if: a) Flight is nonmaneuvering and straight and level or wings level in a normal climb; and b) The direction of flight is in one direction only—back to front or front to back.

18

That’s only when flying over the crowd though, most flying at an airshow is over the display area not the spectator area.

19

Why are you arguing this? I wasn’t asking about flying over people, I was asking about flying over the runway. Which, as you have helpfully shown us, is done as low as 50’ AGL. So where is the argument??:confused:

20

Well heck fire, I ddin’t even respond to your post. What I posted addresses the thread and the parameters for jet aircraft flying over houses, people, trained whales, etc… don’t take it personal. :wink: