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saberrider

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Unread post09 Jul 2018, 13:44

https://m.facebook.com/story.php?story_ ... 5972190263 Gums and other real pilots are welcomed to give a reason for this .
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magnum4469

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Unread post09 Jul 2018, 19:25

:wtf: Looks like he landed too fast, over aerobraked, exceeded 15 degrees AOA and scraped the ventrals, speed brakes, and stab. I would bet he also dinged up the turkey feathers too... Not smart, :bang: the airplane will fly again but will have a lot of damage to repair and cost some big bucks!! :doh: :doh: :doh:
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saberrider

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Unread post10 Jul 2018, 04:28

But is not hard to go past 15 AoA in landing configuration ? You are not limited by FLCS ? I mean why he is wallow after touchdown .Gust or wet runaway ?
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magnum4469

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Unread post10 Jul 2018, 17:05

I watched it over again and paused during multiple areas. He clearly was way too fast, thus looks like he flew it onto the runway. Too much speed he creates excessive sink rate yet still carries too much speed to stay on the ground, trying to arrest sink rate he exceeds 15 degrees AOA. Basically getting himself into a landing PIO trying to keep it on the ground. You can see if you pause and restart video that his main gear come off the ground at least 4 times while he is trying to land. This is when he takes out the ventrals, speed brakes and striked the right stab. I can't tell from video but I would bet he also struck the turkey feathers. As far as a wet runway would not have caused this, and neither would a gust. You can tell by the approach that starts out flat then dives towards the runway that he was carring way to much speed.
I believe it must have been a new pilot, the F-16 gives you so many tools, one of which is while in the landing configuration your HUD displays a landing staple(AOA) by the flight path marker. I believe it is 11 to 15 degrees all you have to do is keep the flight path marker in the middle for the perfect approach speed. There is no math in public to do, it computes AOA based on flight computer imputs and will give you the correct AOA to fly regardless of fuel and stores on board.
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35_aoa

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Unread post11 Jul 2018, 02:03

Under optimum landing conditions, yes, it isn't difficult to not exceed 15 AoA. But that "15 AoA" limit doesn't take into account extra strut/main gear compression during excessive sink rate touchdowns. In other words, if you flare too high, and don't catch the sink with timely power addition, you could scrape the boards at 15 AoA or possibly even slightly less if it were really heinous. Or in that scenario, if you just reefed back on the stick to try and catch the sink without adding power, the effect would be similar......still excessive sink rate, plus you also rotated past 15 degrees most likely. Both are ways to do this. Additionally, if you are ham fisted in your aerobrake (i.e. you over rotate), there is another opportunity to scrape things.....ive seen it done badly enough to scrape the tailpipe.

Without seeing the HUD or whatever, it is hard to say with certainty, but my guess is that this guy over rotated in the flare to catch a settle (kinda hard to see since the camera angle takes the horizon/runway out of view for a few seconds), and exceeded a full slow/15 AoA in catching it......ie the second scenario I mentioned. The first impact was coincident with his touchdown, so I don't think it is the last one I talked to, over rotating in aero brake (unless maybe he thought he was already on the ground when he wasn't).

Edit: I think magnum is probably on the money. To me it is hard to tell if he is definitively "too fast", the Viper does have a pretty fast approach speed if there is much fuel remaining, but I agree it looks "flat" in terms of attitude at the beginning, which would tend to suggest fast/not 11-13 AoA.
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saberrider

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Unread post11 Jul 2018, 06:00

35 aoa What is sync rate for F18 and F16 ?
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saberrider

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Unread post11 Jul 2018, 06:07

But exceed 15 * AoA immediately after touchdown will get airborne quickly don't you ?Looks to me like he has @15 *AoA before touching the ground
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35_aoa

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Unread post11 Jul 2018, 07:34

saberrider wrote:35 aoa What is sync rate for F18 and F16 ?


I'm not really sure what you are asking here?
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35_aoa

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Unread post11 Jul 2018, 07:35

saberrider wrote:But exceed 15 * AoA immediately after touchdown will get airborne quickly don't you ?Looks to me like he has @15 *AoA before touching the ground
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Not if the engine is at idle or something less than required to overcome physics :|
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saberrider

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Unread post11 Jul 2018, 08:32

35_aoa wrote:
saberrider wrote:35 aoa What is sync rate for F18 and F16 ?


I'm not really sure what you are asking here?

What VVI number is showing ( feet per minute) when F16 and F 18 glides @ 3 degree's glide slope in landing configuration?
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magnum4469

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Unread post11 Jul 2018, 19:49

Saberrider,
Disclaimer, the number I'm using are not exact, just an example to help explain.
There is not set in stone VVI for a 3 degree glide slope, it is all based on speed. Think of it this way, a C-172 will travel on final around 70kias and if there is a headwind component of 20kias his track over the ground will be 50kias. Now to maintain a 3 degree glide slope while traveling 50kias ground speed since the C-172 is not traveling that fast over the ground his VVI will not need to be very high, maybe 150fpm.
Now lets take a clean F-16 with 1000lbs of fuel, they are not flying an airspeed per say but an AOA usually 11-13AOA. Lets just say that is approx 155kias and with that same 20kias headwind component his ground speed would be 135kias. Now to maintian that 3 degree glide slope since he is traveling approx 2.5 times as fast as the C-172 he would need somewhere around 500fpm VVI.
So lets now look at a fully loaded F-16, two tanks, 4 AAM, ECM pod, lightning pod, TERS, and 1000lbs fuel. Lets just say that it is 6,000lbs of extra weight. Rule of thumb I believe is 5 kias for every 1000lbs. 6X5=30Kias so 155+30 is 185 but minus that 20 kias headwind componet give the speed over the gound 165Kias. With that increase to maintain a 3 degree glide slope he will need 550-600fpm.
And in the last example lets take the same full loaded jet but get rid of the headwind, now his ground speed is 185kias, in order to maintain that same 3 degree glide slope since he is faster yet he will need a higher vvi around 700 fpm.

Here is the catch, to land a F-16 you need to touch down around 11-13AOA while on glide path, anything faster and it will not stay on the ground and get airborne and bounce, anything slower and you will do damage to the jet.

Don't confuse VVI with sink, the faster you go down the glide slope the higher your VVI will be but you are still only on a 3 degree glide slope, excessive sink would be if you dipped below the glide slope.

Also AOA gets a lot of people confused, it is the angle between the chord line and the relative wind.

So to land a F-16 the pilot will fly 11-13AOA while maintaining a 3 degree glide slope until he gets to the flare. There is no set speed or VVI, as these will alway change with aircraft weights and wind condition. In the visual would looking to touch down 500ft down the runway.

As for that video the pilot was way too fast, as the aircraft became airborne at least 3 more times before finally staying on the ground.
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saberrider

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Unread post12 Jul 2018, 05:48

Wow ,thanks man for details , I believed you must have always the same SINK rate approx. the same VVI on final in and 11 AOA. But in F16 set for landing configuration and trimmed for 1 G the speed reduction until 11AoA don't give perfect speed /VVIvfor that specific gross weight?
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35_aoa

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Unread post12 Jul 2018, 06:30

What magnum said is primarily true. VSI (or VVI) varies in order to fly a set glideslope, but that variance is normally mostly due to winds......with the caveat that if you are at a higher gross weight, or landing at a higher density altitude field, your increased approach speed certainly factors in as well. If you have a stiff headwind or decreased airspeed to maintain AoA at a lower weight, you will fly a lesser VVI to maintain that glideslope......in other words, if you are making less speed over the ground due to headwind, then holding the same VVI you would use with no headwind would result in going below target glideslope (for comparison sake, say the aircraft is at the same weight and density altitude in both cases). You are approaching the runway at a slower rate, so your descent rate must be lower too if that makes any sense. Opposite is true for a tailwind (more groundspeed), or if you are heavy flying a faster than normal "on speed"(same AoA but with increased lift requirement, i.e. more speed) approach.

AoA vs IAS comparison is related to your weight. If heavy, you are flying faster while "on speed" than you are when light. Approach speeds vary significantly depending on gross weight. As an example, a heavy Rhino might be 140-150 knots on speed while a lightly loaded/typical landing weight Rhino will be more like 120-130 knots. Like I mentioned, in a same winds comparison, you need higher VVI at heavier weight to maintain the same GS than you did at a lighter weight.

I don't really recall what a typical VSI/VVI was in a viper, but once you enter the flare, I seem to recall that 500 fpm sink rate at touchdown was about as much as you could have prior to causing damage to the undercarriage, effectively not flaring......most of my landings were much less than that. Could be wrong on that specific number, it has been a while now.

I get what magnum is saying about sink versus VVI, but ultimately they are the same thing. In his example of flying down through glideslope, you are flying a higher VVI than needed to maintain GS, and that is an unnecessary sink rate. But as a counter example, if you need to maintain a 1000 fpm descent rate to maintain a 3 deg glideslope due to for example, a tremendous tailwind, while you may be on a reasonable 3 deg GS, if you touch down in a viper at 1000 fpm descent rate, you will cause damage. This is of course fixed in the flare, even in that extreme scenario. I think this is where magnum might be talking to.

Hopefully I made that as confusing as possible :)
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Unread post13 Jul 2018, 05:17

I'm trying to figure out when and where this happened? Is the aircraft Greek? Is this recent? Anyone know? :bang:

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