Fatigue life in Aircraft (particularly Fins)

[forthbridge]

HI all

A few questions on this......

1: In the Hawk fleet, will a Reds aircraft have'higher' fatigue for a given number of hours than a 'standard' (non display) aircraft?*
*is the standard flying less harsh on them?
2: Does the 'Standard' (IE non red arrow) Hawk display incur about the same 'fatigue' as a reds Aircraft per hour?

(Being uneducated in this stuff, I assume that as long as an aircraft is flown within it's 'envelope' fatigue should match hours so to speak, although obviously an airframe that gets thrown about is under more stress...)

Now the difficult bit...

3: Not knowing how many Hawks are gone to attrition, but, putting operating costs aside, how is the RAF (and RN and Quinetic) able to field so many Hawks for display - yet - the Tornado (F3 at least) is seemingly getting past it?

(MY memory may be dodgy but IIRC the Phantom was performing right up until retirement? (Probably totally wrong there..! :roll: )

4: Apart from it being cheaper to operate a Hawk that a Tornado - what extra stresses on front line aircraft are incurred compared to the aircraft of the Hawk fleet? Are the hawk generally a better airframe for handling fatigue? Is there something specific in the Tornado design that increases fatigue?

5: Finally (you will be glad to know!!) - if the F3 aircraft are getting worn out,* why is this (apparently) not also the case with the bombers?** Being low level aircraft do they not incur more fatigue? Have they undergone remedial work against this and the F3 has not?
*Do the F3s not just sit at a nice FL350 and shoot the breeze with tanker pilots over the radio?
(IN all seriousness - my assumption is the F3 was/is meant to be a long range interceptor, not designed to get in a turning battle with a Mig29 or whatever, and is it not generally the case that there is less stress on an aircraft at high level?)
**although the bombers are also not taking many starring roles these days...

[Prop Jockey]

A few years ago when I was fortunate to spend a day with the reds I remember discussing fatigue with one of their engineers. They've been pretty pleased with the Hawk and it had only showed up one significant sign of fatigue compared with the a normal service aircraft. I can't remember the details but its in the rear fuselage. The reason for it is that quite a lot of the display is flown with the airbrake extended. The guy I spoke too didn't mention anything else to me.

Cheers

Rich

[ukmil]

it is all down to aircraft design paramters. some are designed from the start with fatigue in mind, the hawk being one.

Being small and carrying small amount of payload, they are easier on the Fatigue. Yes an aero dislay will incure more FI [fatigue index] than say a trainer at valley, but the FI is montitored by the design authority every day. The designers such as BAE also fly their 'test' beds way beyond what the users do, to see when failures occur

when an aircraft such as the F3 is released to service, it's expected fatigue life id measured in FI [fatigue index]. this is a percentage figure, and when it reaches 100 FI, it should be scrapped, as that is when BAE have decided it is no longer safe to fly. However, the BAE test beds are way beyond this, without issues, so they can, extend the life, if the MOD coffs up enough cash.

At least 1/3 of our current F3 fleet are currently flying around on around 4000 hours, and already on aroun 103 FI, so they are past when it was intended for.

the main difference between all, is the intended use. From the start, the GR4 flies low, and pulls very little G force, which is was 'eats' FI.

The F3 was designed as a stand off platform, and was meant to fly high, and long range, and never inteded for dogfighting, so it;s FI was set low.

However, this has proved to be ineffective, as it is often used in close quarterbattles

This is why, doing aeros, eats FI, in say the F3, as its range from 0-100FI is low compared to a hawk, which is designed for hard tight turns.

i hope this has helped

[forthbridge]

Cheers for that - just the explanation I was after! :flying:

[petermcleland]

In the case of the old Venoms, they fitted "G Count Meters" these had little ticking counters that incremented each time a certain G figure was exceeded. There would be counters for 4G, 5G, 6G etc. etc...As our aircraft neared the end of their available "G Counts", they would be relegated to high level cross county flights and similar exercises that used less counts than for instance low level dusk strikes. The worst sortie for adding seriously to an aircraft's "G Counts" was to give it to me for a solo aerobatic display.

This got fairly serious towards the end and they could not afford the "G Counts" for me to practice...So I had to do my practices in the Vampire T11 and just the actual display in the Venom.

Finally the end came and they cut them all up and we went off to England in a Britannia and picked up our Hunter FGA9s from Stradishall.

[forthbridge]

In the case of the old Venoms, they fitted "G Count Meters" these had little ticking counters that incremented each time a certain G figure was exceeded. There would be counters for 4G, 5G, 6G etc. etc...As our aircraft neared the end of their available "G Counts", they would be relegated to high level cross county flights and similar exercises that used less counts than for instance low level dusk strikes. The worst sortie for adding seriously to an aircraft's "G Counts" was to give it to me for a solo aerobatic display.

Very interesting!

Perhaps just as well that there is no similar code in Flightsim models - or I imagine some of your fleet would be needing replacements..... ;-)

[ukmil]

indeed, that has advanced now, and the Tornado uses the same technology. it counts, g levels and differing wingsweeps. we then use that data, along with aircraft fuel weights and loads to convert that into Fatigue index. so as you say, straight, level flight is much better, but short combat/aeros runs will eat FI.

so aircraft such as airlines, VC10's etc, that go up, level then down can clock 1000's of hours, but fighters will only clock small amounts

this is evident with the Tornado. the GR is currently lifed upto 8000 Flying hours, but most are way below 100FI, but the f3, ome of ours that have gone over 100Fi have only clocked 3500 hours.

part of my job, is to manage what we use our fleet for, so ones that are high on FI, we only allow the crew to fly low G sorties.

[forthbridge]

Fascinating stuff, one tends to think of aircraft fatigue in terms of numer of landings and such like only (well I do) - this is a nice eye-opener on the intricacies of keeping high-performance aircraft aloft

all this sets off the brain cells.. what would the performance at airshows be like for Spits and hurricanes if there were a few brand spanking new models kicking around...... ;-)

[ukmil]

this is also what is against the current vulcan project. if and when it flies at airshows, what it can do will be severly limited, as they will not be allowed to throw it around.

[SkippyBing]

On one of the numerous hold overs I had during flying training I helped gather data to see if the fleet of Squirrels at Shawbury were flying the sort of sortie profiles that had been originally assumed. As the Squirrel doesn't have a g count gadget (I think the Merlin might be one of the first military helicopters to have something along those lines) no one really knew if the assumptions made to generate the fatigue index were correct. Basically I sat in the back with a laptop running a program that let me enter what was being done and it assumed we kept doing it until I told it to stop and do something else and it then generated a log file which went into some massive spreadsheet.
Interestingly the Lynx IPT got a bit huffy when 702 painted two aircraft in the display scheme for the Black Cats as they pointed out it would massively skew the fatigue on those two aircraft compared to the rest of the Lynx fleet as they would now be doing all the display flying whereas before it would have been spread out over the whole fleet.
I also remember being told the Phantoms the RAF got second hand from the RN had much more restrictive limits than the FG1s they'd received direct as 892 had basically used 90% of the fatigue life in 10 years, which is I guess what happens when you know your aircraft has a hard out of service date! On a similar note there're a couple of Sea Harrier F/A2s you shouldn't try and get airborne again!