Shoreham R31 in a 152

[Okavango]

WX looking like would need to use R31 today - what's this like in a 152? Fairly short but also concerned about any windshear etc. Thanks.

[Fuji Abound]

31 is fine and not particularly known for shear. I would have thought it is fine.

[IO540]

Anytime there is any wind, there is wind shear...

Wind shear is the variation of the wind with height.

[Genghis the Engineer]

Let's see:

Take-off
Runway 31, length 425m + 190m starter extension = 615.
Cessna 152, standard take-off distance 410m. x 1.1 grass factor x 1.33 safety factor = 600m.
600m <625m.

Slightly marginal, but okay so long as you use all of the runway starter extension.

Landing
Runway 31, length 425m, but no landing extension.
Cessna 152, standard landing distance 366m x 1.1 grass factor x 1.43 safety factor = 575m.

425m<<575m !!

If flying out to land somewhere else, yes. If planning a routine landing onto 31 at Shoreham - NO.

Yes, of-course, an experienced pilot who knows their aeroplane should be fine, but anything goes wrong and the insurance company, (other) aircraft owners and AAIB will be less than polite about it - so why do you need to?

G

[Miserlou]

IO540,
A wind gradient, you mean, being a change in the speed and/or direction due to friction with the surface.
Wind shear being a sudden change in wind speed and/or velocity.

[IO540]

It's only a question of degree.

In the world of gas flow, there is no such thing as "sudden".

The reason wind shear can cause crashes is because some planes (notably jets) are unable to increase their airspeed fast enough to overcome the loss of airspeed due to the headwind reducing on final approach.

Pistons are usually more able to cope with wind shear, though I know of a TB20 owner who was trying to land with a surface wind of about 55kt (straight down the runway) and busted the nosegear (and the prop, etc). With a 55kt reported surface wind you can expect a big loss of airspeed - even between the top of the pole where it is measured and the surface.

[Cough]

Gengis,

Nice answer, may I expand for a laugh?

To make the landing public transport legal you would have to reduce the landing distance by 23%. According to the 152 POH that I saw, I remember a 10% reduction in landing distance per 9kt HWC. I asume that this is the reason for Oka asking about R31 in the first place. So to get that reduction in landing distance, lets factor in enough headwind to get the 23% reduction:

so 2.3*9 = 21kt.

Now the CAA safety sense leaflet (http://www.caa.co.uk/docs/33/ga_srg_09webSSL07.pdf) says that you may only use a half of the headwind component for landing distance performance corrections. So we must factor the minimum HWC:

21 * 2 = 42kt HWC.

For a POH approach speed of 54kt, that gives a ground speed of 12kt. I wager the ground roll in these circumstances may be around 20m? (or 4% of the LDA):E

Love numbers!

ps. If anyone out there approaches an airfield with a 42 kt HWC at 54Kt indicated, could you contact me first so I can:

1. Get the AAIB on site to record the crash:eek:
2. Call the fire service for you...
3. Clear the undershoot so no innocent parties are hurt!

Be safe! Never do anything in this game you couldn't explain to your mum, and then a judge.

[Genghis the Engineer]

Very good Cough.

Aged about 25 (which sadly was a long time ago) I was in the scarily responsible position of managing a number of training and fast jet aeroplane performance and handling trials (occasionally from the back seat). Fortunately I had a very experienced boss who was very ready with helpful advice, and was working with some very very high ability pilots, who were more than happy to point out bluntly if I was about to screw up (or just had done!)

One bit of advice from my boss that I've repeated often in my mind was: "Before making any decision, always run through your mind the phrase 'at the subsequent board of enquiry'".

I've been lucky not to make too many aviation cock-ups since, but the biggest was certainly one where I'd failed to heed that excellent bit of advice, and I try hard never to forget it again. It was, incidentally, also related to landing performance calculations.

G


N.B. I don't have a C152 POH to hand, but don't recall it actually allowing for proverse factors, only adverse ones. Can anybody confirm or deny?

[Okavango]

This flight actually went well and i'd guess I used about 1/3-1/2 of R31 including ground roll. Regarding Ghengis' figures:

Landing
Runway 31, length 425m, but no landing extension.
Cessna 152, standard landing distance 366m x 1.1 grass factor x 1.43 safety factor = 575m.

Is the safety factor not just for public transport ops? I don't recall that in the PPL and it would preclude going to a lot of smaller airfields.

[flybymike]

A 575m runway length requirement for a C152 would put airfields like Netherthorpe out of business.

[Mark1234]

Maybe you do things differently round here, but I'd rather expect to find takeoff weight, density altitude, and headwind component present in both the takeoff and landing calcs? They tend to have a rather marked effect on the distance used!

[Gertrude the Wombat]

Maybe you do things differently round here, but I'd rather expect to find takeoff weight ... present in both the takeoff and landing calcs?
For a 152 I would expect to use max weight, with no allowance if I took off a bit lighter. Obviously for airliners the actual weight is taken into account.

At what size aircraft does the actual weight start to get used (routinely, not just by people who are trying to shave a couple of metres off their requirement)?

[Mark1234]

Interesting (by the way, that wasn't supposed to be a flippant or sarcastic post, though in retrospect it probably read that way).

I got my PPL under a different regime (australia); I've always used everything on every takeoff calculation - headwind, tailwind, surface (short grass hard base / long grass, soft base). As standard a certain amount of margin is mandated to be built into the POH, which I wouldn't touch - POH figures all the way.

Then having arrived at a final number, my personal practice was/is to make a judgement call on how close to that number I'm willing to go, based on a whole bunch of factors: currency and familiarity, confidence in the numbers, stability of any headwind, predictability of the surface (grass in particular being a very variable animal - encompassing anything from almost as good as concrete to absolute treacle - how a single grass factor can be published I don't know!)

I'm no sky god, but personally I'd rather know what the theory says, then fudge some safety onto that, than use a very conservative estimation tool that gives a factored number - and make a go-no-go on that number; My thinking is that at least I have a feel and awareness for how hard I might be pushing it vs the other system where I'm not making the call. Of course, if you get down to making decisions based on single digits it's probably time to take a long hard look!

[Sir George Cayley]

I wonder where the Cessnas I used to fly got their performance from? I don't recognise any of the figures as 'real'

Solo with 2 hrs fuel, 150s and EVEN 152s are quite agile.

Load any a/c to mauw and start factoring and you can find excuses not to operate all of them.

Don't get drawn in by declared distances on grass aerodromes. In golfing terms the playable often extends beyond the limits imposed from the Belgrano.

Have a look at Derby:eek:

Be safe, but be real - and have fun.

SGC

[SkyHawk-N]

It's only a question of degree.

In the world of gas flow, there is no such thing as "sudden".

The reason wind shear can cause crashes is because some planes (notably jets) are unable to increase their airspeed fast enough to overcome the loss of airspeed due to the headwind reducing on final approach.

Pistons are usually more able to cope with wind shear, though I know of a TB20 owner who was trying to land with a surface wind of about 55kt (straight down the runway) and busted the nosegear (and the prop, etc). With a 55kt reported surface wind you can expect a big loss of airspeed - even between the top of the pole where it is measured and the surface.

Have you ever experienced flying through a windshear? Not a reducing headwind but one where there is a significant, SUDDEN change of wind speed AND direction, even a complete 180 degree change in direction?

If you have experienced it you would know that there is much more to it than what you alude to in your response above. All aircraft types can be severely affected by it, and at anytime, not just when landing.