35 ft, 50 ft heights question

[md-100]

1. Why a height of 50 ft above threshold is considered for calculation on landing performance?

2. Why a height of 35 ft above ground is considered for calculation on climbing performance ?

3. Why a height of 50 ft above obstacle is considered for landing and climbing performance on small airplanes?

Thanks

[john_tullamarine]

Like most things in the regulatory environment, one needs to put lines in the sand. There need not necessarily be a sound, theoretical basis for such boundaries .. and, on some occasions, a good old finger in the wind engineering WAG is used in lieu ....

(a) according to a tale which I heard about 15 years ago during a training course .. the source was one of the instructors .. and a quite elderly chap at that time ... This chap was a long ago FAA engineer, who was around as a young chap in those ancient times. According to his tale, an early military demonstration of, as I recall, a Curtis machine, involved flying in and out of an Army parade ground. The particular parade ground was surrounded by a line of trees whose height was in the order of 50ft. The early CAB folk adopted that as being as good as any figure, on the basis that the need was to have some provision for notional obstacles to be addressed during the takeoff and landing. This then became the standard for light aircraft.

(b) at some much later stage, the heavies moved to 35ft screen. Unfortunately, I have never seen any historical discussion on the specific reasoning. However, one could presume that this had some basis in the more rigorous takeoff treatment accorded to this class of aircraft. The 50ft landing screen remained.

(c) at a yet later stage, as the Industry moved to account for contamination on the runway surface, a compromise for heavies was reached in adopting a 15ft screen for takeoff for wet runways.

Perhaps others can provide additional tales relating to the 35ft history ?

[411A]

Regarding takeoff performance, in the earlier days of jet transport flying, the screen height during takeoff was lowered from the standard fifty feet (for piston transports) to 35 feet because...many of the early model jet transports at the time (B707's/DC8's) could not meet the fifty foot requirement with the runway/clearwar lengths available at the time.

So, alter the regs (CAR4b) so they do fit.

Simple as that.

[Old Smokey]

I never thought that the day would come when I would verify an uncertainty posed by the redoubtable John_T, but it is not a tale. It is absolutely true that the U.S. Army adopted a standard tree height of 50 ft, and instituted a takeoff to 50 ft requirement as a result.

So too, 411A has correctly provided the origins of the latter day 35 ft requirement.

To add one more, the 35 ft requirement reverts back to the standard 50 ft if the takeoff involves a curved procedure. This is to account for the wing tip being lower than the usual lower point of the aircrfat in a banked turn. With the advent of the A380, this too, may change.

Regards,

Old Smokey

[md-100]

thank you, i thought the heights was more about computations and studies.

[john_tullamarine]

.. another finger in the wind one (from the same fellow) was the 70 mph/61 kt SE stall speed ... they had to pick something and that was an engineering guess for a figure thought to offer some reasonable chance of surviving an engine-out off-field landing ...

[targaman]

An interesting side issue to this discussion is that the minimum screen height of 35 feet or 15 feet on wet runways is a demonstrated capability by:
A test pilot crew
A known runway surface condition.
New tyres and brakes.
An accurate known (weighed) weight & balance.
The knowlwdge of an engine or critical system failure prior to the event.

If we, as line pilots, encounter the same engine failure or critical system failure at or after V1 and we don't make the screen height due to:
A non test pilot crew.
An unknown runway surface.
Old tyres and brakes.
An innaccurate weight & possible balance problem.
We didn't know the failure was about to occur.

the ensuing accident will probably be classed as pilot error!

The fact that aircraft aren't crashing everytime an engine or critical system fails at or after V1 is a testament to the reliability of modern engines and the risk mangement calculation that the critical failure at maximum weight etx etc is 10 to power of X.

Yer pays yer money and yer takes yer chances.

[4PON4PIN]

Mr Targaman, may I suggest that certification may be based on parameters such as test pilot etc. but actual T/O Perf calculations are factored to take into account "average" line pilot, tyre wear etc. :hmm:

[john_tullamarine]

Yes, indeed .. but there is not much fat in the OEI continued takeoff sums FOR a Vef failure.

Fat considerations -

(a) the failure probably will be not precisely at the critical speed

(b) ops are not always TOD1/TOR1 limited

(c) apart from close in first/second segment obstacles, the net/gross margin gives an increasing pad as the takeoff progresses.

The certification process takes some steps to account for average pilots and startle considerations ...

[mutt]

J_T,

Since you mentioned it, do you have a definition of a close-in obstacle and a distant obstacle?

4PON4PIN.

Accounting for brake wear is relatively new, I believe that the MD11 was the first FAA aircraft certified with totally worn brakes.

Mutt.

[john_tullamarine]

.. a rubbery thing, Mutt ... but a useful description for separating bumps which are either "near" (for the purposes of the discussion) or "far" (likewise) .. and the two, naturally, will vary ...

[OzExpat]

Just to add a bit to targaman's thoughts, one can probably also factor in the number of aerodromes where the take-off path is benign. Then, comparing that result to the number of aerodromes where the take-off path is not so benign, you'd have to reckon that there's a reasonable chance of getting away with a (somewhat) poorly executed OEI continued take-off.

Can't add anything meaningful to what j_t has said about near and far obstacles. However, for the benign take-off path scenario, take-off performance is based on the Type A chart, which should take care of the problem. In the not-so-benign scenario, performance engineers will crunch the numbers for OEI procedures, for client airlines, thereby also (hopefully) also eliminating the problem of having to define near and far obstacles.

[mutt]

The close-in and distant obstacle question came from IOSA, something on the lines of.."how do you tell your crews that performance data accounts for close-in and distant obstacles".. So i was just trying to discover if there is an actual definition of both.

Passed the audit, so dont have to think about it for another two years... :)

Mutt.

[Old Smokey]

Yes, that was pretty rubbery John_T, near is near, far is far (and never the twain shall meet?) The best interpretation that I've found is from a BAe P/E who specified "near" as the zone beyond the end of TODA where 1st segment ducks below the obstacle-clear gradient subtended from the end of TODA, and the actual 2nd segment gradient therefrom then intersects with the required OCG, at or before the first obstacle beyond the end of the 1st segment. Where no significant obstacle exists, this is the zone below the nominal 1.2% obstacle-clear plane subtended from the end of TODA. Anything beyond that is "far".

To add a word or two about inbuilt fat, it's appropriate that this arose in a discussion about screen heights, as this is the territory where the least fat occures. Plenty for the all engines continued Takeoff with distance to screen height increased by 15%, ever increasing obstacle clearance in the 2nd/3rd segments where the Gross Vs Net delta pads us well. A little bit of fat for the RTO with reverse thrust being a bonus, but this is gone altogether for the wet runway case, a 'fat-free' environment. The worst of all 'fat-free' environment is, of course, the OEI continued Takeoff from Vef until 2nd segment is underway, with no fat for the TODR to screen height, and no Gross Vs Net buffer in the 1st segment.

Small wonder that the most accidents occur in the 'fat-free' zone.

Audit? - So that's where you've been Mutt. Thought we'd done something to offend you.:O

Regards,

Old Smokey

[mutt]

Old smokey...

This message is actually from Changi... enjoying the duty free as i pass through...

Mutt.

[Old Smokey]

Mutt, Looks like that beer may have to be rescheduled to sandier climes....

This message is actually from Hong Kong.....

Looks like we're incompatible:rolleyes:

Regards,

Old Smokey

[mcdhu]

........and if I may resurrect this one by asking you perf experts how much margin has been taken away from us with the advent of the laptop calculation of performance (Airbus) compared with the old RTOTs where minor details such as the headwind -ignored - and temps - rounded up. Are we we really looking at clearing obstacles by the regulatory minimum - if that's the limiting regime we're in - assuming EO at the critical point? If so, to quote Sgt Wilson; ''Do you think that's wise?''

Thank you,
mcdhu

[john_tullamarine]

Can't speak to a given laptop program without do some testing on it .. but, in general, all the laptop should be doing is giving you the certification data you could get from the AFM or certification programs.

Keep in mind that we look at half the headwind and 1.5 the tailwind.

Rounding the OAT a degree is not going to make a large difference to the real world experience. Keep in mind that the certification workup normally takes test weight to 1-2 percent.

Other than for a bad day and close-in obstacles, the aircraft, in an OEI situation, should have a progressively increasing clearance above the bumpy bits as the takeoff climb proceeds.

[mutt]

J_T,

The programs that mcdhu is talking about use the first principle method of takeoff calculation, they are extremely precise and can offer an increase in takeoff weight of around 8,000 kgs (B747-400) over the older calculation method (model tables).

The effect of rounding up temperatures depends on how much the temperature spread is, again for the B744 we are looking at 1,500 kgs per degree C. So rounding up 1 degree isnt too bad, but rounding to the nearest 5 wouldnt be wise.

mcdhu You are achieving the regulatory minimums, that much is guaranteed. Whilst it appears that the comfort margins are reduced, you have the benefits of certification changes that are more representative of the actual aircraft operation, for example, Accel/Stop distance accountability for worn brakes and the acceleration of the aircraft with thrust after V1 rather than maintaining a constant speed.

Additionally, your airline performance engineers now have the ability to account for line up distance, stopping margin, multiple slopes and runway surface condition. These were never available through the AFM.

So to answer your question, "is it wise?".... My answer is yes.

Mutt.

[john_tullamarine]

Mutt,

I am a little hard pressed without knowledge of the particular software .. but, are you suggesting that this program "permits" weights in excess of that certified in the AFM ?

How is that argued in court during the enquiry ?

[mutt]

J_T,

Note the edit above.... On older aircraft we used "model tables" for calculating takeoff weights, these were based on the AFM but were conservative.

The newer programs are actually part of the AFM. The B777 has no takeoff performance data in the AFM but a reference to the AFM-DPI (Digital Performance Interface).

Boeing offered the DPI for the B744, this increased the takeoff weight by 8000 kgs over the older method, as the AFM is the only certified performance data for that aircraft, Boeing would have had to validate the resulting data against the AFM.

Now I'm off to enjoy some Guinness.....

Mutt.

[mcdhu]

Thank you, both, for your input.

The Airbus program is pretty sophisticated and allows you, among other things, to insert (temporary) obstacles and to vary the R/w length to take account of any Notams that maybe in force.

Cheers,
mcdhu