What continues to amaze me is the lack of interest shown by the operators, pilots, regulatory authorities, and manufacturers in the failure of the aircraft to achieve the peformance specified in the certified takeoff data.
refer
www.airliners.com for great photos of failure to achieve the performance (4 really neat photos at last count).
The maths required to quickly scare yourself is not highbrow, but appears to beyond the capacity or interest of the aforementioned parties in general.
The discussion at:
http://www.geocities.com/profemery/a..._distance.html
is marked by concern and lack of understanding of the reuirements specified by FAR/JAR25 and other state criteria.
notwithstanding the above, the evidence of compliance or not resides within the grasp, and observation of the pilot. The authorities could readily externally monitor compliance but fail to do so.
A simple case in point: refer criteria for 4 engine aircraft.
requirement is to achieve in normal service 35' at 100/115% (87%) of TODA, so 13% of TODA remains when aircraft achieves 35'. before that, the aircraft must achieve Vr, rotate to liftoff attitude and then climb from liftoff to 35'. training manuals will provide basic timing, as does flight data. these things take a finite period at a given velocity, and therefore cover a given distance. Simplistically, it takes 5-6 seconds at ~90 msec to liftoff post Vr call (450-540mtrs), and 2-3 sec to achieve 35' (180-270mtrs). So, minimum distance is 630mtrs to about 810mtrs before the 87% TODA point, the aircraft must achieve Vr, at heavy weights. Heavy weights for the current aircraft need BIG runways, and therefore have also lots of TODA after the 87%TODA point is achieved. Criteria limits clearway incorporated into TODA to 0.5 X distance from liftoff to 35'... crosscheck for above...)
So...
big airfield: 4000mtrs, what do we get...
13%=520mtrs... distance remaining to end of TODA when we should be at 35'. Stated another way, we travel an additional 520mtrs after achieving 35' before reaching the end of TODA. During this period we climb in second segment ( assuming we didn't hit anything) and average some climb rate X time to traverse 520 mtrs, at V2+10 etc. That definitely gets us above 35' at the end of the TODA.... like about 160'+ etc. ( getting a queasy feeling yet?)
Of the 13%, the 520 mtrs above, part of that can be over the clearway, (0.5 x distance liftoff to 35') which gives from above 70-135mtrs. (AAC data actually gives 180mtrs interestingly, ie very long time to 35' from liftoff). From this we can calculate that the 35' must be achieved some distance from the end of the runway while we use the clearway beyond runway end. Using maximum clearway from AAC of 180mtrs, not from the above calculation, this would occur some 340mtrs from end of runway. That is 35' is qchieved some 340mtrs before end of runway. Great!
To achieve that though we got to Vr sometime before that, ie some 7 to 9 seconds earlier we got to Vr. Simply, to underestimate the answer just use your Vr speed in M/sec X time to achieve 35', and U get an answer like 615mtrs to 790mtrs from this calculation.
Add the above together:
using AAC distance for liftoff to 35' (only for increasing useable clearway, but using minimum distance from data to 35')
minimum: 615+340= 955mtrs
maximum : 790+340= 1130mtrs.
So what?
This indicates where U should be when U get the Vr call at heavy weights, and what sort of height U should be when U get to end of TODA. Simplistically, having a coaming cuttoff of the endzone markers or centreline lights at 3000' to run is approximate limit case. A worked example for a given Jet is such that the limit occurs 2.3 seconds before coaming cuttoff of the 3000' markers.
Only problem is above URL, photos, personal experience and observations are inconsistent with above on many occasions.
Implications are only this, refusal point is not where it was supposed to be. Stopping may be problematic.
Go case is compromised, how much? Don't know, I am not paid to answer that, the regulators, and manufacturers are...
What do pilots do about it? evidently, nothing. Begs some questions regarding ethics. Don't expect your company to thank you for any observations on the above. Don't expect the regulators to do their job either.
Want to be really annoyed? Look in your flight manual, the one by the manufacturer, and have a look at the basis for the performance data. Consider that against the advise given in training, company policy and experience on how U conduct a takeoff. Read very carefully the wording, and where the wording changes slightly obfuscating the matter.
Personally, the risk is not the issue to me personally, but we sell the passengers a contract with every ticket, that we collectively will provide safe passage compliant with the law. Do we achieve that by silence of our observations.
