MysteriousMrX

Ok.... so again, we’ll assume ACARS works. It says we need 3500’ to stop it, 5100 for the 60% of the runway number. Which of those do we add the 20% to?! Let’s start with that to un-confuse me.

2StgTurbine

The reason your questions are so hard to answer is you are getting numerous performance topics confused.

The 60% rule is for DISPATCH only. Before you depart, the dispatcher makes sure that there is at least one usable runway that you can land WITHIN 60% of (you divide your actual landing distance by 0.60).

But once you get in the air, that number no longer matters. The dispatcher was using forecasted conditions. You as a pilot have more information. You have the current ATIS and braking action reports. Because you have more accurate information than the dispatcher did hours before when they made the release, the FAA does not require you to have a 67% safety margin (remember that the DISPATCH rule was to land WITHIN 60%) that's over the actual landing distance; they require only a 15% margin. That is what the ACARS number should be giving (but since I am not at your airline I have no idea what your company is using for performance). I have never worked at an airline that gave ACARS landing data with the 60% dispatching rule.

Another important thing to consider is the assumptions based in the performance calculations. Your company may differ from the ones I worked at, but usually the dispatcher assumes you are landing the plane like a test pilot. You cross the threshold at Vref and touch down with minimal flare/no floating a few knots below Vref. Then you apply max wheel braking with no reverse thrust credit. Since no one actually lands like that, the landing performance you get through ACARS normally has more realistic assumptions or allows you to select reverse thrust credit, braking level, wind additives to your approach speed, etc.

Now when it comes to QRH landing data (again your airline may differ), those corrections are normally based on the ACTUAL landing data (that 60% rule is only for your release). Some airlines have a separate performance manual for abnormal situations. The numbers in those usually include a safety margin in the numbers already.

As you can see to answer your questions, I had to make a lot of assumptions about your airline, so therefore, my answers may not be very helpful. That is why I told you to write an ASAP so your company can fix this. All this information should be in your FOM, the manual your dispatchers use, or in a section of the QRH where it teaches you how to use that manual.

501D22G

The advice you've received so far is as accurate as can be with what is known;

I'll add a few of my thoughts which hopefully add to the discussion:

The 79,000 LB number looks like a nice round number that could be the manufacturer's max structural takeoff weight. A "never exceed" weight that has nothing due to performance calculations. Just an upper limit.

In other words, a CRJ-900 departing a 15,000 foot runway on a cold dry day flying a fairly long segment in flat terrain might be able to weigh that amount.

Many times that max structural weight for takeoff is never reachable because of other factors. They usually include:

DEPARTURE
EN ROUTE
LANDING

Let me expand on these:

departure - limits due to the departure runway TORA, TODA, ASDA, and then climb limits due to surrounding terrain upon departure, based on takeoff power/thrust setting, LE/TE device setting, ambient conditions, and other factors like anti-skid, auto-spoiler, auto-brakes (insert whatever equipment that matters here)

en route - limits due to terrain clearance during the flight to meet driftdown performance when OEI (only certain parts of the world that have tall-enough mountains where it matters)

landing - a weight that includes destination/alternate runway limiting weights based on the -900's performance for the approach & landing;
stopping on the pavement - LDA
OEI approach climb (missed)
all-engine landing climb (go-around)
you must ADD your planned fuel burn to get you there.

quick example: Peoria to O'Hare you might burn what, 1000 lb?
If your max landing weight for ORD runways are 73,500 lb, you can't weigh more than 74,500 in Peoria, even if you have a long runway, it's cold, etc - you cannot plan to land overweight. The 79,000 lb number doesn't apply.


These weights are calculated for planning purposes at release time and the LOWEST (most limiting) of the 3 is what will be your "max takeoff weight" for that leg.

My educated guess is you saw the 79,000 lb limit, compared it to your actual weight of 74,200 -but that doesn't tell the whole story.

The 74,700 lb number is likely the "more correct" limit.

I'm not familiar enough with the -900, so perhaps a "flex" thrust setting that was still good enough for your airplanes current load was part of that "more correct" limit calculation. If more bags/people/support animals/whatever showed up, you would likely get a new limiting takeoff weight calculated (remove the flex, use max thrust, flap 8 instead of 20, different/longer runway, etc) to give you the elbow room for the new airplane weight.

Many/most operators use a reduced thrust/power for takeoff for engine longevity, perhaps a flap setting that allows for lower tire speeds, when the people/cargo that show up isn't a large number. Maybe this is why it looked to you why you "were really close" to going overweight when really you had more wiggle room.

They do this to reduce wear & tear, etc. When the weights are higher, you do the very best the book will allow.

Hopefully you see that Bombardier's max structural takeoff weight, max structural landing weight, which are sometimes "salesperson" numbers - quickly become meaningless in this context.

Sorry for wall of text, I hope this helps.

AZFlyer

As alluded to above, this could also be an issue of being confused by max flight planned take-off weight vs max runway takeoff weight.

MysteriousMrX

again, I think I’m overthinking it and confusing myself! But thanks for the responses.
The max weight may indeed be something to do with the runway but it was DFW and all runways are very long.

But on ACARS landing data, surely everyone gets the same info: which includes the unfactored length and the longer 60% factored length. As perplexed as others are as to why i don’t know this, I’m equally perplexed no one can answer it either?! Do we add the 20% to the bigger or smaller number?!

If ACARS is no-comm.. I guess we could take the landing distance for our weight off the speed cards. That number will be less than the release numbers that include the 60%. HOWEVER, once again do we add the 20% penalty for whatever QRH procedure requires it to the smaller or bigger? .. now let’s say it’s a snowy icy nasty day. Release says we need way longer distance to stop, right? And now we have QRH saying to add 20% .. adding it to the smaller number doesn’t do anything. So I guess we always add to bigger number??

2StgTurbine

Your questions have been answered. I will try to simply it. Once you take off, the 60% rule does not matter, that number is only used to plan the flight.

When it comes to QRH corrections, that depends on your airline's specific procedures. Odds are your airline has this in writing in one of your manuals. If you cannot find it, call one of your OE training captains or email your training department since no one on here knows what airline you are talking about.

sigler

I will give it one last shot- once you’ve departed, there is no smaller number and big number. Those do not apply anymore. Throw them away. Forget about them. There’s only one number, which is your in-flight landing distance (your company may call it something else). One number, one distance, based on actual conditions, that will be the reference to which you add any factors you may have due to failures.

MysteriousMrX

Ok. So why ACARS gives two, I’ll ask the training department.
But with no ACARS, using speed cards only, we only have that number. Isn’t that based on dry perfect conditions? What if it’s snowy and slushy and crappy? And then we need an extra 20% for whatever QRH scenario happened.

I guess what brought all this to mind was the recent “runway excursions” by Southwest and Mesa and Skywest, and how maybe some things weren’t calculated right.

Twin Wasp

As pointed out above, without knowing what type of aircraft you are talking about it's hard to give answers. Looking at my Boeing QRH there are landing distance tables for the two approved landing flap settings. Each of those tables has 5 sections for different runway conditions. Each of those sections are further divided to account for 6 different brake settings. There are then another 25 tables to cover various system malfunctions. All of these tables, normal and non-normal, have corrections for altitude, temperature, runway slope, wind and reverse thrust. I would think most manufacturers provide the same information if it applies to their aircraft.

cougar

Airplane manufacturers generally provide two distinct Landing data sets: Certified data and Advisory data.
Certified data provides landing distances as required by regulations (FAR parts 25 and 121). Its sole use is to determine landing distance requirements prior to dispatch. As stated earlier, this demonstrated landing distance is determined during flight testing. The demonstrated landing distance parameters are: 50' TCH at VREF, 1000' touchdown, max manual braking, and no reverse thrust.
The Certified FAR Dry distance is 1.67 times the demonstrated landing distance. The Certified FAR Wet/Slippery distance is 1.15 times the FAR Dry distance. But again, this only applies prior to dispatch.
Advisory data provides landing distance capability for different runway conditions and braking configurations. It meets JAR OPS and FAR requirements, as well as SAFO 06012 and TALPA ARC. Advisory landing data is used to determine landing distance to make operational decisions.
SAFO 06012 was a direct result of the Southwest accident at Midway. Prior to the TALPA ARC recommendations, the advisory landing data provided assumed the same as the demonstrated landing distance flown during testing, 1000' touchdown, and no safety margin additive.
The TALPA ARC recommendations were to add a 15% safety addictive to the advisory landing distances, and a more realistic touchdown (1500'). It is also now required to accomplish landing distance assessment prior to arrival.
As such, the normal landing distances in the (Boeing) QRH now include the 15% safety margin and assume touchdown at 1500' (7 second flare distance). Note the non-normal landing distances do not include a 15% additive.
To accomplish the required landing assessment, use the QRH advisory landing distances to compare the applicable landing distances from the QRH against the runway LDA (landing distance available). At a minimum, the max manual braking landing distance must be less than the runway LDA to land. No additional factor needs to be applied.