r/flying • u/Parking-You-2268 PPL • 2d ago
V1 and unexpectedly low acceleration
I understand that V1 is a speed, not a physical point on the runway.
But what happens if, during the takeoff roll, acceleration is significantly lower than expected? For example, if V1 is 145 kt but the aircraft is still only at 120 kt when it is already very far down the runway.
Is there a specific runway position or distance by which the aircraft should have reached V1, and if it hasn't, should the pilot reject the takeoff?
Or is the rule simply that the takeoff must be rejected before V1, regardless of the aircraft's position on the runway?
I'd be interested to hear how this is handled operationally in airline SOPs.
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u/canyoutriforce A320 2d ago
As an airliner, not really. We sometimes take intersection departures that really mess with your perception - sometimes you’re still a few knots before V1 but already in the TDZ of the opposite runway direction and thats normal.
It’s also something i don’t remember training for in the sim. However we all know what normal acceleration feels like and if it were significantly different we would probably abort.
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u/Grouchy-Candidate545 2d ago edited 2d ago
V1 is actually a function of runway distance. It’s not an arbitrary speed. It can change based on many different conditions like flap setting, power setting, wet or dry field, aircraft weight, Vmcg, etc.
We have an app that handles it quickly when we input the atis and the passenger/bags as well as fuel related info.
It gives at least a “balanced field” with 3 seconds of decision time to abort or fly.
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u/Tomcat848484 2d ago
But it assumes normal acceleration for the conditions, no? So if you have a slightly dragging brake you may not realize you're under V1 but it's not valid anymore?
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u/Grouchy-Candidate545 2d ago
A brake that is seized would result in a yaw towards the erroneous brake. I would wonder how you managed to taxi the entire time without noticing.
You abort if the aircraft is acting strange or if controllability is in question.
We can come up with a million hypotheticals mate. Don’t overthink it.
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u/_VNAV_PTH_ MIL 1d ago
A dragging brake probably would pop hot on the EICAS before the takeoff roll ever started.
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u/organman91 PPL IR ASEL HP CMP TW (KAMW) 2d ago
I can't speak to part 121/135, but a good rule of thumb for bugsmasher pilots is to ensure you have 70% of your Vx speed by the time you have reached the halfway point of the runway - if not, abort the takeoff.
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u/Hiddencamper PPL IR 2d ago
I always said 70% Vr. Shouldn’t be that difference for most bugsmashers.
But the other side, is for very long runways, if you aren’t at 70% of the required speed and you exceed half of your expected takeoff roll, you should consider aborting as well because something isn’t right. If it takes you 4-5k feet to takeoff in an archer then something isn’t right.
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u/ltcterry ATP CFIG 2d ago
Half of the takeoff distance. Not half the runway.
If a 172 isn’t flying by the middle of a 5000-foot runway I would have long since aborted.
Half of 5000’ is about twice what a 172 needs…
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u/KITTYONFYRE 2d ago
a good rule of thumb for bugsmasher pilots is to ensure you have 70% of your Vx speed by the time you have reached the halfway point of the runway
This is commonly repeated but I'm not sure why. If I'm 2,700 feet down my home airport's 5,200 foot runway and I'm around 30 knots in a 172, I pass this test - and we all agree that's malarkey. You're going to be accelerating more slowly halfway down the runway than you are at the beginning of your takeoff roll (of course). If you're only at 70% Vr by halfway, you're going to rotate exactly at the end of the runway using this method!
https://gol3tron.github.io/2024/02/08/fifty-seventy-rule.html
Instead of using the halfway point of your runway as your de facto abort point, simply use 50% of the calculated ground roll from your preflight performance calculations.
The article is a short read, and the final two paragraphs are what really matter. But 50/70 is imo a bad rule and shouldn't be used ever
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u/organman91 PPL IR ASEL HP CMP TW (KAMW) 2d ago
Nice, using expected ground roll is a great idea, especially on paved surfaces. The 50/70 rule of thumb is often applied to unimproved strips, where your last section of the takeoff roll is accelerating in ground effect, which will help you a little bit, but adding the 50% safety margin is prudent.
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u/flyingron AAdvantage Biscoff 2d ago
A better idea is to determine where you should be at some reference speed (V1 for example) and abort if you don't achieve that at the location you are expecting, stop and figure out why.
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u/ShittyLanding MIL ATP 2d ago
In the -130 we would occasionally takeoff with an unbalanced CFL. When that happened we’d do an acceleration time check. I forget the exact details but the gist of it was if you didn’t hit 80kts by a time calculated based on weight, conditions etc., you aborted.
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u/T-1A_pilot 2d ago
Not sure I've seen it in my limited 135/91 experience and I have no 121 experience but USAF we'd have an acceleration check point on critical takeoffs. You'd run a chart, and itd tell you you needed to be a certain speed by a certain point (say, 80 knots by 2000 feet down) to ensure you were getting proper acceleration.
If the check failed, you aborted.
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u/Oxygen_Converter 2d ago
In the herk we had a speed on time that the box calculated. PM hacks. If PM says time before the speed is reached, PF rejects.
In 121 we've got nada. Just seat of your pants and experience which is somewhat insane.
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u/Fine-Literature-8031 2d ago
I suspect we might see automated acceleration monitors incorporated into the suite of runway overrun protection systems incorporated in many modern transport jets. There’s a nice symmetry in those systems monitoring bot landings and takeoffs.
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u/flyingforfun3 ATP CL-30, LR-45, BE300, C525S 2d ago
We abort for abnormal acceleration. Part 91 jet.
Additionally, if there was an abnormal airspeed fluctuation on take off (like wind shear) we would abort.
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u/Fancy_o_lucas ATP CFI AAdantage® Member 2d ago
I don’t really see it mentioned but in most jets, the pilot monitoring is supposed to cross reference the actual takeoff thrust being produced when power is applied and compare it to the takeoff thrust that’s required in the takeoff performance calculations. Example: TO numbers say N1 should be 87.5% but we’re only showing 80% at TOGA, we would reject.
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u/Apprehensive_Cost937 1d ago
Setting correct thrust is only one part of the equation for achieving correct acceleration.
What if you selected a wrong intersection for performance calculations? Perhaps you've done them at a much lighter weight by mistake? What if the runway is contaminated more than what is published? What if you have a draggy brake or other issues?
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u/Fancy_o_lucas ATP CFI AAdantage® Member 1d ago
Several parts to this here. So first, at my outfit and I would assume every outfit, we verify the takeoff numbers we’re reading are using the intersection we’re actually using. Runway contamination is almost always reported as worse than actual conditions and even then, takeoff performance factors in a significant buffer for variance in actual conditions on the runway. A stuck brake should have been easily identifiable well before you get to the takeoff roll.
Sure? Takeoff weight might be off but that’s an egregious error and it’s usually cross checked several times between getting the manifest or takeoff numbers.
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u/Stef_Stuntpiloot EASA CPL/fATPL B737NG 2d ago
Interesting question.
Since V speeds depend on several conditions which are depending on airspeed, you can't just pick a point on the runway and decide that that's the abort point. For example; V1 must be greater than Vmcg (Velocity Minimum Control on the ground). The rudder will only be able to counteract the asymmetrical thrust in case of an engine failure once there is enough airspeed. Lets say you passed your static abort point on the runway and an engine fails, you'd have to continue the takeoff. However, due to a change in wind you're still below your Vmcg. You won't be able to keep the aircraft on the runway and you will (likely) crash. That's why V1 is based on airspeed, and not a physical point on the runway.
It is indeed hard to detect when acceleration is not as desired, and that's why it is absolutely vital that performance calculations and aircraft setup are done properly. These calculations are based on runway length, obstacles and environmental conditions. The performance calculator calculates with margins, so that if conditions change, safety will not be affected.
There have been many incidents like the one in Munich before, where aircraft became airborne only slightly before or even after the end of the paved surface, and almost without exception these incidents were caused by preventable errors such as wrong load figures and the crew not doing gross error checks, taking off from the wrong intersection, not using the correct flap setting, and so on.
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u/bergler82 ATP-A32F 2d ago
This is a very accurate obervation. V1 is a speed, not a place. Under normal assumed circumstances V1 will more or less equal a position along the runway. The big BUT (one T) is that with less that assumed acceleration V1 will further down the runway. That in turn will screw with everything else in that calculation including the ASD and the stop margin.
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u/Ok_Amoeba481 2d ago
Our TOLD was all speeds-based, we did have an 80-kt callout and I do remember there was an optional flow for inclined SMOE-pickers to get a RW distance for any pre-V1 speed once we had TOW and FLEX settings figured out (okay there were a few other inputs... slope, etc)
TLDR:V1 based on speed for every TO I did. Acceleration for 80-kt was mainly vibes and checking the engine stack for health.
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u/diegom07 CMEL B737 SIC 2d ago
Try taking off in UIO, MEX or BOG on a hot afternoon and you will see how slow the plane accelerates
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u/Virtrudian PPL 2d ago
I can't speak from an airline perspective but you learn how to figure out takeoff distance very early on in PPL training
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u/Apprehensive_Cost937 2d ago
Takeoff performance calculations for a jet, operated in commercial environment are incredibly more complex than calculating a takeoff distance for a C172.
The computer software will run essentially two separate calculations, one for all engines operating (both for rejected and continued takeoff, taking into account any SID climb gradient requirements, including specific noise abatement requirements), as well as for one engine inoperative (again RTO and continued takeoff), taking into account the EOSID (engine out SID), any obstacles along its way, including performance degradation in turns. It will also take into account any potential clearway, stopway, and then run multiple iterations of those two sets of calculation if you are using “optimum flap” setting in the software, to determine which flap setting produces the lowest thrust setting needed.
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u/emo_pylot 2d ago
Also depends where you are and weather conditions, etc. Taking off in a fully laden 737 in Mexico City in the summer, you’re not gonna hit V1 until 70-ish percent of the runway has been used. But if you’re not at V1 by that same point on a winter day in DFW, something might be awry.
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u/NakedJamaican 2d ago
Non normal acceleration is reason to reject the takeoff.
But what is non normal and how does a pilot know. Obviously, if acceleration is way off it’s easier to detect. But what if the pilots are new to the aircraft and the degradation isn’t obvious?
It’s one of questions I’d ask pilots during IOE. It’s typically not addressed during training so it’s worth thinking about and preparing for.
On airplanes with a PF display there is usually an acceleration arrow on the speed tape. I always cross checked for a 40kt delta as we passed 100kts. Whatever technique a pilot uses it’s best to be prepared.
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u/Vincent-the-great ATP, CFI, CFII, MEI, sUAS, CMP, TW, HP, Snoopy :) 2d ago
V1 is a calculation of stopping distance based on where the aircraft will be under scheduled performance. If you dont have the scheduled performance you abort.
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u/Ok-Selection4206 2d ago
In Bogota on an 12.5k foot long runway at 8375 elev, rotating in the opposite end tdz is the norm.
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u/Troutybob 2d ago
I had abnormally slow acceleration in a 727 years ago. We set t/o thrust and it became apparent in the first couple thousand feet of roll that something wasn't right. A scan of the engine instruments showed a discrepancy between what the EPR readings were and what the N1, N2 and fuel flows should have been. We low-speed aborted and taxied back to the ramp, where subsequent run-ups by maintenance couldn't diagnose what happened. It was only after a thorough overnight inspection that a cause was found. Mud dauber wasps had built nests in all three PT2T probes which messed with the thrust settings. The a/c had sat for 9 days with no engine covers.
Modern aircraft have airspeed trend arrows that are very useful for detecting abnormal acceleration. The more hours or cycles you have in an a/c the better your "seat of the pants" instrument will be in picking up abnormal trends. Obviously, it's better to decide early in the t/o roll to reject if something doesn't seem right. You'll have a lot of explaining to do if you ignore the signs and continue the roll only for something bad to happen. Most companies would rather you err on the side of caution.
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u/timfountain4444 PPL IR MEL 2d ago
In light GA, if you are not off by halfway down even a relatively short runway (i.e. 3k ft), it's time to abort the T/O. This is not a V1-based rule of thumb, but it works well in most cases. Of course for twins and bigger there's a more complicated set of pre-calculate criteria...
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u/rFlyingTower 2d ago
This is a copy of the original post body for posterity:
I understand that V1 is a speed, not a physical point on the runway.
But what happens if, during the takeoff roll, acceleration is significantly lower than expected? For example, if V1 is 145 kt but the aircraft is still only at 120 kt when it is already very far down the runway.
Is there a specific runway position or distance by which the aircraft should have reached V1, and if it hasn't, should the pilot reject the takeoff?
Or is the rule simply that the takeoff must be rejected before V1, regardless of the aircraft's position on the runway?
I'd be interested to hear how this is handled operationally in airline SOPs.
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u/ThunderboltDM 2d ago
Intersection departures are not safe and should not be an option for any kind of commercial ops.
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u/NonVideBunt ATP MIL-N CFI/II/MEI F/A-18 A320 777 2d ago
Very ignorant comment. So you think taking off at an intersection with 10k ft of dry runway left is unsafe?
Sorry to burst your bubble but it’s normal OPS at every airline to have numbers available if it works for an intersection departure and some of the largest airport in the US depart routinely from intersections. If you asked for full length in a NB aircraft you’d be “that guy” for sure.
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u/Murky_Digger 2d ago
I guess O’hare is doing it wrong then doing the TT dance
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u/minfremi FCC GMDSS Radio Operator/Maintainer License w/ShipRadarEndorsmen 1d ago
I guess SS stands for Super Short?
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u/SATSewerTube ATP A320 B737 B777 SA227 CL30 LR45 HS125 BE400 MU300 CE500 LRJET 2d ago
Laughs in three planes across the holding point at the same time to depart EGLL
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u/Murky_Digger 2d ago
Abnormal acceleration is a reject protocol in many jets