To explain further, V1 means they cannot stop in time if anything happens. Therefore they must commit to taking off. So why wait for a positive rate? Shouldn’t it be retracted as soon as they see a difference in altitude? From my understanding, is that the landing gear deployed during a crash landing can rupture the fuel tanks??
Why do pilots wait for a positive rate to put the gear up, When V1 means “commit to takeoff”?
2$\begingroup$ V1, you're still on the ground! $\endgroup$– FattieOct 28, 2019 at 2:06
$\begingroup$ This question could be clarified by saying "So why wait for the VSI to show a positive rate of climb?"-- if that's what you meant. Arguably, if you see any indication of change in altitude in the correct direction in the altimeter hands, digital altitude readout, symbolic tape depiction of altitude, etc, you are in some sense seeing an indication of a positive rate of climb, so your question as written is a little unclear. The distinction between seeing a positive change in altitude indication and seeing a "positive rate" is not obvious. $\endgroup$– quiet flyerAug 9, 2020 at 23:43
$\begingroup$ @Fattie well aware. What I mean is why wait to put the gear up when V1 means if a problem occurs it’s an inflight emergency. $\endgroup$– George Clooney In a MooneyAug 15, 2020 at 1:58
V1 is the speed at which you are committed to take-off, it doesn't mean you are off the ground. You can't retract the gear at V1 because you are still rolling on it heading down the runway.
Vr is the speed at which you rotate the nose up to get into the air. Again you are still rolling down the runway with your nose in the air, you can't retract your wheels if you are using them.
V2 is the speed at which you are able to climb with one engine inoperative. It doesn't mean you are in the air yet, you can't retract the gear.
And just because the main gear come off the ground doesn't mean a gust won't push you back down, so usually you leave it down until you are reasonably assured that you won't touch again. It's a lot gentler on the aircraft if you touch the wheels than if you rub the skin off the aircraft. Positive rate is pretty much the only indication of this since there is a little bit of a lag in the VSI, by the time it shows positive rate you are far enough off the ground that you really don't need to worry about touching again.
Shouldn’t it be retracted as soon as they see a difference in altitude?
The best indication is not the altimeter, it is the VSI. It reacts faster than the altimeter.
For GA aircraft, a lot of pilots won't retract the gear until "there isn't any runway left". This means that taking off on an 8,000 foot runway, they would be airborne by the 1500 foot mark and leave the gear down in case the engine fails they can just put it back down again.
What you really don't want to happen is for the gear to be retracted prematurely. When you are rolling down the runway you get "light on your feet" and the gear retraction mechanism can pull the gear up before the aircraft is ready to fly. The squat switches may be unloaded so they won't stop the system. Best thing to do is to wait for the positive rate, verify visually and then retract. Personally I leave it down until I'm 500' below pattern or no more runway left.
$\begingroup$ Thank you for your answer! I appreciate the knowledge. $\endgroup$ Oct 27, 2019 at 5:50
$\begingroup$ The radar altimeter should be faster than the VSI but since the attitude of the aircraft can also affect the reading this is not really reliable. It would also show increasing figures as the terrain below you drops off, doesn't mean that you're actually climbing... And you sated correctly the barometric altitude is way too slow and so the vertical speed indicator is used as the best option. $\endgroup$– JanOct 27, 2019 at 14:33
In all big air craft, the retraction cycle involves the opening and extension of many bay doors where the gear is stored. Practically it means that initially you will experience an increase in drag when the gear is retracted. We do not want this when close to ground.