First we take flaps 1 then 2 after that we lower the gear followed by flaps 3 and full. My thinking is that this will avoid any GPWS warning is that it or there is something else also to it.?
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$\begingroup$ The procedure is somewhat similar in small GA aircraft with retractable gear. I was always taught that the sequence was "flaps-flaps-gear-flaps" and then another gear check on short final. $\endgroup$– Ron BeyerSep 7, 2017 at 14:43
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3 Answers
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You would want to start with flaps/slats. The landing gear slows the aircraft down like an airbrake, and you'll need a higher lift coefficient to retain enough lift. Slats are a safety feature that allow for a higher angle of attack for the main wing, flaps provide a higher $C_L$ by themselves. So you start with a higher $C_L$ and the safety of the slats, then deploy the gear which slows you down, then delay more flap/slat at the lower velocity.
Deploying full flap/slat first and then deploying gear is also not a good idea: speed will be slow already at max flaps, gear deployment brings a significant further deceleration which may have an impact on safety, definitely on continuity throughout the landing process. You are in a queue with lots of others at a busy airport, and want to keep the array of craft flowing without sudden decelerations.
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$\begingroup$ I know each aircraft will have Vmax limits for each config. Is the flaps/slats limit usually higher than the gear limit? And is the gear limit usually higher than flaps full? $\endgroup$– TomMcWSep 7, 2017 at 17:45
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$\begingroup$ I reckon the procedure is just for Best Practise. It could probably be done in a different order without breaking the aircraft, but the procedure of the OP makes the most sense. $\endgroup$– KoyovisSep 8, 2017 at 6:04
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$\begingroup$ Found an A320 manual online. VLE/VLO are 280/250kt and VFE is 230kt for the 1st position (slats only). So you're correct, it's not due to design limits $\endgroup$– TomMcWSep 8, 2017 at 17:48
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There is not a technical reason. The procedure gives the best and the most optimum deceleration rate on the approach. Think of this. With flaps 1 already set, your S speed (minimum maneuvering speed with flaps 1) will be at like 185 knots (aircraft weight at about 63 tonnes), which is pretty close to 200 knots. If you put the gear down after flaps 1, a lot of unnecessary thrust has to be added to maintain this minimum safe speed. But if you put in flaps 2 before the gear, the aircraft can be safely decelerated to about 145 knots. To maintain this low speed, a lot less thrust will be required. Hence, if you put the gear down after flaps 2, a large thrust bump would not be needed, making the ride comfortable and more efficient.
The other reason is that in all transport category aircraft I have flown, the landing flaps come the last in configuring sequence. In A320, both flaps 3 and flaps FULL are approved landing settings. So, putting the gear down before the final setting makes more sense. The reason why the final flaps come last is to prepare the aircraft for a go around. So, it is always a good practice to put down the final stage of flaps only when you are fully committed to the landing. Flaps are quite detrimental to go around performance. That is why in Airbus, when you initiate a go around, the pilot flying calls out: GO AROUND, FLAPS (i.e command pilot monitoring to take flap up by one stage). When I flew Dash 8s the call out was: GO AROUND, MAX POWER, FLAPS 15 (if the landing is performed at flap 35). As you can see, the flaps up command always comes before the landing gear.
The real question here is, can you modify the sequence? The answer is yes, you can as long as you maintain a safe operation. I have had to lower the landing gear without any flaps twice in my time in A320. One time, we were high and fast (about 15 miles to the runway) and we realised, that if we do not act up soon, we will end up in an unstabilised approach. So, we put the gear down, and extended the speed brakes. The gear generates a lot of drag and together with the speed brakes, the speed bleeds off quite fast. As the speed came to flaps 1 speed we put down flaps 1 and configured flaps to FULL in sequence. The next time it happened, there was a notorious tail wind pushing us on the approach which made the deceleration difficult. Again, selecting the gear down first helped us to ensure a stabilised approach and to avoid an unnecessary go around. Thus, within a safety margin, you can always change up the configuring sequence if it is ever required. The best is to always follow the SOPs, but as a pilot you should always know what to do if something happens outside of the book. That is the mark of a good pilot.
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this is speculation, but if you ask why we lower gear between F2 and F3, I think Koyovis is getting quite close to the potential answer:
Each configuration has a reference speed: Green Dot, S speed, F speed. The biggest difference is between S and F ( ie after deploying flaps 2) , that is: this is where aicraft will decelerate the most from around the 180+ kts S-speed to the ~140 something F-speed.
This can take quite a while to decelerate, so it might be a good ideea to use this time to lower the gear (since hydraulics are now free) and even use that extra drag from gear to decelerate.
Mind you, you can still lower the gear at the same time as lowering the flaps, it's just extra strain on the hydraulics so most people will try to avoid making that habit.
Also, remember it is not prohibited to lower the gear much earlier ( eg. if you require extra drag on the approach), even in clean config.There used to be a limitation of FL250 for deploying gear, but that has recently been removed.
You could also delay gear down until after flaps full if there is an operational necesity. The only real limitations is to be below Vloe (250kts/M0.60) and you should try to get gear down preferably before GPWS kicks in (750ft)
