737-400 - Early Gear Drop

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Member for

8 years 4 months

Posts: 16

I have recently been travelling on a 734, and I have noticed a peculiarity when landing. On at least two occasions on the aircraft, the gear have dropped early. Very early. The aircraft hadn't even turned onto the base leg one time at KPSP and the familiar thump could be heard before the wind could be heard against the gear. Another time the gear also went down very early, landing at KSFO. Does anyone know why this would be? It wasn't just a one off occurrence. It seems almost like procedure, and surely it's inefficient. On an unrelated note, I was on a Q400 landing on runway 16R some time ago at KSEA and the flaps didn't even start to be lowered until past KBFI, which I'm sure you can see is quite close to the runway. Does anyone know why these two occurrences occur?

Thanks!

-DMac10121

Original post
Profile picture for user Bmused55

Member for

16 years 11 months

Posts: 10,625

The gear is sometime used to slow the aircraft down. It's possible they were told to reduce speed to maintain separation? I'd imagine that was particularly the case at SFO. Busy airport that. I've sat through a few videos of approaches to SFO and speed reduction calls are frequent. Sometimes it's challenging to slow the aircraft down in time to comply.

Member for

8 years 4 months

Posts: 16

Yes, I suppose that would be the most likely option. There really isn't much else it could be. It's only odd that they would lower the gear to slow down, as at that speed the gear probable shouldn't be dropped anyway. I personally, at least for one occasion, didn't see the spoilers go up, as that is a common option to slow down when approaching the airport.

Profile picture for user atr42

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13 years 4 months

Posts: 338

Also adds to drag if you might be a bit high & hot.

Profile picture for user Deano

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17 years 1 month

Posts: 2,623

It's not that odd to lower the gear if fast. What you might have been seeing is exactly how bmused described it. In most, if not all modern aircraft you have "landing flap settings". In other words, a degree of flap that is certified to be used for landing (Flap zero is not one of them for obvious reasons). Based on this, an aircraft, if given speed control will have to slow down and in doing so there will be a flap extension schedule for each stage of flap based on weight (amongst other things). If you extend even the first stage of landing flap without dropping the gear first then you will get the gear warning horn. I think the 737 series landing flap settings start from Flap 15 onwards, so you can go to flap 10 without the gear and thereafter you have to drop the gear before any more flap can be extended without the horn going off. A typical example could be this scenario:

Flap 5 extension speed: 210kts
Flap 10 extension speed: 188kts
Flap 15 extension speed: 173kts

If they were told to maintain 160kts then you see they have to extend flap 15, but because Flap 15 is an approved landing flap setting they will have to take the gear between flap 10 and 15 to prevent the horn going off. If ATC gave them any speed below the flap 15 extension speed, even as far out as 15 miles then they would have to take the gear early to enable them to extend flap 15. Clear as mud?

atr42

You are technically wrong about the Q400. The Q400 is a CAT C aeroplane just like the Airbus 320 series and 737 series. This CAT level is based on it's Vref speed and the Q400 falls in the same threshold speeds as the airbuses and Boeings. We can keep up with anything whilst being vectored in terminal airspace and the only difference in the flap and gear settings on the Q400 is down to the design of the wing. It doesn't have a swept wing, but a high aspect ratio wing meaning we can have a fairly low manoeuvring speed before we have to drop the gear "pre landing flap".

Rgds

Dean

Profile picture for user atr42

Member for

13 years 4 months

Posts: 338

Thanks Deano
Adds to my knowledge.