I realise that the 737-800, especially at high landing weights is a somewhat slippery machine, but at lower altitudes with TAI on, I'm getting a ridiculously poor rate of descent.

 

On my last flight, I was trying to descend to platform altitude using FLCH. Passing 4000 feet, with flaps up, TAI on doing 220KIAS, I was getting 300fpm! The aircraft was not desceleratig and N1 was approximately 37%. 300fpm does not comply with the minimum rate of descent in the UK and just doesn't seem normal.

 

I'd be interested to see if anyone else has this problem.

 

Many thanks,

 

Martin Neep.

What are your landing weights when you experience these issues?

 

What weather add-ons are you using, if any, and in what type of weather do you experience these issues?

I always fly with clear skies, I find that the weather engine in FSX is very unrealistic and causes some strange behaviour. Surface temperature was set to -1C, dew point -3. Nil winds. QNH, 1013.

 

Landing weight was 58T if I remember correctly.

Are you only experiencing these issues at certain airports? I have occasionally experienced flotation on approach to airports such as KSFO.

 

Have you tried replicating the exact same flight without using anti-ice?

That seems to be pretty bad, but not totally unreasonable. Were you also slowing down at the time? Was it both EAI and WAI? Speed-brakes in or out? Winglets? Headwind, tailwind? Also 58 tons isn't high, infact it'd probably be a bit lower, if not around average...

 

Either way, flaps 10, gear down was always a sweet spot that worked well for me.

 

Regards,

Ró.

While TAI will increase flight Idle N1 it, shouldnt make THAT big a difference on your Performance.

 

Although if you always fly with clear skies why do you need TAI?

 

Cheers

Michael Zigford

Speedbrake down, only TAI, no airframe anti icing and the aircraft was not decelerating.

 

When I switched TAI off, descent returned to a more normal 800-1000 fpm.

Hmm, that is pretty bad...

While TAI will increase flight Idle N1 it, shouldnt make THAT big a difference on your Performance.

 

Precisely! Either I'm doing something wrong, or the flight dynamics with TAI on have been modelled incorrectly.

I did two landings today with TAI on on descend with no problems. Moreover, when approaching EGGP I was 1000 feet below descend path for some reason...

I did a flight earlier into EYVI with both TAI and WAI on and was going down at about 800fpm, steady speed....

4000ft, 220knots, why did you not have some flaps out? Anti-ice requires bleed air, which requires higher thrust levels, which means you need more time at deceleration altitude, or you need more drag. If I am using Anti-ice, I usually plan on using at least one additional notch of flaps. I would use flaps 1 instead of clean, flaps 5 instead of 1 and so on.

4000ft, 220knots, why did you not have some flaps out? Anti-ice requires bleed air, which requires higher thrust levels, which means you need more time at deceleration altitude, or you need more drag. If I am using Anti-ice, I usually plan on using at least one additional notch of flaps. I would use flaps 1 instead of clean, flaps 5 instead of 1 and so on.

 

If you know where you will need anti-ice, go to your DES page on the FMC and the LSK6 to select forecast. On the RLSK1 level, you can enter altitudes you anticipate TAI on and off. The FMC will account for the lower descent rate and start your TOD out a little earlier accomadate this. This way, no spoilers or flaps needed and the corporate guys are happy about the lower fuel consumption.

 

For the OP, 300 fpm certainly seems low with TAI, but 800-1000 fpm is also a bit on the low side at 220 KIAS. Not sure why you're having this problem. I do find the 738 a bear to slow down, but not like you are right now. Do remember the phrase "you can slow down or go down, but not both".

 

Eric Szczesniak

It's not normal. Check to make sure that you're really at idle. It should be closer to 1000fpm, even with anti-ice.

The EEC will increase the engine idle speed with TAI on. This will reduce your rate of descent. If you look in the FMC descent forecast page, you can set an altitude below which TAI will be on and this will allow the FMC to calculate an appropriate path.

 

However, 300fpm is way too low. Normally clean speed is around 1000 to 1200 fpm so with TAI you could expect 800 - 1000 fpm. To be honest I am not entirely sure, that is a bit of a guess, but 300 is too low.

 

FLCH? You mean LVL CHG?