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I am just another one of the passionate flight simmers trying to understand more about Anti-ice system mechanics. I am talking about the Boeing 737-800 here if it makes a difference.

What bothers me is: Why is turning on TAI systems having negative impact on descent performance? Given a constant speed that we want to keep a shallower descents can be experienced.

Let's imagine a descent with thrust levers in IDLE position. From my research about aircraft systems and jet engines I would assume that turning on TAI would (in that order):

get more bleed air -> less air to burn -> turbine turning slower -> decreased N1

But from what I can experience and read on the web N1 is actually increased instead. I can guess the aircraft it trying to compensate something - but what is it if we're in idle descent? Also, I am assuming the decreased vertical speed comes from somehow increased thrust, which I would also suspect to be lower (or at least kept on the same level as before TAI) not higher than originally.

I understand this is a complex system and each part has impact on each other but maybe you can point me in the right direction ;)

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2 Answers 2

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When turning on the Anti-Ice, the demand for bleed air increases and the EEC (Electronic Engine Control) will increase the target N1 to provide enough pressure by injecting more fuel. From the Boeing 737 NG FCOMv2 (7.20.5 Engines, APU - Engine System Description, emphasis mine):

In the normal mode, the EEC uses sensed flight conditions and bleed air demand to calculate N1 thrust ratings. The EEC compares commanded N1 to actual N1 and adjusts fuel flow to achieve the commanded N1.

This impacts the descent performance as explained in the Boeing 737 NG FCTM (4.24 Climb, Cruise, Descent and Holding):

The use of anti-ice and the increased thrust required increases the descent distance. Therefore, proper descent planning is necessary to arrive at the initial approach fix at the correct altitude, speed, and configuration. The anticipated anti-ice use altitude should be entered on the DESCENT FORECAST page to assist the FMC in computing a more accurate descent profile.

Note: the above quote is for the 737 NG models. The older 737 classic series does not increase thrust:

Since all engines have been manufactured or modified to use a single idle speed in flight, the use of TAI does not increase thrust as it does in other models.

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  • $\begingroup$ Thanks for the detailed answer. However, I still don't understand where does this increased thrust come from. If N1 is increased in order to restore original amount of air in the combustion chamber to prevent flame out, why is there an increase in thrust? The same amount of air is now burned, and additional air is redirected to TAI. Where does the thrust come from? Does it come from increased bypass air as a consequence on increased N1? $\endgroup$
    – fafaldo
    Commented Nov 14, 2019 at 13:22
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    $\begingroup$ N1 is (more or less) directly proportional to thrust. An increase in N1 will therefore also increase thrust regardless of how much bleed air is used. You will however need more fuel to create the same amount of N1 because some of the airflow into the core engine is used for the bleed and does not enter the combustion chamber. Therefore less air goes through the turbine. Bypass air however is unaffected and this is creating most of the thrust in a high-bypass turbofan. $\endgroup$
    – Bianfable
    Commented Nov 14, 2019 at 13:25
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    $\begingroup$ So to sum it up to make sure I got it right: idle N1 is already at the border value, providing air to the combustion chamber that is absolutely necessary to prevent flame-out. So When we start getting less air to burn because of increased bleed usage, we need to start burning more fuel to get the N1 not only to the original value, but higher to provide the same amount of air into the combustion chamber as before. So because of that amount of thrust produced in the combustion chamber will more or less stay constant, but the increased thrust comes back from the increased bypass air? $\endgroup$
    – fafaldo
    Commented Nov 14, 2019 at 13:55
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    $\begingroup$ @fafaldo Yes, that sounds about right to me ;) $\endgroup$
    – Bianfable
    Commented Nov 14, 2019 at 14:00
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Turning on Engine Anti-Ice causes the engine idle N1 to automatically increase so that it can supply additional bleed air and not flame out.

This means that for descent planning, an earlier descent should be planned since idle N1 is now producing higher thrust.

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