BE200 - Power Settings Interfere With ILS??

[User546]

This afternoon I was browsing thru my King Air 200 manual, and under the APPROACH section of the NORMAL PROCEDURES, it has a caution box which says:

"Propeller operation in the range of 1750-1850 RPM should be avoided, as it may cause ILS Glide Slope interference."

Someone, PLEASE explain that one to me. Thanks.

 

[DaveJ]

Let's see... 1800 RPM, 3 bladed prop that means a 90 Hz rate of a prop blade passing in front of a glide slope or localizer antenna. Guess what frequencies are used to modulate the g/s and loc signals? 90 Hz and (I think) 140 Hz.

They're worried that the 90 Hz 'signal modulation' caused by the prop blade interference could distort the real 90 Hz signal modulation and cause the localizer or glide slope indicator to read incorrectly.

 

[NotFurloughed]

Nice

Good answer, good answer....is he right?...anyone?

We'll be watchin' you....

 

[snoopy_1]

Caravan manual says the same thing.

 

[mattpilot]

so basically i should stop shooting my ILS approaches in the 172 at 2700 RPM (a little bit past redline ) ??

 

[User546]

so basically i should stop shooting my ILS approaches in the 172 at 2700 RPM (a little bit past redline ) ??

OK, you made me curious about something else. With the theory that was given earlier (very good by the way, THANKS!) should I assume that would be the case with any 3 bladed prop, such as a Cessna 182 / Beech Baron? Or is this unique to the King Air you think?

(With the exception of course of the Caravan as Snoopy mentioned)

 

[mattpilot]

OK, you made me curious about something else. With the theory that was given earlier (very good by the way, THANKS!) should I assume that would be the case with any 3 bladed prop, such as a Cessna 182 / Beech Baron? Or is this unique to the King Air you think?

(With the exception of course of the Caravan as Snoopy mentioned)

I'm thinking the same thing.... why shouldn't it be the same regardless of the number of propellor blades?

But then again, today is the first time i ever heard of a prop interfering with an ILS frequency



**edit, say you have a 4 bladed prop - should you then try to avoid 1350 rpm?

 

[banned username 2]

say you have a 4 bladed prop - should you then try to avoid 1350 rpm?

Yes, I would recommend avoiding 1350 RPM unless you are on the ground, because if you aren't, you will be shortly!

 

[Flyingdutchman]

Let's see... 1800 RPM, 3 bladed prop that means a 90 Hz rate of a prop blade passing in front of a glide slope or localizer antenna. Guess what frequencies are used to modulate the g/s and loc signals? 90 Hz and (I think) 140 Hz.

They're worried that the 90 Hz 'signal modulation' caused by the prop blade interference could distort the real 90 Hz signal modulation and cause the localizer or glide slope indicator to read incorrectly.

Good ole JAA ATPL textbook knowledge!

 

[RipCurl]

90Hz & 150Hz.
LOC: 90 Hz left side, 150Hz right side
GS: 90Hz upper lobe, 150Hz lower lobe

Edit - Ref: AIM Figure 1-1-7

 

[mattpilot]

Yes, I would recommend avoiding 1350 RPM unless you are on the ground, because if you aren't, you will be shortly!

point taken .... but it was a technical question with regards to frequency interference.

 

[dhc8ap]

BE200 Props

Is it the same for a 4 bladed Hartzell 200? Would they have the same frequency modulation as a 3-blade?

 

[DaveJ]

A four-bladed prop would cause the same interference at 1350 rpm. The magic number is 5400 blades per minute (which is 90 blades per second or 90 Hz).

 

[User546]

So if I'm flying a four-bladed King Air 200, shooting an approach, I shouldn't worry about the above mentioned interference?

Because it should then only interefere at 1350 RPM's, which we all know is not a do-able power setting for an approach. (Well, a successful ending approach that is.)

 

[mattpilot]

So if I'm flying a four-bladed King Air 200, shooting an approach, I shouldn't worry about the above mentioned interference?

Because it should then only interefere at 1350 RPM's, which we all know is not a do-able power setting for an approach. (Well, a successful ending approach that is.)

On the subject of 4-bladed props .... should one also avoid 2250 RPM ? As that would interfere with 150 Hz ... ??

 

[KiloBase]

I would think it also has to do with the placement of the receiver antennae.