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Significance Of 800ft RA  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (4 years 1 month 1 week 21 hours ago) and read 2523 times:

Just trying at the minute to brush up on my tech knowledge of the 737NG. I've noticed that in relation to a number of systems, especially the ILS and Flight Director system that when passing 800FT RA certain changes take place.

Is this just on the 737 or all Boeings/other makes?

Was just wondering what the significance of 800ft, as in why 800

Many Thanks

2 replies: All unread, jump to last
User currently offlineBoeingGuy From United States of America, joined Dec 2010, 4402 posts, RR: 8
Reply 1, posted (4 years 1 month 1 week 7 hours ago) and read 2366 times:


Help me out with this. I'm not familiar with much of anything happening at 800 ft RA on Boeing airplanes. What system effects are you speaking of?

The 787 has some alert aural takeoff and landing inhibits that start and end at 800 ft, but those are usually at 400 feet on other models, and optional at 400 ft on the 787 (e.g. Caution aurals are inhibited between 80 knots and 400 or 800 feet during takeoff).

I'm not familiar with 800 feet having any significance on the ILS our Autoland system. 1500 feet and 200 feet are altitudes in which certain things happen.

User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Reply 2, posted (4 years 1 month 1 week 2 hours ago) and read 2308 times:

Just copied and pasted some bits were 800ft is mentioned. Any ideas would be greatly appreciated.


For dual autopilot operation, both VHF NAV receivers must be tuned to the ILS
frequency and both autopilots must be selected in CMD prior to 800 feet RA.

Autopilot (A/P) Disengage Light
Illuminated (red) –
• flashes and tone sounds when autopilot has disengaged
• reset by pushing either disengage light or either A/P disengage switch
• steady for any of following conditions:
• stabilizer out of trim below 800 feet RA on dual channel approach

If a generator is lost during a F/D TO or GA, or while in dual F/D APP mode
below 800 feet, the FCC on the unaffected side positions the F/D command bars
on both attitude indicators. If the F/D MA light on the affected side had been
illuminated, it extinguishes upon electrical bus transfer.

• the A/T remains in THR HLD until 800 feet RA. A/T annunciation then
changes from THR HLD to ARM and reduction to climb thrust can be
made by pushing the N1 switch.
Note: During a reduced thrust takeoff, a second press of the TO/GA switch
below 800 feet RA will change the thrust limit mode to GA and N1
reference bugs to increase to full GA thrust, thrust levers will not be in
motion. A second press of the TO/GA switch above 800 feet RA, thrust
levers advance toward full GA thrust.

800 ft RA
mode engages

One VHF NAV receiver must be tuned to an ILS frequency before the approach
mode can be selected. For a dual A/P approach, the second VHF NAV receiver
must be tuned to the ILS frequency and the corresponding A/P engaged in CMD
prior to 800 feet RA.

800 Feet Radio Altitude
The second A/P must be engaged in CMD by 800 feet RA to execute a dual
channel A/P approach. Otherwise, CMD engagement of the second A/P is

Illuminated (amber) –
• in–flight -
• SPEED BRAKE lever is beyond the ARMED position, and
TE flaps extended more than flaps 10, or
• radio altitude less than 800 feet
The SPEEDBRAKES EXTENDED light provides an indication of spoiler
operation in-flight and on the ground. In-flight, the light illuminates to warn the
crew that the speed brakes are extended while in the landing configuration or
below 800 feet AGL
Landing Gear Indicator Lights (top)
Illuminated (red) –
• landing gear is not down and locked (with either or both forward thrust
levers retarded to idle, and below 800 feet AGL).

Red light illuminated –
• landing gear is in disagreement with LANDING GEAR lever position (in
transit or unsafe).
• landing gear is not down and locked (with either or both forward thrust
levers retarded to idle, and below 800 feet AGL).

Aural Indications
A steady warning horn is provided to alert the flight crew whenever a landing is
attempted and any gear is not down and locked. The landing gear warning horn is
activated by forward thrust lever and flap position as follows:
Flaps up through 10 –
• altitude below 800 feet RA, when either forward thrust lever set between
idle and approximately 20 degrees thrust lever angle or an engine not
operating and the other thrust lever less than 34 degrees. The landing gear
warning horn can be silenced (reset) with the landing gear warning

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