737-800 Holding time??

[endofeng]

Hi guys,

Can anyone tell me why the 737-800 with the FMC programmed to hold with 1 min legs and a slack wind would fly nearly a 7 min hold??

Wind: Slack
Hold: DAYNE MAN, Inbound 312, RHTurns, 1 min legs
A/C: Holding at FL70, spd 215kts as per FMC
FMC: 1 min legs at DAYNE, update 10.8

I've not noticed this on the -300, but scratching my head to why the -800 would do nearly twice the standard 4 min hold!!

Any info much appreciated!

[B737900er]

If this a flight sim question then you have a bug in the program.

[Johnny Tightlips]

Is there not an alternate holding at DAYNE when the MCT VOR/DME is u/s? It has 3.1nm legs. Maybe the FMC uses this as the default for some reason?

[endofeng]

B737900er - errr no it's not!

It was definitely programmed with 1 min legs, I was PNF and did it myself!

Very strange, I've flown the 737 for over 6 years and never experienced this before!

May have been a bug, I'll check again next time I have enough fuel to hold for 15 min

[de facto]

scratching my head to why the -800 would do nearly twice the standard 4 min hold!!

Was MCT VOR inop?

[endofeng]

Defacto - No, hence my confusion, backed it all up raw data too!!!

#confused

[Waldo]

Did you enter the hold at a speed faster than 215KTs. I believe the FMC will adjust the hold size depending what speed you passed the fix.

[Jinkster]

Where is BOAC when needed?

[Crankshaft]

...standard 4 min hold!!

First of all, a B737 will never make a 4 minute hold unless you're flying very, very slowly.
This is because, unlike a light aircraft, we can't achieve a rate 1 turn while limited to 25° bank.

At a density altitude of about 7-8000 feet your TAS would be close to 250 kts if you fly 215 indicated. Each 180-turn would then require about 1 1/2 minute which totals the hold to about 5 minutes. And that is only if you really fly the 25° bank.

So the question is why the aircraft flew 2 more minutes for a complete hold.
It depends on what the hold was calculated to and what was really the case.

What altitude were you at and what altitude was the hold calculated for?
Was there a significant deviation from ISA?
Was the hold executed while still descending through a level with a different wind?
What bank angle did you actually achieve?
(I often see the 737 fly LNAV turns on departures and arrivals with a lot lesser bank angle than you would expect, even when you have entered correct speeds and altitudes, with a resulting overshoot).

The magenta line is calculated for whatever ambient conditions the computer thinks will prevail by the time of execution. Any difference from this and the aircraft will still fly the magenta line regardless of actual conditions.

EDIT: Waldo also has a very good point above. Did your hold speed change while already entered the hold?

[MrHorgy]

I despair at my initial partial on my IR, because I flew 1min 20 secs outbound, rather than 1 minute and didn't pass that section, when in real life aircraft seem to do whatever the FMC wants it to!

[de facto]

So if if the VOR was operative,It means that 2 type rated pilots were just staring at their CDU rather than flying their aircraft,,quite worrying..

[endofeng]

Defecto, your comment is neither valid or helpful!!

In actual fact we aviated then communicated our observations to ATC who were happy for us to continue with our slightly longer than published hold, then we continued to navigate using a mode enabling me to concentrate on the big picture - such as how much fuel we had! It was deemed by myself as the Commander the safer option at that time owing to several other factors!

And as I said earlier it was backed up with raw data!!! We were also under RADAR control above our MSA, with neither Pilot transfixed on their ND's..

Thanks for the constructive comments, it would seem there are possibly many factors affecting the length of the hold, on this particular day we entered the hold at hold speed at FL80 (standard obviously set). The QNH was low if I remember correctly!

Again thanks for the feedback.

[Checkboard]

When you say if flew "nearly a 7 min hold" - What exactly are you timing?

[endofeng]

Checkboard - Over the beacon Start Stopwatch, out turn, outbound, in turn, inbound then back over the beacon - check stopwatch = nearly 7min!

Ie one full hold!!!

[de facto]

In actual fact we aviated then communicated our observations to ATC who were happy for us to continue with our slightly longer than published hold, then we continued to navigate using a mode enabling me to concentrate on the big picture - such as how much fuel we had! It was deemed by myself as the Commander the safer option at that time owing to several other factors!

I agree using Lnav holding above Msa under radar will reduce workload,however you did enter a non standard hold albeit agreed upon by ATC and then started to work out how much holding fuel you had...
How about a brief before descent concerning holding fuel?possible alternates..or was it not deemed important by then?
So you were both concentrating on the same things? How about one flies and the other(captain) makes those last minute decisions?
I dont want to seem too harsh but it seems some just didnt work out well on that flight...dont you think?

[endofeng]

Ohh Defecto.....

How you have come to your conclusions from what brief explanation to my query I have given is beyond me!

Let me try to explain!

It is my companies SOP to do a full brief prior to TOD. I adhere to this SOP (and some!), as do my FO's. On the day a full brief was given encompassing all eventualities regarding AC serviceability/Fuel for Holding/MSA/Approach(and published hold)/Landing/GA's/Diversions. My FO on the day (an exemplary example and about to upgrade) did a splendid job of briefing. We covered all bases!, so in answer to your comments - we did!

I agree using Lnav holding above Msa under radar will reduce workload,however you did enter a non standard hold albeit agreed upon by ATC and then started to work out how much holding fuel you had...

We entered the hold with approx 15 min of holding fuel. We entered the standard published hold at DAYNE which was crosschecked in the FMC and backed up with raw data. Do you really think we didn't assess the fuel situation? Of course I did, as I said it was briefed and deduced from the Fuel on Board how much time we had before we either committed to MAN or diverted......this is standard practice, so no we weren't having a bad day - more a standard one!

On entering the hold it soon became obvious we were flying a longer than expected 4 min hold which was quickly conveyed to ATC, who were happy for us to continue - which in turn lessened the workload enabling me to concentrate on the big picture. At no point was either Pilot 'out of the loop' or transfixed on Nav Displays.

So you were both concentrating on the same things? How about one flies and the other(captain) makes those last minute decisions?
I dont want to seem too harsh but it seems some just didnt work out well on that flight...dont you think?

No we were not both concentrating on the same things, I was PNF/PM my FO was flying the aircraft. There were no last minute decisions, we had a plan, its covered in the brief! The only anomaly was the longer than expected hold which was quickly resolved via ATC - and a mute de-brief point on the ground - but did get me asking the question why the aircraft held for that time over standard expectations - hence this thread about the holding timing of the boeing 737-800......

So please, can you stop trying to read into my query as a bad day at the office - it wasn't, it was a very nice day out with a query at the end - no drama - QUEEN!!!

Again, many thanks for the constructive stuff

[Oakape]

First of all, a B737 will never make a 4 minute hold unless you're flying very, very slowly.
This is because, unlike a light aircraft, we can't achieve a rate 1 turn while limited to 25° bank.

This is correct. The regs in our part of the world cover it by stating 'a rate 1 turn up to a maximum 25 degree angle of bank'. I would imagine all regs say something similar.

We were in the hold a few days ago at FL200, 220 kts indicated & the inbound turn was taking 2 minutes & 5 seconds. The outbound turn was just under 2 mins. The difference is because the FMC draws a perfect racetrack pattern. It does the downwind turn at 25 degrees AOB, draws parallel inbound & outbound legs & adjusts the other, in to wind turn, AOB to suit.

I have never had ATC complain, because they shouldn't. In fact, I have had some ATC comment on how nice the patterns look on their screens.

[Denti]

Wasn't there a known bug in the 737 FMC that sometimes holds were calculated too big? I remember even the odd "UNABLE HOLD AIRSPACE" scratch pad message and a technical bulletin about it. Dunno if it was since fixed to be honest. If that message appeared we simply flew the hold with primary autoflight modes (heading/VORLOC) and sometimes tried to re-enter the hold which usually worked out fine. Haven't flown a hold in a few years now, over here they like their transitions instead of holds.

[de facto]

If that message appeared we simply flew the hold with primary autoflight modes (heading/VORLOC) and sometimes tried to re-enter the hold which usually worked out fine

Alleluia