how high can you get in an approach on the 737
 

Hi guys

Sorry if this have been discussed before.

Let say you are shooting an approach on a 737 Cl ou NG the controller brings you quite high and clears you for the approach.

As per your experience how do you know if you can still make it or not ( request a 360 ) in other words what are the "high " tolerance of the 737

thank you

Depends on company sop, by what altitude must you be stable and what is the maximum sink rate.

With a normal load, slight headwind on a 3deg gp at 6,5nm out blazing in at 190-200kts you're usually stable by 1000' if you are with flaps 5 and start getting the a/c dirty but it varies alot from time to time. If its a hot summer day with alot of thermal activity it gets more difficult. The 737 is not forgiving.

thank you

Company SOP requires to be stable by 1000 ft IMC and 500 ft VMC

Gear and full flap will allow 3000fpm (normal operating limit) at around 140kts groundspeed. You can work it out.

3000fpm @ 140kts - does that qualify as a steep approach?

(THY at AMS didn't quite achieve that level of performance:
It went from 140 kts @ 700 ft to 90 kts @ 400 ft in 27 seconds in LDG config @ idle, equivalent to 1800 fpm at constant speed).

I read of one Singaporean 737 captain that was very high and instead of going around (real men don't go around, syndrome) he simply whacked on full aileron left and right alternately in order to get flight spoiler drag. Passengers were upset understandably.

He still was 900 ft high over the threshold continuing with the wing waggle technique until his F/O had enough of this crap and shoved open the thrust levers thus forcing a go-around. The F/O saved the day on that occasion. The captain was later killed in another accident.

The moral is if the aircraft is clearly uncomfortably high on glide slope then don't risk crashing just to big note yourself in front of the co-pilot. Go around early - not later. The later you leave the decision to give it away and go-around, the more is the temptation to believing you can just sneak it in and continue the unstable approach. It is commonly known as piss poor airmanship. There is no shortage of cowboys who are tempted to give it a go.

HN39 - it is about 4-5 times the normal ILS slope. I call that steep. The difference is this is planned, THY was not.

BOAC - Having been involved with qualifying aircraft for steep approach, I too thought it was steep. What do you do to descend more steeply than THY at idle?

Remember they started 'on the glidepath' so the rate would be building - and I'm afraid I don't think your calculation is meaningful.

We'll have to differ then. The performance of the 737 demonstrated in the accident is quite similar to that of the aircraft I referred to, i.e. an L/D of around 6.5 in the landing configuration at Vref.

I don't understand the point you are making. Over 27 seconds it went from a 3 degree Vref approach to a stall. There is no way you can get anything from an 'average' RoD. Show a plot at 27x1 second intervals and we can talk. I don't think the RoD was particularly 'steep' there anyway until the very end. Just in the wrong place. Also they had full power at a high pitch angle to cushion.

EDIT::

In those 27 seconds the RoD was approximately constant at 11.11 fps. At the average airspeed of 115 kts the flight path gradient was 5.72 % (3.28 degrees).

The airspeed dropped at an approximately constant rate of 1.85 kt per second (0.0972 g).

The average drag-to-weight ratio is then 0.0572 + 0.0972 = 0.154 and the lift-to-drag ratio is 1/0.154 = 6.5

The argument is about the rate of energy dissipation.

Total energy is the sum of potential energy per unit mass (height) and kinetic energy (speed). Within certain limits, potential and kinetic energy are interchangeable.

The rate of dissipation of total energy is a function of thrust minus drag per unit weight. With the engines at idle, thrust is negligible, so the rate of energy dissipation is determined by drag-per-unit-weight, the reciprocal of L/D.

An L/D of 6.5 at zero thrust produces either a flight path angle of 8.8 degrees (15.4%) at constant speed or a deceleration of 3 kt/second (0.154*g) at constant height, or anything in between.

What is the basis of your 3000 fpm?

- see posts 2 and 4. The normal max in most airlines is 3000fpm within 3000' ft of MSA. The only way of achieving that without accumulating too much KE is as described.

The point that I'm making is that full flaps, gear down, idle thrust at 140 kts will give you a steady rate of descent of 2200 fpm.

At 3000 fpm your KE will increase at the rate of 68 kt per minute. Perhaps you should use the airbrakes.

P.S.
I'm pursuing this discussion only because I don't feel it would be good advice to the OP to accept an ATC request that requires him to descend in the landing configuration at 3000 fpm.

I agree with your caution but better to know how to do it when it happens, and it does.

Appears that we have two discussions in parallel ?

(a) steep descent prior to conducting a normal ILS or similar.

This is something we often had to contend with on the Classic in Oz. One watched what ATC was doing to the profile like a hawk as the aircraft got closer in ... and the calculation below was run constantly in the mind so that the need for an orbit was known throughout ..

Easiest way to plan for it was to

(i) start from the desired configuration and speed spun up in the slot at one's desired distance to run to touch,

(ii) work the calculation backwards

(iii) include an appropriate distance/height (level always made it much easier all round) for landing onfiguration change and slow down to final approach speed

(iv) figure on about 1nm/1000ft gear down, approach flap, idle thrust and a little below limit flap speed - our SOP rules required us to be spun up with land flap regardless of height .. which defeated the aim of the exercise .. hence approach flap to get down. The airline had been a heavy user of B727 for which land flap with idle thrust was frowned upon by all. Habits die hard ...

Generally worked out real fine and just needed the pilot to massage speed a knot or two up or down during descent to stay on the desired profile. Mind you, the pilot had to be able to do simple arithmetic.


(b) steep approach once on the final approach.

This depends on the SOP configuration sequence.

In general we rolled over into the final descent (eg ILS) at 3000ft agl 210kt clean with a requirement to be at final approach speed in the landing configuration and spun up by 1500ft.

Not much room for excess height but the sequence generally worked like a dream unless there was more than a modest tailwind with which to contend. That just meant starting with some flap and the appropriate speed for that flap.


If the approach were commenced in a part flap and reduced speed configuration, eg as per standard OEM, one could trade some initial excess height without compromising the final gate requirement.

As a general rule, not a good technique as it just loaded the crew up and increased the chances of an unsatisfactory outcome with the need for a miss .. which wasted a lot more time than requesting an orbit first time around.

Hi John,

Not really, we're discussing:
(a) steep descent prior to conducting a normal ILS or similar.

We just have three gradients in reply to the original post:

BOAC: 0.78 NM/1000 ft
JT: 1 NM/1000 ft
HN39: 1.07 NM/1000 ft

Maybe someone can be interested to try it in a simulator?

NB JT's 1/1000 is based on approach flap not full, so may well be closer to my figure.

It 'worked out really fine' for the B727. Would it work out equally well for a B737?

My comments were relating only to the 737. For the 727, the boards did us proud. For the 737, however, they were pretty useless.