Glideslope unserviceable, a displaced threshold and a missed-approach gradient that cannot be met on one engine
You are the pilot in command of a twin turboprop, landing weight 6,200 kg, Vref 115 kt, arriving at an aerodrome of elevation 320 ft in moderate rain. The ILS to runway 05 is serviceable only as a localizer: a NOTAM says the glideslope is unserviceable. The chart gives ILS DA 540 ft (220 ft above touchdown) with RVR 550 m, and LOC-only MDA 780 ft (460 ft above aerodrome elevation) with visibility 1,600 m. The LOC missed approach requires a climb gradient of 4.6 per cent to 4,000 ft. At the current temperature and weight, the aircraft's all-engines gradient is 7.2 per cent and its single-engine gradient is 2.8 per cent. Another NOTAM displaces the runway 05 threshold by 400 m, reducing the landing distance available to 2,000 m, and the runway is wet; your factored landing distance required on a wet runway is 2,050 m. Runway 23 has no instrument approach, and circling is not authorised at night. Weather: wind 230/12, visibility 2,500 m in rain, broken at 600 ft, QNH 1010. You have 55 minutes of holding fuel and an alternate 18 minutes away.
1. Can you commence the localizer approach, and what minima and visual references apply?
Read the worked answer
Yes, the localizer approach can be commenced.
With the glideslope unserviceable, the ILS DA of 540 ft and RVR 550 m are no longer available. You must revert to the LOC-only minima: MDA 780 ft, visibility 1,600 m. Using the ILS DA here would put you 240 ft below the applicable minimum.
Weather against minima
- Ceiling 600 ft above aerodrome, which is above the MDA of 460 ft above aerodrome. On the approach you should break cloud at about 600 ft and be visual before reaching the MDA.
- Visibility 2,500 m in rain against a required 1,600 m - acceptable, but rain reduces slant visibility and the runway lights must be clearly identifiable before descending below the MDA.
- Wind 230/12 against runway 05 gives a slight crosswind component of about 11 kt; check it against the aircraft limit.
Technique
- Fly the localizer on the published step-down altitudes, using DME or timing to identify the final approach point and the missed approach point, since there is no glideslope to define the vertical path.
- Level at the MDA and do not descend below it until the runway environment is in sight and the aircraft is in a position to land normally.
- Manage the descent rate so the aircraft is not diving to the MDA; a stabilised, continuous descent is safer than a late, steep correction.
First conclusion
The approach itself is legal and flyable. That does not end the decision, because the landing itself may not be.
2. What is the effect of the displaced threshold and the wet runway on your decision, and what would you do about it?
Read the worked answer
The arithmetic
- Published LDA is reduced by the 400 m threshold displacement to 2,000 m.
- The factored landing distance required on a wet runway at 6,200 kg is 2,050 m.
2,050 m is greater than 2,000 m, so the aircraft cannot be planned to land on runway 05 in these conditions. The margin is only 50 m, which is precisely the sort of small excess that gets waved away at the end of a long day - but it is a hard limit, not a guess.
Options
- Reduce landing weight: to bring the factored distance below 2,000 m you would need to shed several hundred kilograms, more fuel than you can burn without eating into your reserves. Not realistic.
- Use runway 23: no instrument approach, and circling is not authorised at night, so it is not available as an instrument arrival.
- Divert to the alternate, 18 minutes away, with 55 minutes of holding fuel - comfortably within range.
- Wait for the runway to dry: not credible in continuous moderate rain.
Decision
Divert. Discovering on short final that the runway is too short for the conditions wastes the fuel and commits you to a missed approach from low level in rain. The landing-distance calculation belongs in the approach briefing, not in the flare.
Lesson
The NOTAM displacement is the kind of item that is easy to read and easy to forget; it must be applied to the landing performance before the approach is commenced.
3. How does the published missed-approach gradient affect your briefing and decision, and how does it combine with the other constraints?
Read the worked answer
The comparison
- Published requirement: 4.6 per cent to 4,000 ft.
- All engines at 6,200 kg and the current temperature: 7.2 per cent - acceptable.
- One engine: 2.8 per cent - well short of the requirement.
What that means
The published missed approach guarantees obstacle clearance on the assumption that the aircraft can achieve 4.6 per cent. On one engine this aircraft cannot, so an engine failure during the missed approach would leave you unable to follow the protected path. The gradient is not a planning nicety; it defines whether the go-around is a safe manoeuvre.
Briefing
- State the required gradient and the aircraft's capability out loud before the approach.
- Brief the single-engine case: identify the safest area to turn towards (away from rising ground), the best single-engine gradient speed, and the drift-down altitude.
- Accept that on one engine obstacle clearance is not assured, and decide in advance whether that risk is tolerable at this aerodrome at night in rain.
Combining the three constraints
The minima can be met, the gradient can be met on two engines, but the landing cannot legally be made because of the displaced threshold and the wet runway. That single fatal constraint makes the whole approach pointless. The correct decision is to divert before starting the approach, with the fuel state intact and the missed-approach gradient risk avoided entirely.