Holding on a star
HOLDs
While descending via the STAR you get the following radio call “SKW100 ready to copy hold instructions”. You tell ATC you are ready after you note your landing fuel which indicates 4200 Lbs. “SKW100 hold WEST on the 270° bearing FROM RUNNN, EFC 1745Z time now is 1615Z”. Well with that kind of EFC here goes my commute flight home. But what can you do? This is the job.
Before I finished reading back the holding instructions my partner (PF) is already in the MCDU working away. I look up at my MFD and RUNNN looks to be approximately 20 miles ahead. Shortly after ATC was done with us I heard them say the same thing to the aircraft behind us and then the one behind them and so forth so on. We were at FL250 descending via the arrival and the STAR showed RUNNN to be a window crossing (at or below 17000’ and at above 14000’). I realized that ATC did not give us an altitude to hold on. To make myself useful while my partner is diligently building the hold I keyed the mike and asked ATC for a hold altitude. “SKW100 cross RUNNN at 15,000, hold at 15,000’, speed at pilot discretion, you are going to be at the bottom of the stack”. I ready back the altitude and thanked the controller.
FLY the PLANE (SPEED)
I find my flying partner reaction to be typical of most pilots I fly with. Their first reaction is always to dive into the FMS to building the hold. I would argue that building the hold although important should be done after you “fly the plane”. That includes managing your speed, altitude and flight path to the hold. Of course that is true when you are given the hold in adequate time. First step should be to slow down. This does few things such as buy time and perhaps save little fuel not to mention comply with the FAR that require you to slow down to holding speed three minute prior to entering the hold. Our SOPM say that we should hold at a minimum of 210 knots above 6000 feet but never less than green DOT (Figure X).
The FAR say that min holding speed above 14000’ should be no more than 265 knots. The FAR max hold speeds provide adequate obstruction protection based on aircraft turning radius for that speed. Going any faster in the hold you might not clear obstruction and possibly terrain. So for our hold altitude of 15,000 feet now we have the hold speed range (FAR maximum 265 knots, and SOPM minimum 210 knots, respecting Green DOT). To conserve fuel the Hold speed selected should be the slowest speed within said speed range considering environmental factors such as turbulence, wind aloft, ice, etc. I recommend to the PF to go to 210 knots manual speed (or speed intervention) while we work on building the hold in the FMS (make sure you get ATC clearance if you are going to slow down early).
FLY the PLANE (ALTITUDE)
Altitude should be the second step. Go ahead and bug the assigned hold altitude in your altitude preselect window. In your decent, you should be on a lateral course with LNAV as your active lateral mode and PTH as your vertical mode. Notice that VNAV is already planning on crossing RUNNN at an arbitrary altitude within the charted window altitudes (not 15000’). In the scenario above the holding FIX altitude happens to be a window altitude (between 17000’ and 14000’). The vertical track the FMS calculated is most likely crosses RUNNN closer to 17000’ and what VNAV will use when in PTH. This is typical because the VNAV algorithms are designed to keep you as high as possible without violating any crossing restrictions. So unless you make you hold altitude a hard altitude (15000’) in the FMS the aircraft will get to the fix at the wrong altitude.
Now, it is important to understand the clearance. The one we got was ”HOLD AT” which means we have to be at 15,000’ BEFOREor AT RUNNN. That is not the same clearance as ”hold at RUNNN...., descend to 15,000“. With the latter you can arrive at RUNNN at any altitude and descend in the hold to 15,000‘. The bad news is that there are automation threats in either clearance.
With the “old AT“ clearance the threat is staying in PTH and on vertical track and subsequently finding yourself high and in green FPA (the automation threat) after you drop 15,000 on RUNNN. A return to PTH, or FLCH if you are above the vertical track, is needed which requires FMA and vertical profile awareness. Remember the plane does not slow down and go down at the same time so getting to the proper altitude is not a set a forget maneuver. Pay attention and fly the plane all the way to the hold.
The threat with the “descend to 15,000” (ie descend in the hold) clearance is related to leveling off at the wrong altitude and not continuing the descent after crossing RUNNN. Typically you get this clearance when it is not practical to descend to the assigned hold altitude before reaching the fix (you are too high). Although dropping a hard altitude of 15,000 on RUNNN in the FMS will helps with properly exiting the hold it will not help with the descend to the hold as the plane can’t PTH down in the hold. Unless a vertical mode is selected the FMA vertical mode will change from PTH to ALT (the automation threat) and the plane levels off at that same random altitude between 17,000 and 14,000 that VNAV was planning to cross RUNNN at.
Hold Components
With the speeds, vertical track and altitude preselect handled I hear my partner ask me to “confirm” the hold. The hold looked good on the MFD PLAN page however lookng at the FMS hold page I noticed that the speed shown is too low and the leg lengths is not added. The default speed that the FMS enters on the hold page is max endurance speed which in most cases is too slow (well below the required min speed of 210 knots). So, I ask my partner to enter 210 knots and adjust leg length to 1.5 MIN. According to the AIM a standard hold requires Right turns and outbound leg length adjusted to provide 1.5 min inbound leg above 14000’.
Note that if you change the LEG TIME in the FMS HOLD page the LEG DIST field will change to reflect the proposer distance and vice versa. So it make sense that you can pick one OR the other but not both. Not hearing direction of turns, leg times or distances in our hold clearance we decided to go with the AIM and SOPM guidance (Figure X). If you are not sure (and always a good idea) quarry ATC. After adjusting the fields in the FMS HOLD page fields, I note the landing fuel shown on the MFD MAP page progress strip and say “activate”.
HOLD page
Well to this point we took care of flying the plane but before we call the HOLD setup complete we need to contend with the EFC time we are given. Going back into the FMS hold page with the “EFC TIME” field blank, we note the “LAST EXIST” and “HOLD AVAIL” times displayed (Figure X). It is important to understand what the FMS unmodified times displayed on the hold page represent. The “LAST EXIST” time include the time you must exit the hold after you: 1- fly to hold, 2- hold for the amount of time shown in the “HOLD AVAIL” field,3- fly to the destination and miss, then 4- fly to the alternate (if one is entered in the RTE page) and land with the reserve fuel you entered on Perf Init Page 1.Operational variables such as routing, weather, etc. may require that you leave the hold earlier then indicted. You can certainly guess an EFC that will get you to your alternate with more than min reserve. To do so, enter a new time in the “EFC TIME” field and let the FMS, Apply and Activate to calculate your new landing fuel, both at the destination (MFD Progress strip) and alternate (Perf Data Page). Keep repeating until you get the landing fuel number you are happy with. I call that the Trial & Error method –Or-- you can choose to take the more scientific approach.
How Long Can I Hold (BARH)??
Reference landing fuel you noted before you “activated” the hold. Subtract your current fuel on board from your landing fuel will yield your BRUN from current position to the destination (you can also ask dispatch for that burn from hold point to the destination if not on the release). On the release look up your ALT fuel and required reserve. The ALTfuel is the fuel required to fly from the destination to the alternate. The reserve fuel is the minimum fuel you will be landing with at your alternate. It is acceptable to factor in more fuel then the required RESERVE but you must never plan on landing with less than your release reserve. We will use the BAR acronym to delineate the amount of fuel we must exit the hold with in order to fly to the destination, then to the alternate and land with the reserve (rounded up) fuel.
BAR = BURN + ALT + RESERVE
HOLD Fuel = FOB – BAR
Example:
Assume that you have 5500 Lbs. of fuel on board. The progress strip was showing you landing at the destination with 4700 Lbs. So your BURN = 800 Lbs. (5500 – 4700). The Release is showing that you need 700 Lb. to miss at your destination and then fly to the alternate to land with reserve fuel of 2450 Lbs. You decide to increase the reserve fuel from 2450 to 3000 lbs at you alternate.
BAR = 800 + 700 + 3000 = 4500 Lbs.
This means that in order to land at the alternate with 3000 lbs., you must exit the hold when your FOB reaches 4500 Lbs. But the fuel gage is showing 5500 Lbs. at the moment, this means:
HOLD fuel = 5500 – 4500 = 1000 Lbs.
So the amount of fuel you can use in the hold is 1000 lbs. The last step to answer the “How long you can hold” question by converting the HOLD fuel in lbs to minutes. The fuel flow in a hold will vary based on altitude and other environmental factors. For planning purposes however, a good rule of thumb number to use is 3000 Lbs./Hr. Of course this number will need to be validated once your established in the hold and an adjustment to the hold time may be required. Using the 3000 Lbs./hr mean that you can hold for 20 minutes. Add 20 minutes to the current time in ZULU and update your EFC time with ATC.
Once established in the hold and in low AOV, fill in the required fields on the HOLD page in ACRAs and send to inform OCC/Dispatch.
Unplanned Diversion (Plan B)
NOTE: It is important to note that sometimes the required fuel to the diversion field is more that the fuel needed to the destination airport. In this case the FMS calculated LAST EXIT and EFC times must be reduced to allow for the diversion airport (Plan B).
It is always good practice to ask ATC the reason for the hold. If the certainty in continuing to the destination is high, after a reasonable delay, and you have a planned alternate then no plan B is required. In this case your alternate becomes your diversion field once your LAST EXIT (or calculated EFC) time is reached. But if there is uncertainty as to whether the destination or planned alternate will be available/usable (for example an unanticipated airport security breach) AND/OR you have no planned alternate, a plan B is required (unplanned Diversion).
While you request and wait for a Diversion field from dispatch you may look for alternatives. After checking the suitability (runway lengths, WX, NOTAMs..etc.) of nearby airports you decided on a diversion field. Using the NAV function on the MDCU, go to CROSSING POINTS and select PPOS DIRECT. After entering the diversion field 4 letter identification code the Present Position Direct function of the MCDU will show you the landing Fuel at that field. This is based on leaving the hold right now and proceeding direct to that airport unitizing your current fuel flow. Figure X showing you landing in DEN with 6400 Lbs. of fuel provided you left the hold now.
Unplanned Diversion (Present Position Direct)
Assume that you want to land in KDEN with 3000 Lbs instead of required reserve of 2450 Lbs. That leaves you with a hold fuel of 3400 Lbs. (6400 – 3000). Using the rule of thumb of 3000 Lbs./Hr you can hold for 1 hour & 8 minutes. Hopefully dispatch has provided you with diversion airport information including fuel burn (present position/holding fix to your diversion) by now that confirms your calculations.
Exiting the HOLD
Although there are other ways to exit a hold (going direct to a fix for example) using the EXIT HOLD function on the MCDU is the best way to exit a hold. Using the previous hold example, if the subsequent clearance was “exit the hold, cross RUNNN at 15000’ then descend via the STAR” it is important to note a key VNAV behavior that might lead to an altitude violation. Magenta FLCH will NOT respect the altitude crossing ahead if that altitude is the same as the present aircraft altitude and should not be selected. To comply with given clearance above, PTH is the proper mode. To get PTH, set the bottom altitude on the arrival in your altitude preselect window, select EXIT HOLD, ensure that you are in LNAV and magenta ALT and wait. Note that the TOD and RUNNN may now coincident. Reaching TOD, VNAV will transition to PTH. If you select magenta FLCH in error this will cause an early descent (prior to reaching RUNNN/TOD) even though you are in VNAV and have 15000’ set in the FMS thus violating your clearance.
Remember
Entering and exiting holds are areas of high AOV. At high altitudes with high winds aloft, high ISA or a heavy plane require heightened awareness. Ensuring you are flying the proper speeds and altitudes is critically important to ensure terrain clearance and prevent potential CFITs or slow speeds events. Although fuel planning and communication with company, Flight Attendants and passengers is important it should only be done after you are established in the hold at the proper altitude and speed and in a low AOV (ie. both pilots in the GREEN).